CHARMM

Table Of Contents

Previous topic

Low Energy Path OPTmization

Next topic

Monte Carlo Minimization/Annealing (MCMA)

This Page

..py:module:: mc

Monte Carlo

The Monte Carlo commands in CHARMM have been designed to allow construction and use of an almost arbitrary move set with only a few atom selections. This goal is accomplished by providing a pre-defined set of move types which can be combined to specify the allowed movements of an arbitrary CHARMM molecule. Speed and flexibility are gained by separating the bookkeeping associated with a move (MOVE subcommands) from the actual application of that move to the molecule (MC).

Syntax for MOVE and MC commands

[Syntax MOVE < ADD | DELEte | EDIT | READ | WRITe | LINK > ]

MOVE ADD  1{ MVTP move-type } nsele{ SELE...END } -
           [ WEIGht  1.0 ] [ DMAX      1.0 ] [ TFACtor     1.0  ] -
           [ FEWEr   0/1 ] [ NLIMit      1 ] [ LABEL move-label ] -
           [ opt-spec    ] [ mini-spec     ] [ hmc-spec         ]

           where nsele, the number of SELE...END statements,
           depends on move-type

move-type (nsele)::= < RTRN rig-unit ( 1 ) |   ! Rigid translations
                       RROT rig-unit ( 1+) |   ! Rigid rotations
                       CART          ( 1 ) |   ! Single atom displacements
                       TORS          ( 2 ) |   ! Simple torsion rotations
                       CROT [PIMC]   ( 1+) |   ! Concerted torsion rotations
                       HMC           ( 1 ) |   ! Hybrid Monte Carlo
                       VOLU rig-unit ( 1 ) >   ! Volume scalings

rig-unit ::= < BYREsidue | BYALl | BYHEavy | BYATom >

opt-spec ::= [ ARMP   -1.0 ] [ ARMA      0.0 ] [ ARMB        0.0 ] -
             [ DOMCf  -1.0 ] [ ANISotropic 0 ]

mini-spec::= [ MINI                      < SD (default) | CONJ > ] -
             [ NSTEps    0 ] [ NPRInt      0 ] [STEP         0.2 ] -
             [ TOLEner 0.0 ] [ TOLGrad   0.0 ] [TOLStep      0.0 ] -
             [ INBFrq   -1 ]

hmc-spec ::= [ NMDSteps  0 ] [ TIMEstep  0.0 ] -
             [ MEUPdate  0 ] [ MEFActor  0.0 ] [ MEWEight 0.0 ]

MOVE DELEte < MVINdex move-index | LABEL move-label > -

MOVE EDIT   < MVINdex move-index | LABEL move-label > -
            [ WEIGht prev  ] [ DMAX        prev ] [ TFACtor prev ] -
            [ NLIMit  prev ] [ opt-spec         ] [ mini-spec    ] -
            [ hmc-spec     ]

prev ::= previous value

MOVE WRITE [UNIT -1]

MOVE READ  [UNIT -1] [APPEnd 1]

MOVE LINK  < MVI1 move-index  |  LAB1 move-label > -
           [ MVI2 move-index ] [ LAB2 move-label ] [ GCMC ]

[Syntax MC]

MC [ NSTEps          0 ] [ ISEEd   prev ] [ IACCept   0 ] [ PICK      0 ] -
   [ TEMPerature 300.0 ] [ PRESsure 0.0 ] [ VOLUme prev ] [ ACECut 0.01 ] -
   [ INBFrq          0 ] [ IMGFrq     0 ] [ IECHeck   0 ] -
   [ IUNCrd         -1 ] [ NSAVc      0 ] [ IMULti   -1 ] -
   [ RESTart           ] [ IUNRead   -1 ] [ IUNWrite -1 ] [ ISVFrq    0 ] -
   [ IARMfrq         0 ] [ IDOMcfrq   0 ] [ gcmc-spec   ] [ wl-spec     ]

gcmc-spec ::= [ GCBF  0.0 ] [ MUEX  0.0 ] [ DENSity 0.0 ] [ NGCTry 0 ] -
              [ GCCUt 0.0 ] [ NOTBias 1 ] [ RGRId  -1.0 ] [ INSPhere ] -
              [ INSX  0.0 ] [ INSY  0.0 ] [ INSZ    0.0 ] [ INSR 0.0 ] -
              [ XMIN  0.0 ] [ YMIN  0.0 ] [ ZMIN    0.0 ] -
              [ XMAX  0.0 ] [ YMAX  0.0 ] [ ZMAX    0.0 ]

wl-spec   ::= [ IWLRead -1 ] [ IWLWrite  -1 ] [ NWLFrq  0 ] [ WLINcr 0.0 ] -
              [ WLTOl  0.0 ] [ WLUPdate 0.0 ]

MOVE

The MOVE subcommands are associated with construction of the move set.

The primary MOVE subcommand is MOVE ADD, which determines all of the locations in a subset of atoms to which a move type can be applied. For each location (or “move instance”), MOVE ADD also determines the rotation axes and centers, the moving atoms, and the relevant bonded terms. Thus, each call of MOVE ADD results in a group of move instances of the same move type (the number of instances is stored in the substitution variable ?NMVI). By repeatedly calling the MOVE ADD command, the user can employ several different types of moves in conjunction, which typically yields the most efficient and complete sampling.

The available pre-defined move types are rigid translations (RTRN), rigid rotations (RROT), single atom displacements (RTRN), rotations of individual torsions (TORS), concerted rotation of seven (or, in the case of a chain end, six) torsions (CROT) to deform the system locally (Dinner, 2000; Dinner, 1999; Go and Scheraga, 1970; Dodd et al., 1993; Leontidis et al., 1994), hybrid Monte Carlo propagations (HMC) (Duane et al., 1987; Mehlig et al., 1992), and volume scaling moves for constant pressure simulations (Eppenga and Frenkel, 1984). Each of these can be applied to an arbitrary subset of atoms using standard CHARMM SELE...END statements.

MVTP rig-unit nsele Description
RTRN BYALl 1 The entire atom selection is rigidly translated.
RTRN BYREsidue 1 The residue containing each selected atom is rigidly translated. If more than one atom in a residue is selected, each counts as a separate move instance.
RTRN BYHEavy 1 Each heavy atom and its associated hydrogen atoms are rigidly translated.
RTRN BYATom 1 Each instance is a displacement of a single atom by a random vector distributed uniformly in an ellipsoid (see the description of the ANISotropic keyword). For historic reasons, the CART keyword is a synonym for RTRN BYATom, but use of the former is discouraged since the moves are not actually based on Cartesian coordinates.
RROT BYALl 1-2 The entire first atom selection specifies the rigid body of atoms to be rotated, and each of the atoms in the second atom selection is an allowed rotation center. The second selection need not be a subset of the first, so rotations around atoms outside the rigid body can occur. If no second atom selection is given (or one is given, but no atoms are selected), the rotations are made around the center of mass of the first atom selection.
RROT BYREsidue 1 There is only a single atom selection, and each selected atom is a center of rotation (around a randomly selected axis) for its residue. If more than one atom in a residue is selected, each counts as a separate move instance.
RROT BYHEavy 1 The hydrogens attached to a heavy atom are rigidly rotated around the heavy atom. If the FEWEr keyword is set to 1 (the default), a move instance is counted for each selected heavy atom with at least one hydrogen atom attached (whether or not the hydrogens are selected does not matter). If the FEWEr keyword is set to 0, all heavy atoms are counted to permit straightforward linking with RTRN BYHEavy move groups.
TORS   2 The two selections define the middle atoms (JK in IJKL) of the rotatable torsions. If the FEWEr keyword is set to 1 (default is 0), the directionality of the selection will be ignored and each rotatable bond will be included only once in the move set (such as to rotate the end with fewer atoms). Otherwise, each rotatable bond will be included either once or twice depending on whether the atom selections match the bond in only one direction (JK) or both (JK and KJ). Only torsions in the PSF are enumerated.
CROT   1+ The first atom selection defines the “backbone” along which the 7 (or in the case of a chain end, 6) dihedrals lie. Each additional pair of selections defines non-rotatable bonds. The first bond in a set of 6 or 7 is the driver torsion. Non-rotatable bonds are not allowed at the third or fifth bonds following the driver (counting only rotatable ones). Note that there is no checking for whether bonds selected to be rotatable are indeed so. NLIMit is the number of torsions in addition to the driver torsion that are restricted by the maximum rotation (DMAX); values of 0 to 5 are possible. In general, setting NLIMit greater than or equal to 1 is recommended since it speeds up the root finding process and moves with large changes to the torsions tend to be rejected anyway. Concerted rotations of path integral “polymers” require the PIMC keyword.
HMC   1

The selected atoms are propagated with the specified TIMEstep for NMDSteps molecular dynamics steps. The change in total energy is used for the acceptance criterion. SHAKE constraints are respected. The standard fixed atom list is ignored, but note that, in the present implementation, selections that move only small parts of the system will be inefficient. The non-bond list update during dynamics is separate from that used in MC and is controlled by a common variable set by the DYNAmics command. To suppress updates of the non-bond list, it is necessary to issue a dummy dynamics statement prior to MC: DYNAmics NSTEps 0 INBFrq 0 NSAVC 0.

If IACCept=2, the dynamics take place on a transformed potential (Andricioaei and Straub, 1996; Andricioaei and Straub, 1997); use of the Tsallis method with HMC requires that the TSALLIS keyword be included in pref.dat during compilation.

If IACCept=0, sampling can be enhanced using the method introduced in Andricioaei et al. (2003) by setting MEFAactor, MEUPdate, and MEWEight to non-zero values. MEFActor is the csi multiplicative factor above Eq. 18 in the above paper, MEUPdate is the frequency of updating the bias vector, and MEWEight is the weight each new dynamics step carries in the bias vector (dt/tl, where dt is the timestep and tl is the averaging period).

VOLUme rig-unit 1 Volume scaling moves. Changes in ln V are selected uniformly from the allowed range, and the scaling is around the image center. The possible rigid units are the same as for RTRN and RROT. If a “mixed” scaling move is desired (e.g., solvent atoms are scaled by residue while solute atoms are scaled individually), it is necessary to couple two or more “pure” scaling moves (see MOVE LINK).
GCMC   1 Insertion and deletion moves for grand canonical Monte Carlo. Move instances are identified in the same way as RTRN BYREsidue above. In other words, there is one instance per selected atom in a grand canonical molecule (which must be a single residue); selecting more than one atom in a residue will waste memory. Moreover, translation and rotation moves of grand canonical molecules should be linked to the GCMC move group to avoid wasting time moving inactive atoms (see MOVE LINK). See MC and the Examples section below for further details on grand canonical Monte Carlo.

In addition, MOVE ADD associates with each group of move instances a set of parameters.

The values of the following parameters are used in all MC calls.

WEIGht The relative weight of that group of move instances in the complete move set. The probability of picking a group of move instances with weight w_i is w_i/(sum_j w_j) where (sum_j w_j) is the total of all the WEIGht values.
DMAX The initial maximum displacement of each instance in a group. Translations are in angstroms and rotations are in degrees. In cases where anisotropic automatic optimization is to be performed, the initial assignment is isotropic.
TFACtor A multiplicative factor to scale the TEMPerature in the acceptance criterion. TFACtor is not used in assigning the initial velocities in HMC moves.
LABEL An optional tag for the group of move instances. Only the first four characters are retained. All sets of move instances are also given an integer index which can be used instead.

The following optional parameters are associated with automatic optimization of the volumes from which individual move instances are chosen (the timestep in HMC moves). The two available methods are the Acceptance Ratio Method (ARM) and Dynamically Optimized Monte Carlo (DOMC); both are described in detail by Bouzida et al. (1992). The latter has the advantage that it allows optimization of anisotropic volumes.

ARMP ARM target probability of move instance acceptance.
ARMA, ARMB

Parameters to avoid taking the logarithm of zero in ARM:

DMAX(new) = DMAX(old)*ln(ARMA*ARMP+ARMB)/ln(ARMA*obsP+ARMB)

where obsP is the observed probability of accepting that move instance.

DOMCF

The F factor in DOMC:

DMAX(new) = DOMCF*SQRT[(d2ave*TEMP)/Eave]

where d2ave is the observed average square of the displacement and Eave is the observed average change in energy (both averages are done over all moves, not just those accepted). DOMCF is used for the anisotropic version of this equation as well. In the event that the square root of a negative number must be taken, the routine branches to ARM optimization, so ARMA, ARMB, and ARMP should be set even if one plans on using DOMC.

ANISotropic DOMC anisotropic optimization of the volume from which the moves are chosen. If ANISotropic is 0, it is off (isotropic) and, if ANISotropic is non-zero, it is on. At present, only 3D translation moves (RTRN and CART) allow anisotropic optimization.

The parameters NSTEps, NPRInt, STEP, TOLEner, TOLGrad, TOLstep, and INBFrq are associated with minimization prior to application of the acceptance criterion (Li and Scheraga, 1987) and have the same meanings as for MINImization (see minimiz.doc). Note that the INBFrq used for minimization (set in MOVE) is distinct from that used for Monte Carlo (set in MC) even though the command-line keywords are the same; moreover, INBFrq and NPRInt access common variables associated with minimization directly and thus are not stored with the rest of the move set by MOVE WRITE. During the minimization phase of a move, all atoms that have not been constrained with CONS FIX are considered mobile. At present, the minimization algorithms available are steepest descents (SD) and conjugate gradients (CONJ); in the case of CONJ, the following parameters are fixed: NCGC = 100, PCUT = 0.9999, and TOLIter = 100. It is important that the user keep in mind that MC with minimization does not satisfy the detailed balance condition (microscopic reversibility) and thus should be used only for conformational searching, not calculating equilibrium averages. Minimization following HMC moves is not allowed.

MOVE DELEte allows the user to delete a group of move instances. The group to be deleted is the first that matches the four-character tag specified by LABEL or the integer specified by MVINdex; if there is a conflict, the LABEL is used.

MOVE EDIT allows one to change the values of the parameters associated with a group of move instances. The matching rules are the same as those for MOVE DELEte (as a result, the LABEL parameter itself cannot be changed with MOVE EDIT). Any parameter not specified retains its current value. If a positive value is specified for DMAX, all move instances will be reset to that (isotropic) value; if a negative value (default) is specified, the maximum displacements retain their current values. If DMAX is not specified and the ANISotropic flag changes such that anisotropy is no longer allowed (when it was previously), the maximum displacements are assigned as the geometric mean of the eigenvalues of the matrix used to calculate the allowed ellipsoid from the unit sphere.

MOVE WRITe writes out the current move set to a formatted file opened with OPEN WRITe CARD.

MOVE READ reads in a move set. If APPEnd is 0, existing moves are eliminated; otherwise they are preserved and the new moves are appended. MOVE ADD calls can follow to expand the move set further.

MOVE LINK links two existing moves such that they are always performed together before applying the acceptance criterion. For example, one might wish to perform both a rigid body translation and a rigid body rotation of a butane molecule in the same MC step. The first move group [specified by either its label (LAB1) or index (MVI1)] remains “active”, while the second move group becomes “slaved” to the first. In other words, a move from the second group can no longer be selected by itself (as a result, only the WEIGht parameter of the first move group matters). At present, move instances within the groups are matched by indices, so the two move groups must have the same numbers of instances. MOVE LINK can be called repeatedly to create a chain of moves. In the example mentioned above, one might also want to change the central dihedral of the butane molecule in the same MC step. In the second MOVE LINK call, the second move group from the first call would become the first move group, and the new move group would be the second:

MOVE LINK LAB1 RTRN LAB2 RROT   !Resulting chain is RTRN->RROT
MOVE LINK LAB1 RROT LAB2 DIHE   !Resulting chain is RTRN->RROT->DIHE

Moves can be decoupled (in the reverse order by which they were linked) by specifying only a single move label (LAB1) or index (IND1):

MOVE LINK LAB1 RROT             !Resulting chain is RTRN->RROT
MOVE LINK LAB1 RTRN             !All moves are treated separately

If minimization before applying the acceptance criterion is desired, it must be associated with the first move group in the chain (RTRN in the example); other minimization parameters will be ignored. By the same token, only the TFACtor for the first move group will be used. Linking is not allowed with Hybrid Monte Carlo moves (HMC).

The GCMC keyword is used to specify that a move group involves changes to the coordinates of molecules that are inserted and deleted during grand canonical simulations. MOVE LINK GCMC suppresses moving “ghost” molecules, and thus saves simulation time. In contrast to other calls to MOVE LINK, no chain of move groups is constructed, and the non-GCMC move group is still active (not slaved).

Please note that the MOVE LINK command is presently under development. Consequently, the syntax might change in future versions. Moreover, its compatibility with certain other features, such as automatic optimization of the move limits, is not guarranteed.

MC

The MC command performs the loop over the appropriate number of Monte Carlo steps. Each step consists of (1) randomly picking a group of move instances (weighted), (2) randomly picking an instance from that group (unweighted), (3) calculating the energetic contribution of the moving atoms and their images, (4) applying the move, (5) calculating the energetic contribution in the new configuration, (6) applying the acceptance criterion, (7) if necessary updating the statistics for automatic optimization of the move limits, and finally (8) performing any desired I/O (at present, only trajectory writing is enabled).

NSTEps The number of loop iterations. Each pick of a single move instance and subsequent application of the acceptance criterion counts.
ISEEd The seed for the random number generator. If it is not specified, it is unchanged, so that a script can be seeded once initially and then loop over an MC command and yield different behavior with each call.
TEMPerature The absolute temperature in degrees Kelvin.
PRESsure The pressure in atmospheres.
VOLUme The starting volume for constant pressure simulations. It is only necessary to specify if the images are created by an IMAGe TRANsformation rather than the CRYStal command.
INBFrq

The non-bond list update frequency.

  • If INBFrq = 0, the list is not updated.
  • If INBFrq < 0, a heuristic is applied every -INBFrq steps; the list is updated if any atom during a checking step moved more than 0.5*(CUTNB - CTOFNB).

Note that a call to ENERgy or UPDAte must be made before MC to initialize parameters for non-bond list generation.

IMGFrq

The image list update frequency.

An image update will force a non-bond list update.

  • If IMGFrq = 0, the list is not updated.
  • If IMGFrq < 0, the list is updated if a heuristic non-bond list update is done; this option should be used only if INBFrq is also negative.
IECHeck

The total energy check frequency.

  • If IECHeck = 0, the energy is not checked.
  • If IECHeck < 0, the energy is checked if a heuristic non-bond list update is done; this option should be used only if INBFrq is also negative.

The difference between the MC running total and the current total is printed in the Delta-E column of the table.

NSAVc The frequency of writing out the trajectory. If NSAVc is 0, no coordinates are written.
IUNCrd The I/O unit for trajectory writing.
RESTart If present, this keyword indicates that the run is a restart.
IUNRead The I/O unit from which to read the restart information.
IUNWrite The I/O unit to which to write the restart information.
ISVFrq The frequency of writing the restart information.
IARMfrq The frequency of updating the move size by ARM. Note that this counter runs separately for each move instance. If IARMfrq is 0, the move size is not updated.
IDOMcfrq

The frequency of updating the move size by DOMC. Note that this counter runs separately for each move instance. If IDOMcfrq is 0, the move size is not updated.

If both IARMfrq and IDOMcfrq are non-zero, IARMfrq takes priority.

PICK

Flag for method of selecting moves from the move set:

  • 0 = Random move group and random instance (default)
  • 1 = Try each move group and instance sequentially
  • 2 = Random move group but sequential instances within a group

At present, the PICK flag is considered to be an unsupported feature and may be changed without backwards compatibility in future versions.

IACCept

The acceptance criterion to be used.

  • If IACCept is 0, Boltzmann (Metropolis) weighting is used.
  • If IACCept is 1, multicanonical (constant entropy) weighting is used (in which case TEMPerature is ignored).
  • If IACCept is 2, Tsallis (generalized) weighting is used.
  • If IACCept is 3, Wang-Landau version of the multicanonical algorithm is used (recommended over IACCept=1).

The following optional parameters are specific to particular non-canical acceptance criteria.

EMIN The estimated minimum energy of the system in Tsallis MC.
QTSAllis The Tsallis q parameter (see Andricioaei and Straub, 1997).
IMULti

The I/O unit for reading in the multicanonical weights. The file format (subject to change) is:

CHARMM title
Emin  Emax   Nbin
i     E_i    ln[n(E_i)]
       .
       .
       .
Nbin  E_Nbin ln[n(E_Nbin)]

Note that MC closes this file, so that it must be reopened before each MC call with multicanonical weighting.

IWLRead

The I/O unit from which to read the initial guesses of the histogram and free energy for Wang-Landau MC. The file must begin with a CHARMM title (lines starting with “*”) followed by the data in three columns: (1) the indices of the arrays holding the accumulated histogram and free energy surface, (2) minus the free energy divided by kT (-bF), and (3) the accumulated histogram (n). In other words,

CHARMM title
i     -bF_i    n_i
        .
        .
        .
Nbin  -bF_Nbin n_Nbin
IWLWrite The I/O unit to which to write the histogram and free energy for Wang-Landau MC. The format for the result file is similar to that for the initial guess, except that it does not have a title section.
NWLFrq The frequency of checking the flatness of the histogram for Wang-Landau MC. A value on the order of 100000 is recommended.
WLINcrement The initial value of the amount added to the free energy at each step of a Wang-Landau MC simulation. It will be halved automatically when the criterion specified by WLUPdate is met.
WLUPdate The criterion for checking if the accumulated histogram is flat. CHARMM MC compares this number with the ratio of the standard deviation of the accumulated histogram to its average. A smaller WLUP will give more accurate results for the free energy but will require more simulation time. Reasonable choices are typically between 0.01 and 0.05.
WLTOlerance If WLIN reaches WLTO, a Wang-Landau simulation is considered converged and will stop prior to reaching the specified number of MC steps. A value on the order of 10^(-8) is recommended.

The following optional parameters are associated with grand canonical Monte Carlo simulations. Two algorithms for facilitating insertions can be used. The cavity bias method (Mezei, 1980) generates a set of candidate insertion positions at each GCMC step randomly, determines the ratio P_N = cavity sites/total sites, and picks a cavity site for insertion. The acceptance probability is adjusted based on the average of P_N to satisfy detailed balance. Alternatively, one can keep track of the cavities with a grid (Mezei, 1987). The grid-based method is more memory intensive and slower at lower density but is generally more efficient at higher density and in confined geometries such as within the binding pocket of a protein. See Woo et al. (2004) for a discussion of the methods.

MUEX Excess chemical potential.
DENSity Desired density.
GCBFactor B = mu/kT + ln<N>, which sets the excess chemical potential. If DENSity is greater than zero, GCBFactor will be calculated from MUEX and DENSity.
NGCTry Number of points to generate in cavity-biased simulations that do not employ a grid. Simple cavity bias is used automatically if NGCTry is greater than zero.
GCCUt Cutoff distance used for evaluating whether a point in space corresponds to a cavity in cavity-biased simulations.
RGRId The grid spacing for grid-based cavity-biased insertion. Grid-based insertion is used automatically if RGRId is greater than zero
INSPhere Restrict insertion to a sphere of radius INSR centered on INSX, INSY, INSZ. Otherwise, insertions are made in the box spanning XMIN < X < XMAX, YMIN < Y < YMAX, and ZMIN < Z < ZMAX.
NOTBias The number of orientations to attempt when inserting.

The parameter ACECut allows approximation of the ACE screening energy term to accelerate MC simulations which employ the ACE/ACS solvation model. In calculating the total screening energy, as in molecular dynamics, one performs two summations: the first determines the Born radii (b_i) and self energies of the atoms and the second determines the screening energy given the Born radii. In MC, this scheme becomes inefficient. One typically moves only a small part of the system in each step, but one must update all the pairwise interactions (between atoms i and j) in which b_i, b_j, or both change (even if the distance between i and j remains the same). In CHARMM, this problem is overcome by neglecting the contribution to the change in screening energy from atom pairs in which both S_i and S_j are less than ACECut, where S_i = Sum_k.ne.i [E_ik^self/(tau*q_i^2)] (see equations 22, 28, and 31 of Schaefer and Karplus, 1996). For peptides, a choice of ACECut = 0.01/Angstrom has been found to yield close to the maximum increase in speed with errors of less than 0.001 kcal/mol/step. Note that HMC moves and moves involving minimization employ the standard ACE energy routines and thus calculate the ACE energy exactly.

EXAMPLE OF A STANDARD MC SIMULATION

No special actions must be taken during PSF generation to run an MC simulation. Essentially, input files set up for dynamics can be turned into MC input files by replacing the DYNAmics call with a series of MOVE ADD calls (or a MOVE READ call) followed by a MC call. For example, to simulate a peptide in water, one could add to the CHARMM script:

      .
      .
      .

! Standard PSF generation and coordinate input above

! Create the MC move set
! Allow waters to move by rigid translations and rotations.
! Allow anisotropic optimization of the volume from which the
!     translation vectors are chosen.
MOVE ADD MVTP RTRN BYREsidue WEIGht 2.0 DMAX 0.10 SELE (TYPE OH2) END -
         ARMP 0.2 ARMA 0.8 ARMB 0.1 DOMCF 2.0 ANISo 1
MOVE ADD MVTP RROT BYREsidue WEIGht 2.0 DMAX 30.0 SELE (TYPE OH2) END -
         ARMP 0.2 ARMA 0.8 ARMB 0.1 DOMCF 2.0 ANISo 0
! Allow all torsions to move by simple rotations
MOVE ADD MVTP TORS WEIGht 0.1 DMAX 30.0 FEWEr 1 -
         SELE ALL END SELE ALL END
! Allow the backbone to move by concerted rotations with non-rotatable
! peptide bonds and N-CA proline bonds.  If disulfides are present, the
! cysteine phi and psi should be restricted too.
MOVE ADD MVTP CROT WEIGht 0.5 DMAX 10.0 NLIMit 1 LABEL PEPTide -
         SELE ((TYPE N).OR.(TYPE CA).OR.(TYPE C)) END -
         SELE (TYPE C) END  SELE (TYPE N) END -
         SELE (RESNAME PRO .AND. TYPE CA) END -
         SELE (RESNAME PRO .AND. TYPE  N) END

! Seed the generator (for this example, this could be done below)
MC ISEEd 391004

OPEN WRITE UNFOrmatted UNIT 32 NAME example.trj
! Do 20000 steps at 300 K, writing energy 100 steps.
! Update the non-bonded list every 200 and
!     the maximum displacements every 5 picks of that move instance
MC IACCept 0 NSTEp 20000 TEMP 300 -
   INBFrq 200 IECHeck 400 IMGFrq 400 IDOMcfrq 10 -
   IUNC 32 NSAVc 100

In this example, there are four groups of move instances (one for each MOVE ADD call).

It should be mentioned that it is also possible to use moves in MC apart from those which can be generated by MOVE ADD since the MOVE READ command does not do any checking as it reads in the necessary move set information. For example, it is straightforward to make rigid rotations around a pseudo-dihedral simply by changing the pivot and moving atom lists of a dihedral rotation. An understanding of the following section (Data Structures) will aid in manual move creation.

GRAND CANONICAL SIMULATIONS

Some additional actions are necessary for grand canonical Monte Carlo simulations.

Grand canonical atoms are designated as active through the GCMCon array, which can be manipulated with the SCALAR command. A value of 1 indicates that an atom is active and a value of 0 indicates that an atom is inactive. It is suggested that there be roughly twice as many grand canonical molecules as anticipated will be active on average to accomodate fluctuations.

Atoms that block grand canonical insertions in the cavity-based schemes described above are also initialized through a SCALAR array, GCBLocker. A value of 1 indicates that an atom is a blocker, and a value value of 0 indicates that it is not. Time can be saved by excluding hydrogens.

      .
      .
      .

! Generate PSF allowing for extra molecules.
READ SEQUENCE TIP3 432
GENERATE MAIN SETUP NOANGLE NODIHEDRAL

! Read coordinates of starting structure (216 molecules here).
OPEN READ CARD UNIT 1 NAME tip216.crd
READ COOR CARD UNIT 1
CLOSE UNIT 1

! Copy coordinates to uninitialized atoms.  This is important for
! molecular liquids to define the internal structure.
COOR DUPLicate SELEct IRES 1:216 END SELEct IRES 217:432 END

! Set the active and blocking atoms
SCALar GCMC      SET 1.0 SELEct IRES   1:216 END
SCALar GCMC      SET 0.0 SELEct IRES 217:432 END
SCALar GCBLocker SET 1.0 SELEct TYPE OH2     END
ENERGY

! Create the MC move set
! Rigid translations
MOVE ADD MVTP RTRN BYREsidue WEIGht 1.0 DMAX 0.25 -
         SELE (TYPE OH2) END LABEl DISP
! Rigid rotations
MOVE ADD MVTP RROT BYREsidue WEIGht 1.0 DMAX 30.0 -
         SELE (TYPE OH2) END LABEl ROTA
! Insertion and deletion
MOVE ADD MVTP GCMC WEIGht 1.0 SELE (TYPE OH2) END LABEl GCMC

! Link the GCMC moves to the rotations and displacements to avoid moving
! inactive molecules.
MOVE LINK GCMC LAB1 GCMC LAB2 DISP
MOVE LINK GCMC LAB1 GCMC LAB2 ROTA

! Link the translations and rotations to each other for greater efficiency
MOVE LINK      LAB1 DISP LAB2 ROTA

OPEN WRITE UNFOrmatted UNIT 32 NAME gcmc.trj
! Do 1000000 steps at 298 K, writing energy 1000 steps.
! Grid-based insertions with 10 attempted orientations are used.
MC NSTEp 1000000 TEMPerature 298.0 ISEEd 302430 -
   INBFrq 100 IECHeck 1000 IMGFrq 100 IUNC 32 NSAVc 1000 -
   MUEX -5.8 DENS 0.03342 -
   RGRId 0.25 GCCUt 2.5 NOTB 10 -
   XMIN -18.856 YMIN -18.856 ZMIN -18.856 -
   XMAX  18.856 YMAX  18.856 ZMAX  18.856

The trajectory saved contains all of the grand canonical molecules. The inactive coordinates are set to the initialization flag (9999.0D0) before being written. When using the trajectory file, read the trajectory and then delete the inactive molecules:

DELEte ATOM SELEct .BYRES. PROP X .GT. 9998.0 END

The list of active atoms is common, and thus GCMC can be combined readily with other CHARMM features such as dynamics. In addition, it is worth noting that the marriage of GCMC and images is not an entirely happy one; errors arising from insufficiently frequent image updates will be minimized by making the region in which insertion is allowed well within the primary system.

Data Structures

MOVE ADD establishes each of the following pointers for all move types. Each is a pointer to a dynamically allocated array that is n-instance elements long, where n-instance is equal to the number of move instances in that group. In all cases, if the array does not apply to a particular move, it is not allocated.

MDXP This array contains the information about the limits of the move. For isotropic or one-dimensional moves, it is simply an n-instance-long array of REAL*8 elements containing the maximum displacement. If the displacements are to be drawn from an anisotropic volume, the array is a list of pointers, each of which points to an array of 9 REAL*8 elements which make up the matrix that transforms the unit sphere into the appropriate ellipsoid.
IBLSTP

A list of n-instance pointers, each of which points to the list of bonded terms changing under that move instance. For each element in the four-element array QBND (bonds=1, angles=2, dihedrals=3, impropers=4) that is true, there is an element listing the index of the final element containing indices of that bonded term type followed by the list of terms themselves. This list is then followed by a similar one for the next bonded term type with QBND(i) set to true.

For example, if bonds 3, 8, and 10 and angles 16 and 17 were changing, the QBND array would contain (T T F F) and the list would contain (4 3 8 10 7 16 17).

Urey-Bradley terms are handled with the lists generated for angle terms, so do not get their own entries.

IPIVTP This array keeps track of any pivot or special atoms. If there is only one pivot atom, then it is stored in the array. If there are multiple (e.g., 2 for a TORS move and 14 for a CROT move), the list stores a pointer to a list containing the pivot atoms.
IMVNGP This array contains a compact list of the moving atoms. Each element contains a pointer to a list of the following form. The first element in the list is 1 more than the number of rigid groups (NG). Elements 2 to NG contain the index of the last array element with information about that rigid group. The atoms in a rigid group are stored as the first and last atoms in a contiguous range of atom indices.

In addition, it is worth commenting on the CHARMM fixed atom list (IMOVE) here. MC does NOT use the fixed atom list in selecting atoms to move; rather, atoms are held in place by judicious construction of the move set. However, CONS FIX can be used to save memory because MC constructs the symmetrized non-bonded list that it uses for energy calculations from the standard (upper triangle) non-bonded list. Care must be exercised when using this feature to avoid errors arising from moving atoms in the fixed atom list since no checking is done.

Shortcomings

In the interest of computational efficiency, Monte Carlo calls specific energy routines directly, rather than through the main ENERGY routine. As a result, not all energy terms are supported. Those that are supported are bonds, angles, Urey-Bradley, dihedrals, impropers, vdw, electrostatic, image vdw, image electrostatic, QM/MM (MOPAC only), path integral, asp-EEF1, asp-ACE/ACS, NOE constraints, and user (also note that the user must edit both usersb.src and mcuser.src for the user energy to work correctly). All non-bonded calculations can be either atom- or group-based. Terms not listed above are not included in the present implementation.

Only atom-based non-bonded lists can be used in grand canonical simulations.

No warnings are printed for attempts to move a bonded (or patched) residue by rigid translation and rotation.

Attempts to move cross-linked residues will break MOVE ADD if MVTP is CROT. If there is a large drift in the bond energies when bonds lengths and angles are fixed, it is probably because a non-rotatable bond (for example, the N-CA bond in proline) is being rotated by CROT. Someday, a flag might be provided to choose between automatic elimination of such moves and CROT-type relaxation of the cross-link (correct Jacobian weighting is necessary to meet the detailed balance condition in the latter), but such a flag is not on the immediate agenda of the MC developer.

REFERENCES

All studies that employ the MOVE and MC commands should reference:

  • Hu, J., Ma, A. and Dinner, A. R. (2006) Monte Carlo simulations of biomolecules: The MC module in CHARMM. J. Comp. Chem. 27, 203-216.

In addition, studies that employ the CROT moves should reference:

  • Dinner, A. R. (2000) Local deformations of polymers with nonplanar rigid main chain internal coordinates. J. Comp. Chem., 21, 1132-1144.

Grand canonical simulation studies should reference:

  • Woo, H.-J., Dinner, A. R. and Roux, B. (2004) Grand canonical Monte Carlo simulation of water in protein environments. J. Chem. Phys., in press.

Studies that employ the momentum-enhanced hybrid MC should reference:

  • Andricioaei, I., Dinner, A. R. and Karplus, M. (2003) Self-guided enhanced sampling methods for thermodynamic averages. J. Chem. Phys., 118, 1074-1084.

Studies that employ Wang-Landau MC should reference:

  • Ma, A., Nag, A. and Dinner, A. R. (2006) Dynamic coupling between coordinates in a model for biomolecular isomerization. J. Chem. Phys. 124, 144911.
  • Calvo, F. (2002) Sampling along reaction coordinates with the Wang-Landau method. Mol. Phys. 100, 3421-3427.
  • Wang, F. and Landau, D. P. (2001) Efficient, multiple-range random walk algorithm to calculate the density of states. Phys. Rev. Lett. 86, 2050-2053.

The following references may also be of interest:

  • Andricioaei, I. and Straub, J. (1997) On Monte Carlo and molecular dynamics methods inspired by Tsallis statistics: Methodology, optimization, and application to atomic clusters. J. Chem. Phys. 107, 9117-9124.
  • Andricioaei, I. and Straub, J. (1996) Generalized simulated annealing algorithms using Tsallis statistics: Application to conformational optimization of a tetrapeptide. Phys. Rev. E 53, R3055-R3058.
  • Berg, B. A. and Neuhaus, T. (1992) Multicanonical ensemble: A new approach to simulate first-order phase transitions. Phys. Rev. Lett. 68, 9-12.
  • Bouzida, D., Kumar, S. and Swendsen, R. H. (1992) Efficient Monte Carlo methods for the computer simulation of biological molecules. Phys. Rev. A 45, 8894-8901.
  • Dodd, L. R., Boone, T. D. and Theodorou, D. N. (1993) A concerted rotation algorithm for atomistic Monte Carlo simulation of polymer melts and glasses. Mol. Phys. 78, 961-996.
  • Duane, S., Kennedy, A. D., Pendleton, B. J. and Roweth, D. (1987) Hybrid Monte Carlo. Phys. Lett. B 195, 216-222.
  • Eppenga, R. and Frenkel, D. (1984) Monte Carlo study of the isotropic and nematic phases of infinitely thin hard platelets. Mol. Phys. 52, 1303-1334.
  • Go, N. and Scheraga, H. A. (1970) Ring closure and local conformational deformations of chain molecules. Macromolecules 3, 178-187.
  • Leontidis, E., de Pablo, J. J., Laso, M. and Suter, U. W. (1994) A critical evaluation of novel algorithms for the off-lattice Monte Carlo simulation of condensed polymer phases. Adv. Polymer Sci. 116, 285-318.
  • Lee, J. (1993) New Monte Carlo algorithm: Entropic sampling. Phys. Rev. Lett. 71, 211-214.
  • Li, Z. and Scheraga, H. A. (1987) Monte Carlo-minimization approach to the multiple-minima problem in protein folding. Proc. Natl. Acad. Sci. USA 84, 6611-6615.
  • Mehlig, B., Heermann, D. W. and Forrest, B. M. (1992) Hybrid Monte Carlo method for condensed-matter systems. Phys. Rev. B 45, 679-685.
  • Metropolis, N., Rosenbluth, A. W., Rosenbluth, M. N., Teller, A. H. and Teller, E. (1953) Equation of state calculations by fast computing machines. J. Chem. Phys. 21, 1087-1092.
  • Mezei, M. (1980) A cavity-biased (T, V, mu) Monte Carlo method for the simulation of fluids. Mol. Phys. 40, 901-906.
  • Mezei, M. (1987) Grand-canonical ensemble Monte Carlo study of dense liquid Lennard-Jones, soft spheres and water. Mol. Phys. 61, 565-582.
  • Okamoto, Y. and Hansmann, U. H. E. (1995) Thermodynamics of helix-coil transitions studied by multicanonical algorithms. J. Phys. Chem. 99, 11276-11287.
  • Schaefer, M. and Karplus, M. (1996) A comprehensive analytical treatment of continuum electrostatics. J. Phys. Chem. 100, 1578-1599.
  • Tsallis, C. (1988) Possible generalization of Bolzmann-Gibbs statistics. J. Stat. Phys. 52, 479-487.