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Effective Energy Function 1

EEF1 is an effective energy function combining the CHARMM 19 polar hydrogen energy function (with certain modifications, see below) with an excluded volume implicit solvation model. The solvation model is similar in spirit to the Atomic Solvation Parameter approach, but does not use surface areas and is therefore much faster. Latest benchmarks say that simulations with EEF1 take about 50% longer than the corresponding vacuum simulation.

The solvation model assumes that the solvation free energy of each group is equal to the solvation free energy of that group in a small model compound less the amount of solvation it loses due to solvent exclusion by other atoms of the macromolecule around it. The exclusion effect of nearest and next-nearest neighbors (1-2 and 1-3 interactions) are neglected because such neighbors also exist in small model compounds. The CHARMM nonbonded atom and exlusion lists are used for the solvation calculation.

Because not only DG but also DH and DCp data are available, we can calculate the solvation free energy at different temperatures. This calculation assumes a DCp independent of temperature. Therefore extrapolation to temperatures very different from 300 K is not reliable.

EEF1 refers not only to the implicit solvation model but also to the specific modifications and nonbonded options used in CHARMM. The nonbonded options must be: ctonnb 7. ctofnb 9. cutnb 10. group rdie (see example file below).

Three files are needed to use EEF1:

  • toph19_eef1.inp

    This is a modification of toph19.inp where ionic sidechains and termini are neutralized and contains an extra parameter type (CR)

  • param19_eef1.inp

    This is a modification of param19.inp which includes the extra parameter type (CR)

  • solvpar.inp

    This file contains the solvation parameters

When the INTE command is used with EEF1, the number listed under ASP is the amount of solvation free energy that is excluded between the two atom selections. For example, the INTE between atom A and atom B will give the amount of solvation A loses due to B plus the amount B loses due to A. The command “INTE sele all end” will give the amount of solvation free energy excluded, not the total solvation free energy of all atoms. That is, it is not equivalent to “ENERGY”.

EEF1 can be used with images. In that case the ASP energy term refers to the solvation free energy of the primary atoms. This is usually less negative than when images are not present, because image atoms exclude some solvation free energy from the primary atoms.

EEF1 is compatible with the BYCC non-bonded option and the NBACtive command, so that the calculation of non-bonded and solvation energy terms can be limited to specific subsets of atoms in the system.

The analytical expression of the second derivative matrix of the EEF1 potential has now been added. Thus, the normal modes, for example, can now be calculated analytically.

Syntax for EEF1

There are only two EEF1 commands:

EEF1 SETUP [TEMP real] UNIT int NAME solv_param_file


The first sets up the solvation calculation by giving TEMP and reading in the solvation parameters. And the second prints out the solvation of each group. The solvation energy is stored in ETERM(ASP) and reported under the name “ASP”. Obviously, it makes no sense to use both ASP and EEF1. If one wants to skip the solvation term after one has set it up, one can issue the command SKIP ASP.

TEMP is the temperature to which the solvation parameters refer to (default is 298.15). Note that this is unrelated to the temperature at which one runs dynamics. It just determines the solvation free energy parameter values.
PRINT prints out the solvation free energy of each atom/group as well as the solvation enthalpy and heat capacity


  1. T. Lazaridis and M. Karplus, Effective energy function for proteins in solution, Proteins, 35:133-152 (1999)
  2. T. Lazaridis and M. Karplus, Discrimination of the native from misfolded protein models with an energy function including implicit solvation, J. Mol. Biol., 288:477-487 (1999)
  3. T. Lazaridis and M. Karplus, “New View of Protein Folding reconciled with the Old through Multiple Unfolding Simulations”, Science, 278:1928 (1997)


* Example file for EEF1

open read card unit 3 name toph19_eef1.inp
read rtf unit 3 card
close unit 3

open read card unit 3 name param19_eef1.inp
read para unit 3 card
close unit 3

open read unit 3 card name filename.crd
read seque coor unit 3
close unit 3

generate main setup

open read unit 2 card name filename.crd
read coor card unit 2
close unit 2

! The nonbonded options below are part of the model

eef1 setup temp 298.15 unit 93 name solvpar.inp
update ctonnb 7. ctofnb 9. cutnb 10. group rdie

mini abnr nstep 300

!This command prints out solvation free energy for each atom
eef1 print

dynamics verlet timestep 0.002 nstep 1000 nprint 100 iprfrq 100 -
      firstt 240 finalt 300 twindh 10.0 ieqfrq 200 ichecw 1 -
      iasors 0 iasvel 1 inbfrq 20

inte sele resid 2 end sele resid 19 end

!the command below is not equivalent to energy
inte sele all end

skip asp


New EEF1 parameters (May 2004)

Recent work (e.g. Masunov & Lazaridis, JACS 125:1722,2003) revealed that the interactions between some ionizable sidechains in EEF1 are too strong. Also, interactions between hydroxyl groups seem to be too strong. The files toph19eef1.1.inp and param19eef1.1.inp contain empirical adjustments of the partial charges to mitigate some of these problems. We refer to this parameter set as EEF1.1.

In addition, topology files are provided for using EEF1 with the CHARMM22, all atom force field (solvpar22.inp, top_all22_prot_eef1.inp, and top_all22_prot_eef1.1.inp). The standard parameter file can be used with these. The combination of EEF1 with CHARMM22 has not been extensively tested.

Also, DEBYE-HUCKEL screening of electrostatic interactions has been implemented, mostly for development purposes. To use it add the keyword


where xxx is the ionic strength in mol/lt. With this all electrostatic interactions are multiplied by exp(-r/rD), where rD is the Debye length (rD = SQRT(0.0316 Temp/IonicStrength )

Implicit Membrane Model 1

IMM1 is an extension of EEF1 for modeling proteins in lipid membranes (T. Lazaridis, Proteins, 52:176-92, 2003). The implicit membrane is set up like this:

open read unit 11 card name toph19_eef1.1.inp
read rtf card unit 11
close unit 11

open read unit 12 card name param19_eef1.1.inp
read para card unit 12
close unit 12

... generate psf, read coordinates ...

eef1 setup membrane slvt water slv2 chex nsmth 10 width 26.0 temp 298.15 -
              unit 93 name ../ aemp 0.85

... mini, dyna, etc.

The keyword MEMBrane specifies that a membrane is to be modeled. “slvt water” specifies that the exterior solvent is water and “slv2 chex” that the interior solvent is cyclohexane. NSMTH (default 10) determines how steep the transition is at the interface between interior and exterior. WIDTH is the width of the interior region (default 30A). Standard values are to be used here depending on the lipid that one wants to model. Such values can be obtained from experimental data, for example see For example:

DMPC 23.1 A
DOPC 25.4 A
POPC 27.0 A

The last keyword (AEMP, default 0.85) determines the extent of strengthening of electrostatic interactions in the membrane (the smaller, the stronger). This parameter was empirically adjusted to give reasonable membrane insertion energies for model systems.

The above command sets up a neutral/zwitterionic membrane. The effect of negatively charged lipids can be accounted for by using a Gouy-Chapman term in the energy function (T.Lazaridis, submitted). This is done by adding the following keywords:

eef1 setup ....   gouy anfr 0.3 area 70. offset 3.0 conc 0.1 valence 1

GOUY specifies that a Couy-Chapman term is to be used. ANFR is the molar fraction of anionic lipids (e.g., a 70/30 mixture of PC/PG corresponds to ANFR 0.3, which is the default). AREA is the area (Angstrom^2) per lipid (default 70). OFFSet is the distance of the plane of negative charge (usually the phosphates) from the hydrocarbon/water boundary (default 3). CONC and VALEnce is the molarity and valence of the salt (default 0.1 and 1, respectively).


When the Gouy-Chapman term is calculated, the ionic sidechains are given a full charge (they are neutralized otherwise), and this is done by checking the partial charges. If you want to use topology files other the ones provided (toph19eef1.1.inp) it might not work.

It is also possible to include the effect of transmembrane voltage by adding the keyword

VOLT xxx

where xxx is the transmembrane voltage in Volt (default value 0.1). The transmembrane voltage is set up so that it is positive in the +z direction. This term is based on the analytical solution to the Poisson-Boltzmann equation (Roux, Biophys. J, 1997).

The GC and TM voltage energies are added to the Solvation Free Energy (under ASP column). These terms will be printed out if PRNLEV is greater than 9.

An implicit cylindrical, toroidal (parabolic) or circular pore in a neutral membrane can be accounted for by using a modified energy function (T. Lazaridis, J. Chem. Theory Comput., 1:716-722 (2005); M. Mihajlovic and T. Lazaridis, Biochim. Biophys. Acta, 1798:1494-1502 (2010)). The following keywords should be added for the cylindrical, toroidal or circular pore, respectively:

eef1 setup ... rcyl 10
eef1 setup ... rprb 10 aprb 15
eef1 setup ... rcrc 10

where RCYL specifies the cylindrical pore of 10 Angstrom radius; RPRB specifies the parabolic pore with the radius in the center of the pore of 10 Angstrom and APRB defines the curvature of the pore of 15 (which gives the radius at the ends of the pore of 25 Angstrom); RCRC specifies the circular pore of 10 Angstrom radius.