The parmed.charmm package

The charmm package contains classes that can parse a variety of CHARMM files, including PSF files, parameter files, and coordinate files. Parameter files (PAR), stream files (STR), CHARMM coordinate files, and CHARMM restart files.

It is beyond the scope of this site to discuss the details of these file formats, so you are instead forwarded to the CHARMM documentation.

The Protein Structure File (PSF)

The PSF file is the heart of the CHARMM workflow, representing a molecular system and its topology. It contains a list of all atoms with (most of) their atomic properties, and then a list of the covalent bonds and valence bonded terms (e.g., angles, torsions, improper torsions, and coupled-torsions). There are a number of dialects of the PSF file, including that used by CHARMM, those generated by VMD’s autopsf tool, so-called XPLOR PSFs, etc.

The CharmmPsfFile class is capable of parsing all PSF file formats that I have encountered (excluding those with virtual sites and Drude particles, currently).


A chemical Structure instantiated from CHARMM files.

This class is a subclass of Structure, and as such contains the corresponding topology arrays atoms, bonds, … etc. An example demonstrating the use of CharmmPsfFile to parse dhfr_cmap_pbc.psf (a sample PSF file in the ParmEd unit test suite) is shown below:

>>> psf = CharmmPsfFile('dhfr_cmap_pbc.psf')
>>> len(psf.atoms)
>>> len(psf.bonds)
>>> len(psf.angles)
>>> len(psf.impropers)

CHARMM Parameter Sets

CHARMM parameters are spread across three different types of files: Residue topology files (RTF), Parameter files (PAR), and stream files (STR). A CharmmParameterSet (described briefly below) will build a parameter database from collections of all three of these files.


Stores a parameter set defined by CHARMM files.

To instantiate a parameter set, you can pass a list of file names (any iterable will suffice) to the CharmmParameterSet constructor, and the format will be determined from the file name itself. File type recognition follows the rules summarized in the table below:

Filename suffix

File type

.rtf, .top

Residue topology file

.par, .prm

Parameter file


Stream file


Parameter file if par is in the name, RTF if top is in the file name. Otherwise ValueError is raised.

If your file name does not match one of the patterns above, you need to either rename your file or use alternate ways to read in each file (see CharmmParameterSet.load_set() as well as the sections below).

An example is shown processing the sample CHARMM 22 force field files provided in the ParmEd unit test files:

>>> from parmed.charmm import CharmmParameterSet
>>> params = CharmmParameterSet('par_all22_prot.inp', 'top_all22_prot.inp')
>>> params.bond_types[('CE1', 'CE1')]
<BondType; k=440.000, Req=1.340>
>>> params.angle_types[('H', 'NH2', 'CT1')]
<AngleType; k=50.000, THETAeq=1.937>
>>> # Note that the reverse is an equivalent angle
>>> params.angle_types[('CT1', 'NH2', 'H')]
<AngleType; k=50.000, THETAeq=1.937>
>>> params.improper_types[('CE1', 'CPB', 'X', 'X')]
<ImproperType; k=90.000, PSIeq=0.000>

Notice that the dictionaries have keys matching all permutations of the atom types defining a parameter. In the example above, the angle type keys ('H', 'NH2', 'CT1') and ('CT1', 'NH2', 'H') both point to the same AngleType. In fact, they point to the exact same object, so that if you modify that angle type, it is changed for both keys:

>>> params.angle_types[('CT1', 'NH2', 'H')].k = 60.0
>>> params.angle_types[('CT1', 'NH2', 'H')]
<AngleType; k=60.000, THETAeq=1.937>
>>> params.angle_types[('H', 'NH2', 'CT1')]
<AngleType; k=60.000, THETAeq=1.937>

Notice how the force constant has changed from 50 to 60 for both keys.

One thing I’ll point out is that for older force fields (like CHARMM 22, for instance), the various force field atom types (e.g., those for protteins, carbohydrates, etc.) are incompatible with each other—you must pick one pair of parameter and topology files and use that.

For the newer force fields, (e.g., CHARMM 36), you should load all of the parameter files that are relevant for your system (e.g., both the lipid and protein force fields, along with the water and ion stream file, for a lipid bilayer with an embedded protein).

Residue Topology File (RTF)

The RTF file contains a library of residue templates (residue names, the atoms within the residues, their connectivity, and a standard internal coordinate representation of their orientation). It also describes various patches (i.e., small fragments meant to adorn or modify the more typical residues) that are used to modify the residues. This information is not currently processed by CharmmParameterSet. The only information that the RTF file contains that CharmmParameterSet makes use of is the listing of atom types.

As of the CHARMM 36 parameter set, however, this information is now stored in the parameter files, so the RTF files are not strictly required by the CharmmParameterSet for these newer force fields.

When they are required, however, they must be loaded into the CharmmParameterSet before the corresponding parameter file, since the atom types need to be defined before they can be used.

To specifically load a topology file, use CharmmParameterSet.read_topology_file(). Note, this should only be necessary when adding on to an existing parameter set or when the default name recognition does not appropriately classify your file as an RTF.

Parameter File (PAR)

The PAR file contains a database with all of the different parameter types defined between specific atom type names. For newer force fields, they also include the atom type name to type index mapping as well as atom masses.

To specifically load a parameter file, use CharmmParameterSet.read_parameter_file().

Stream Files (STR)

Stream files contain extra information and CHARMM directives. The CHARMM directives are ignored, but any rtf or par sections will be read to augment a parameter set with additional atom types and parameters.

To specifically load a stream file, use CharmmParameterSet.read_stream_file().

Add to an existing parameter set

If you have an existing CharmmParameterSet instance and want to read in another parameter, topology, and/or stream file(s), you can use the CharmmParameterSet.load_set() as a shortcut for reading parameter, RTF, and stream files separately.

CHARMM coordinate files

ParmEd provides classes to parse two kinds of CHARMM coordinate files—standard coordinate files and restart files, summarized below.


Reads and parses a CHARMM coordinate file (.crd) into its components, namely the coordinates, CHARMM atom types, resid, resname, etc.


Reads and parses data, velocities and coordinates from a CHARMM restart file (.rst) of file name ‘fname’ into class attributes

Both of these file types are read-only and can be instantiated directly via the constructor. The coordinates (and for restarts, velocities, accelerations, and old coordinates) are available as attributes to the objects.