You could write a separate frcmod file to extend (or change) the force field parameters rather than modifying the official parameter file. Here is a link describing the frcmod file format:
https://ambermd.org/FileFormats.php#frcmod
Here is an example:
"""
Remark line goes here
MASS
BOND
ANGLE
Os-Cg-Cy 70.00 108.50
DIHE
IMPROPER
NONBON
"""
To incorporate this in the tleap input, you'd load the force field parameters as normal, then add the command "loadamberparams my_custom_params.frcmod" before reading the pdb file.
Although this may not be directly relevant to your situation, one often encounters frcmod files when parametrizing small drug-like molecules using antechamber and parmchk2. The parmchk2 program is used to write a frcmod file that contains guesses for the missing parameters. An example can be found in the tutorial linked below. Note that the default behavior of parmchk2 is to look for parameters missing from the gaff force field; use the --help option for additional info.
https://ambermd.org/tutorials/basic/tutorial4b/index.php
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Received on Mon Jul 07 2025 - 14:30:02 PDT
