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I am using your fftool to convert a xyz file to the lammps input files and ran into some trouble with the IMPROPER keyword in the ff file, it seems the code doesnt read it. Therefor the impropers get added to the list of dihedrals. Also the potential keyword isnt read for dihedral and defaults to opls. I have added the xyz and ff files for reference.

xyz.txt ff.txt

Thank you for your time.

Normally the improper are found automatically based on the connectivity and added to the list of dihedrals. From the standpoint of the simulation code, impropers are no different: in both cases it is a torsion angle between planes 1-2-3 and 2-3-4, only in "impropers" atom 3 is connected to all the others, not 1-2-3-4. In the input files for lammps everything should be in the list of dihedrals. Does this make sense? I am not sure it this was your question. For a specific case, in benzene you should end up with 30 dihedrals, 24 "proper" X-C-C-X and 6 improper C-(C-)C-H

Thank you for your response. In the case that both the dihedral and the improper are calculated with the opls potential, I can see why it wouldn't matter which list they are in. We want to use a diffferent potential for dihedral style and improper style in LAMMPS. So we are using the basic set up provided by the tool and manually changing some things here and there. My question arose due to the examples of fftool and the LAMMPS documentation, where they seem to have a different list for the dihedrals and the impropers. This gave me the idea that the keyword might not be read, since the are added to the dihedral list and the README file doesn't specifiy that.

This is a good suggestion for an improvement, thanks. It should be easy to at least add an option to allow for a separate listing of the improper dihedrals. Maybe then also allow for specific potential energy functions. I’ll try to implement this but can't promise for the next few days.

Hello I have been having the same error, since I am using the OPLS-AA force field, it is necessary to list the dihedrals and the improper ones separately. I would like to know if you were able to add this feature or any tips to fix them?

Thank you for your time.

@ancalome16 could you be more specific? Are these improper dihedrals used to keep chirality or to improve in-plane rigidity? I also use OPLS-AA and don't have problems for in-plane rigidities: the improper dihedrals are specified as I-j-k-l with k being the central atom. LAMMPS does not have opls style for improper torsions, that's why fftool creates only one list.

@agiliopadua Thanks for your answer, what happens is that for the case of molecules that have dihedral angles and improper angles (Toluene) it is necessary to describe each of these separately. What I mean, in LAMMPS I use an OPLS potential with the dihedral and a harmonic potential with the improper ones: dihedral_style opls improper_style harmonic So it is necessary to obtain separately the list of improper and dihedral in the data.lmp file.

Actually, OPLS-AA uses the V2 term (V2/2 [1 - cos 2\phi]) for improper torsions (same as lammps cvff), not harmonic.