How to prepare a system

ParmEd

ParmEd is handful for converting common file formats such as psf/pdb to top/gro (Gromacs) or top/inpcrd (Amber).

import parmed as pmd
gmx = pmd.load_file('topol.top', xyz='system.gro')
gmx.save('system.top', format='amber')
gmx.save('system.inpcrd', format='rst7')
gmx.save('ionized.psf')

Please note that the Gromacs output comes WITHOUT forcefields.

Protein-membrane systems

CHARMM to Gromacs

File conversion from CHARMM to Gromacs is often encountered, because CHARMM-GUI is very convenient for creating a membrane system. There is a script psf2itp provided by CHARMM-GUI (modified by Kev 2019) to convert a psf to topology file for Gromacs with CHARMM36m ff. You may replace the toppar to change the force-field (but still have to be CHARMM). CHARMM36m ff for Gromacs could be downloaded here.

python psf2itp.py toppar.str PSFfile

Protonation states of amino acids

H++

ProPka

PDB2PQR

PlayMolecule (Using ProPka)

Improper dihedral problem in Gromacs' Amber force-field

In ffbonded.itp:

CT  CV  CC  NA       4      180.00     4.60240     2

However pdb2gmx will make

CT  CC  CV  NA

for HID, causing error message of "No default Proper Dih. types". Therefore you have to manually find the improper dihedral terms in the topology (usually topol_Protein.itp made by pdb2gmx) and exchange the second and the third indices. You could find the line number in the error message.

Ligand topology - using Amber force-fields in Gromacs

References: [1] [2]

antechamber -i lig.mol2 -fi mol2 -o LIG.mol2 -fo mol2 -j 4 -at gaff -c bcc -nc 0
parmchk -i LIG.mol2 -f mol2 -o LIG.frcmod
tleap -s -f tleap.all
acpype -p com_solvated.top -x com_solvated.crd -b complex