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CHARMM Developer Guide

This document provides a basic guide for understanding CHARMM’s architecture, implementation, and development protocols and tools. Prospective developers are urged to familiarize themselves with its contents.

CHARMM Implementation and Management

CHARMM is implemented as a single program package, which is developed for use on a variety of platforms. The single source structure makes the program easier to handle and promotes the program’s integrity.

CHARMM was originally written in FLECS, FORTRAN77 and C languages. Before FORTRAN77, FLECS allowed us to use a variety of control constructs, e.g., WHEN-ELSE, WHILE, UNLESS, etc. A FLECS to FORTRAN translator was used to process FLECS source code to produce FORTRAN source. With CHARMM 23, the FLECS source code was converted to FORTRAN 77. CHARMM 23f2 and later versions are fully in FORTRAN, except for some machine-specific codes written in C. All new code should be written in FORTRAN 95.

Since CHARMM version 22, all files are maintained by utilizing software engineering tools. The Subversion utility is used to maintain the CHARMM source code, documentation and other supporting files. The Subversion repository resides on The CHARMM manager controls both the developmental and released versions of the code. He schedules contributions from all CHARMM developers.

CHARMM Directory Structure

CHARMM files are organized in the following directories. UNIX pathnames are used throughout the document. ~/ is the parent directory that contains the CHARMM main directory, ~/cnnXm. nn is the version number, X is the version trunk designator (a for alpha or developmental, b for beta release and c for gamma or general release) and m is the revision number. For example, c24b1 is CHARMM version 24 beta release revision 1.

Directory Purpose
~/cnnXm The main directory of the current CHARMM version. The installation script runs in this directory.
~/cnnXm/source Source files.
~/cnnXm/doc Documentation
~/cnnXm/test Testcases
~/cnnXm/toppar Standard topology and parameter files.
~/cnnXm/support Holds various support programs and data files for CHARMM. See Support Programs and Data Files.
~/cnnXm/tool Contains the preprocessor, prefx, and other CHARMM processing/management tools.
~/cnnXm/build Contains Makefile, module makefiles and the log file of the install make command for each machine in the subdirectory named after the machine type.
~/cnnXm/lib Contains library files
~/cnnXm/exec Will hold executables

Standards (rules) for writing CHARMM code

Because CHARMM is developed by various groups, there are a number of standards to which all contributors must adhere in order for the program to remain modifiable, usable, and transferable. The rules which have been established towards this end are listed below.

  1. CHARMM module and subroutine structure:

    Fortran modules should be used for all code when possible. The general form of a CHARMM fortran module is:

    module SAMPLEMOD
     ! Comments describing the general function of the module
       use chm_kinds
       implicit none
       ! Declarations of public variables
       ! Declarations of private variables
       subroutine SUBROUTINE1(arg1,arg2,...)
       end subroutine SUBROUTINE1
       subroutine SUBROUTINE2(arg1,arg2,...)
       end subroutine SUBROUTINE2
       !----- other subroutines and functions --------------!
     end module SAMPLEMOD

    The subroutines in a CHARMM module might include, for example, setup subroutines, memory allocation/deallocation subroutines, output subroutines, and the principal subroutines for the method.

    A subroutine in CHARMM should have the following general form:

    ! A comment which describes the purpose of this subroutine.
    ! This may include important variables and what their use is
    ! to aid in understanding and modifying the routine.
    ! A description of all passed arrays and arguments if
    ! users need to call this routine.
      use chm_kinds
      use dimens (if dimensioned common blocks are included)
      use number (if commonly used real numbers are used)
      use somemodule, only: var1, var2, func1, sub1
      implicit none
      !---- declare all passed variables here --------
      real(chm_real), intent(in) :: arg1
      integer(chm_int), intent(out) :: arg2
      !---- local ------------
      Declarations of ALL local variables and parameters.
      data statements at end of declarations.
      !----- begin -----------
      Code (liberally documented through comments)
    end subroutine DOTHIS

    Note that data statements, if present, come after all declarations and parameter statements, but before the first line of executable code.

    The use of subroutines outside of modules should be avoided because the compiler does not check their arguments.

  2. All code should be written clearly. Since the code must be largely self-documenting, clarity should not be sacrificed for insignificant gains in efficiency. Variable, function, subroutine and module names should be chosen with care so as to help illustrate their purpose. Avoid using single letter variable names except for scratch variables in simple loop constructs. Comments should be used where the function of the code is not obvious. Define/explain important variables. Use the appropriate “intent” attribute (in, out, inout) in variable declarations wherever possible.

  3. Input/Output

    1. The RDCMND routine should be used to read lines from the command stream. XTRANE should be called to be sure that the entire command line is parsed.

    2. Short outputs, messages, warnings, and error should be sent to unit OUTU (accessed by USE stream) for output.

    3. Any warning and error message should state which subroutine generated it.

    4. PSF and parameter unformatted I/O file formats must remain upward compatible. Use an ICNTRL array element to indicate which version of CHARMM wrote the file. Such upward compatibility must be maintained only across release versions of CHARMM. In other words, a file format for the developmental version may be freely changed until a new release version is generated, at which point all future versions must be able to read it.

    5. Use as many significant digits as needed but not more. Do not use list directed output (use of “*” for format) or print. In particular,

      write(outu,*) var1, var2
      print *, "Value is", e_variable

      should not be used. It makes output unreadable and makes testing on different machines difficult.

    1. All output must be performed based on the PRNLEV value. This is used, for example, to restrict I/O to node_0 in parallel implementations of the code. For example,

      write (OUTU,'(FORMAT)') ITEMs

      should be coded as

      if (PRNLEV.GT.2) write (OUTU,'(FORMAT)') ITEMs

      where N is an appropriate print level (always >= 2; see also Miscellaneous Commands).

  4. All error conditions must terminate with a CALL WRNDIE(...) statement; direct calls to DIE should not be used. The first argument is the severity of the error, the second argument must contain the source file in angle brackets, and the routine name of the location of the error condition. The third argument contains a string describing the error condition.

    call WRNDIE(-3,’<tamd.src> tamd_allocate’, &

    ‘Failed to allocate memory for islct array’)

  1. All error conditions must terminate with a CALL WRNDIE(...) statement;

    direct calls to DIE should not be used. The first argument is the severity of the error, the second argument must contain the source file in angle brackets, and the routine name of the location of the error condition. The third argument contains a string describing the error condition.

    call WRNDIE(-3,’<tamd.src> tamd_allocate’, &

    ‘Failed to allocate memory for islct array’)

    1. Use subroutines chmalloc and chmdealloc from module memory to allocate and deallocate memory.


      module mymod
        use chm_kinds
        integer, allocatable, dimension(:) :: myints
        real(chm_real), allocatable, dimension(:) :: myreals
        subroutine mystart(natom)
          use memory
          integer, intent(in) :: natom
          call chmalloc('mymod.src', 'mystart', 'myints', natom, intg=myints)
          call chmalloc('mymod.src', 'mystart', 'myreals', natom, crl=myreals)
        end subroutine mystart
        subroutine myfinish(natom)
          use memory
          integer, intent(in) :: natom
          call chmdealloc('mymod.src', 'myfinish', 'myreals', natom, crl=myreals)
          call chmdealloc('mymod.src', 'myfinish', 'myints', natom, intg=myints)
        end subroutine myfinish
      end module mymod
    2. If possible, combine memory allocation statements in a separate subroutine. Do the same for memory deallocation.

  2. Arrays should be dimensioned with a constant or integer variable where possible. Do not use (*) when dimensioning arrays passed as arguments; be sure to pass the dimensioning information also. Arrays cannot change shape or type when passed.

    subroutine NEWFORCE(natom,force,fsum)
      use chm_kinds
      implicit none
      integer(chm_int),intent(in) :: natom
      real(chm_real),intent(out) :: fsum
      real(chm_real),intent(inout),dimension(3,natom) :: force
      real(chm_real),intent(inout) :: force(3,natom)

    Check for array overflows.

  3. Error checking in general should be as complete as possible. Consider checking for overflows (reciprocals of very small numbers, exponentials of very large numbers, etc.), square roots of negative numbers, arccosine or arcsine of numbers of absolute value greater than one, etc. Code should contain checks for error conditions where it will not impact performance. Use compiler flags for checking, then remove checking flags for production compiling and code submission.

  1. The code should not use non-standard Fortran 95 features. Such features must be restricted to the machine dependent modules, or encapsulated in “##IF - ##ELSE - ##ENDIF” preprocessor constructs.

    • Do not use obsolescent or deprecated Fortran constructs, only use what is in the current standard.
    • Do not create common blocks, and use the established common blocks only when necessary.
    • Do not use entry points.
    • Do not use computed or assigned goto statements.
    • Avoid “goto” when other constructs are available such as “if,” “case,” “cycle,” and “exit.”
  2. Functions should never be called with a “call” statement.

  3. The generic form of an intrinsic function should be used whenever possible. For example, use SQRT(DP) rather than DSQRT(DP).

  4. Real or integer constants should be defined as parameters.

    real(chm_real),parameter :: ONE=1.0D0, THREE=3.0D0, FIVE=5.0D0, &
    integer(chm_int),parameter :: MAXATM=99999

    See ltm/number_ltm.src for frequently used numbers. Real numbers may never be placed in a calling sequence. All physical constants should be declared as parameters in ltm/consta_ltm.src

    Constants and numbers can be used by adding

    use consta

    to routines or modules.

  5. Routines should have no implicitly declared variables. This means that all variables and arrays, whether passed or not, must be explicitly declared. Each module must thus contain

    implicit none

    as the first statement after any “use” statements and before any declarations. This obviates the need for having the implicit none statement in each of the subroutines contained in the module. A routine that is not contained in a module must have the implicit none statement as the first statement after any “use” statements and before any declarations.

  6. There is no upper or lower case rule in CHARMM except that any variable or subroutine name should have consistent case throughout all of CHARMM code. It is helpful to use all caps for parameter variables.

  7. No tabs should appear in code or documentation.

  8. All strings must be stored in character variables. Use character(len=<n>) for character declarations, where <n> is the length of the string. a “*” may be used for the length of a passed string but this is discouraged.

  9. For routine command parsing, the keyword parsing functions INDXA, GTRMA, GTRMF, GTRMI, and NEXTA4 should be used.

  10. The recommended length for names of frequently used variables is 4-10 characters. Avoid single letter variables, except as indices for simple loops. Avoid overriding standard Fortan words such as SUM or TYPE.

  11. All variables must be declared with a kind. Use the kinds available in chm_kinds_ltm.src, the CHARMM variable kinds (chm_kinds) module. If a new kind is used, add it to the chm_kinds module.

  12. All variables must be initialized before first use. Most initialization is done in the setup or initialization routine in a module.

  13. Other coding conventions make it easier to search through text for particular strings using the SEARCH, fpat, or grep commands. Poorly placed spaces can make it very difficult to maintain code. There cannot be a space within a variable name. Here are some other examples;


    Please Avoid


    GO TO

    |CALL DOSOME(...


    CALL DOSOME (...

    ARRAY(5) = 20


    |ARRAY (5) = 20

  14. The ltm directory contains modules that have no dependencies on other modules, except for the chm_kinds module. Examples are modules containing global parameters and variables.

  15. Avoid duplicating code. If a defect is found in some code, and there are multiple copies of it, the remedy must be applied to all copies. If code is not duplicated, all its users benefit from any improvement to it.

CHARMM Developer Tools

CHARMM is available on a variety of computational devices and we strongly support multiplatform development efforts. CHARMM tools are utility programs/procedures for installation, modification, optimization, etc. The preprocessor PREFX and utility procedures for makefile generation are located in ~/cnnXm/tool. The FLECS to FORTRAN translator FLEXFORT is no longer needed since CHARMM c23f2 and was removed from this and later distribution versions.


Module Makefiles and Optimization

The installation script works with a set of makefiles in ~/cnnXm/build/{machine_type}. These makefiles play the key role in developing, optimizing and porting CHARMM code on the machine you are working with.

  1. Porting to Other Machines

    You may begin with the given set of makefiles for a machine close in the architecture to the one to which you intend to port CHARMM. First you have to decide a name for the machine platform. For example, IBMRS was chosen for the IBM RS/6000 series.

    cp -r  ~/cnnXm/build/{closely_related_machine_type} \

Then delete Makefile in the new build directory and remane Makefile_{closely_related_machine_type} to Makefile_{your_machine_type}. You may have to modify compile commands and compiler flags in the Makefile template.

Study carefully ~/cnnXm/ and modify it if necessary. In most cases, you just need to correct echo messages to address your machine properly. Then issue the command.

  1. Optimization

    Once the makefiles are working properly, you can carry out a compiler level optimization for the CHARMM version. FORTRAN compile macro’s are defined in Makefile_{machine_type}, e.g., $(FC1), $(FC2), $(FC3), etc. Compiler options are bound to these compile macros. You may inspect each module makefiles and set a proper compile command for a given FORTRAN source. For example, the following are the default optimization flags for the c24b1 release. Most of source files are compiled by $(FC2) except

         $(FC0) ehbond.f
         $(FCR) enefst2.f
         $(FCR) enefst2q.f
         $(FC3) enefvect.f
         $(FCR) imnbf2p.f
         $(FC3) imnbfp.f
         $(FC0) nbondm.f
         $(FC0) corman.f
         $(FC3) fshake.f
         $(FCR) fshake2.f
         $(FCR) enbf2.f
         $(FCR) enbf3.f
         $(FCR) enbf4.f
         $(FCR) enbf5.f
         $(FC3) ewaldf.f
         $(FCR) ewaldf2.f
         $(FCR) nbndf2p.f
         $(FC3) nbndfp.f
         $(FC0) qmdata.f
         $(FC0) qmene.f
         $(FC0) qmjunc.f
         $(FC0) qmpac.f
         $(FC0) qmset.f
  2. Generating Module Makefiles

    We have included scripts that find all module dependencies. When you want to create the full set of module makefiles, you may use in ~/cnnXm/tool. UNX

    This generates makefiles in ~/cnnXm/build/UNX, which copies to ~/cnnXm/build/{machine-type} as needed.

  3. Usage Note on makemod

    When you generate files from scratch, the FORTRAN compile macro $(FC2) is used for all source files. In order to set the compiler option for further optimization, you have to modify the module makefiles to set the macro manually.

The procedure for modifying anything in CHARMM

This procedure describes the steps which should be taken when modifying a source file in CHARMM. When you are developing CHARMM source code, always maintain close contact with the CHARMM manager and other developers. Inform them your development plan and which files you are working on. See Checkin Procedure in CHARMM Management System for the procedures to follow when submitting your developmental code to the CHARMM manager.

  1. Get a copy of the current development code. If you are a CHARMM developer and plan to integrate your program into CHARMM in the future, make sure that you obtain the most current revision from Subversion. Check with the CHARMM manager.
  2. Once you obtain the code, you are branching out from the main CHARMM source code control system. You should record details of modification so that you may reproduce them when you check your files in with the CHARMM manager.
  3. While you make modifications and debug them, follow the guidelines in *note Standards::, so that CHARMM code will be consistent. If your modification does not involve any changes in the source file directory structure and makes no changes in USE statements, you may use the module makefiles supplied (with the extension .mk) in ~/cnnXm/build/UNX. If you add/remove any source files, reorganize them, modify any USE statements or are porting to a machine that is not already supported, you have to build the relevant module make files. See *note makemod:: for more information on makemod.
  4. In your local ~/cnnXm directory, you may issue the command to build the library and the executable. See *note Install: (install.doc)Install. Your library is built in ~/cnnXm/lib/{machine_type} and the executable will be in ~/cnnXm/exec/{machine_type}. You may find the log file {machine_type}.log in ~/cnnXm/build/{machine_type}.
  5. If your modification involves a new feature, you should either modify an existing test case or make a new test case to demonstrate and check its operation. See *note testing: (testcase.doc), for a description of the tests currently available. If you add a new test case, update the ~charmm/doc/testcase.doc file. Each new test case should run in 1 second or less on a modern workstation.
  6. If your change involves adding or modifying a command or adding or modifying a feature, modify the existing documentation or if none is available, create new documentation. Make sure that the emacs info program can read the document and the format of your documentation is consistent with other documents. See *note Document::.

How to Document CHARMM Commands and Features

Documentation is an integral part of CHARMM developments. In order to document commands and features under development in a consistent manner, the following documentation format is recommended. All documentation should be accessible (readable) through the emacs info facility. If you do not know how to insert the info directives, ask the CHARMM manager for assistance. If a new functional module is being introduced, a new .doc file should be created. For modifications or extensions of existing CHARMM modules/functions, the preexisting .doc file should be revised.

Each documentation file, with the extension .doc, should contain

  1. One brief paragraph describing the motivation, theory, or procedure relevant to the feature being documented. Here, a few references can be given.
  2. A table of contents of the documentation (to serve as the info menu).
  3. The command syntax.
  4. A complete description of all the commands, sub-commands, and command options. The syntax, defaults and file names involved should be described. A brief description of what the command accomplishes should also be given. The order in which various commands should be invoked should be described. Relevant commands and subcommands can be cross-referenced with a key.
  5. One or two examples involving the concepts and commands described (No output listing).

The same notation should be followed throughout the documentation.

[...]   optional, can be present only once, if at all.

{...}   can be repeated any number of times, must be present
        at least once.

[{...}] or [{...}]  can either be missing or be present any
                    number of times.

n{...}  must be present exactly n times.

<A|b>   either  A or B must be present.

Syntax definitions will use literal keywords such as VIBRan, READ, MINI, VERLet, etc. These are to be typed as such.

Syntax definitions can also use dummy keywords such as atom_name, atom_index and atom_type. The meaning and variable type can be listed just after the syntax notation.

For literal keywords the documentation and examples will use uppercase characters immediately followed by zero or more lower case characters. Dummy keywords will be written in all lower case.

API Documentation

As a complement to the required command and feature documentation, the free Doxygen tool ( can automatically generate low-level documentation from the source code. Doxygen reads declarations of modules, subroutines, and variables, and generates HTML with cross-references showing the relationships between them.

Doxygen also recognizes Fortran comments beginning with !> as text to include in the HTML output. Some examples of such comments are in source/pert/lambdadyn.src.

Before you run Doxygen on CHARMM code, download the Doxygen 1.6 source distribution and apply tool/doxygen.patch as follows:

cd doxygen-1.6
patch -p0 < ../charmm/tool/doxygen.patch

and follow the Doxygen installation instructions. Then the shell command

cd ../charmm
doxygen tool/Doxyfile

generates documentation in a directory named apidoc. A good starting point for browsing is apidoc/html/files.html.

If you have Graphviz ( installed, Doxygen can generate call diagrams for each subroutine. To do this, edit tool/Doxyfile and change CALL_GRAPH and CALLER_GRAPH to YES.

Checkin Procedure in CHARMM Management System

CHARMM is maintained as a single-source software package. The following rules have been established to minimize conflicts and delays and to allow for error-free integration of CHARMM developments from many scientists.

  1. It is always wise to inform the CHARMM manager about your development plan and timetable so that he may better arrange the administrative schedule and also prevent you from duplicating the work of others. The list of files you are working on and the nature of the modifications should be reported in advance.

  2. Developments must be submitted to the CHARMM manager by December 30 for inclusion in the February distributions or June 30 for the August distributions.

  3. Be certain that the submitted developments are based on the most recent development version of CHARMM. The check-in package (see below) should compile out-of-the-box when merged with the base version.

  4. Prior to check-in, run “” after integration of the check-in package with the appropriate base version of CHARMM. Compare the results in the output directory with those obtained when using the base version. If you are introducing new preprocessor (pref.dat) keywords to control the compilation of code for new features (which is recommended), check the results for executables produced both with and without the new keyword(s).

  5. The check-in or submission package should include the following:

    • modified source files (using the CHARMM source directory structure),
    • updated or new documentation (doc/*.doc) files, and
    • updated testcase files

    These files should be assembled in a tar archive conforming to the CHARMM distribution directory structure.

  6. Post a completed project form ( to the CHARMM development bulletin board ( The project form should contain a succinct description of the submitted modifications and new features, the name(s) and institutional affiliations of the developer(s), the date, the base CHARMM version, new preprocessor keywords, and lists of the new and modified files. Upload your tar archive as an attachment to the project form.