MD simulation of an oligomeric protein with 3 ligands - fatal error during addition of ions

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GROMACS version: 2020.1
GROMACS modification: No
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Dear Gromacs community,
I’m quite new to Gromacs, did already two tutorials (lysozme in water and protein-ligand complex from mdtutorials.com) and did an energy minimization of a protein with modifed residues.
Currently, I would like to do a MD simulation of a trimeric protein +3 ligands (c-di-AMP). I’m using the OPLS-AA/M force field from Jorgensen Group and generated the ligand topology using LigParGen. For generating the topology, at first, I generated the topology of the trimeric protein without ligand using pdb2gmx, added the ligand to the .gro file of the protein (also adjusted the number of atoms in the second line) and modified the topol.top file (ligand parameters included directly after including the force field, ligand position restraints file included directly after including the chain topologies of the 3 protein chains and adding the ligand to the [ molecules ] section with # mols 3). Afterwards, I defined a water box (gmx editconf …), filled it with water (gmx solvate …, the converted pdb file also looks fine), but when I want to add ions for neutralising the system (gmx grompp -f ions.mdp -c solv.gro -p topol.top -o ions.tpr), I’m getting the folling error message:

NOTE 1 [file ions.mdp]:
With Verlet lists the optimal nstlist is >= 10, with GPUs >= 20. Note
that with the Verlet scheme, nstlist has no effect on the accuracy of
your simulation.

Setting the LD random seed to 1119388458
Generated 396495 of the 396495 non-bonded parameter combinations
Generating 1-4 interactions: fudge = 0.5
Generated 396495 of the 396495 1-4 parameter combinations

Program: gmx grompp, version 2020.1-Ubuntu-2020.1-1
Source file: src/gromacs/gmxpreprocess/grompp.cpp (line 604)

Fatal error:
No molecules were defined in the system

For more information and tips for troubleshooting, please check the GROMACS
website at Errors - Gromacs

Hence, I guess, that I didn’t probably set up the topology. Do you have an idea, what’s the problem and could you please help me?