[lammps-users] Possible bug in msi2lmp tool

I have noticed a discrepancy in the msi2lmp converter. Specifically, it seems that the dihedral energy constant (K) that is read in from cvff.frc (or other biosym files) is getting reduced by a factor of 4 in the lammps data file. Here is an example using benzene:

From cvff.frc, showing K = 12.0 kcal/mol:

#torsion_1 cvff

E = Kphi * [ 1 + cos(n*Phi - Phi0) ]

!Ver Ref I J K L Kphi n Phi0

!---- — ---- ---- ---- ---- ------- ------ -------

1.0 1 * cp cp * 12.0000 2 180.0000

and from the lammps data file showing K = 3.0 kcal/mol:

Dihedral Coeffs

1 3.0000 -1.0000 2.0000

I am not an expert at C, and have not been able to identify where this conversion is made in the code. I would like to know if I am missing something in the conversion.

Jeff Greathouse

I don't know the inner workings of msi2lmp or maintain it.
It could be some multiplicity factor that is different in
LAMMPS vs the MSI MD code. I.e. some ff list a dihedral
several times with a reduced K, others list it once with
an increased K. But I think it would be the opposite of
what you show. So you may have to dig thru the source code.