Force Field Toolkit
A CHARMM forcefield topology file contains all of the information needed to convert a list of residue names into a complete PSF structure file. It also contains internal coordinates that allow the automatic assignment of coordinates to hydrogens and other atoms missing from a crystal PDB file.
At the beginning of the file is the header, indicated by lines beginning with *s, followed in this case by ``31 1" to indicate the version of CHARMM that generated the file.
The topology file must define the type, mass, and charge of every atom in every residue, so that a PSF file can be constructed.
the mass of every atom type is declared once at the beginning of the file in a MASS statement.
enumerating all of the required angles and dihedrals would be tedious and error-prone, as well as enormously complicated since every combination of residues joined by a peptide bond would require a different set. Therefore, angles and dihedrals are automatically generated for every pair or triple of connected bonds when a segment is built. This autogeneration may be enabled or disabled on a per-segment basis as it should never be used on segments of water, but the default is defined in the topology file:
AUTO ANGLES DIHE
ATb (Automated Topology Builder and Repository)
This server only provide the topology for GROMACS(*.itp)
this link has a video link of introduction of vmd force field toolkit:
Molefacture manual is here.