[gmx-users] Units within Packaged OPLS Force Field File (ffbonded.itp)

Matthew Howenstine mhowenst at nd.edu
Wed Apr 25 20:23:43 CEST 2018


I was just confirming what the units within the
prepackaged \share\top\oplsaa.ff\ffbonded.itp file (within Gromacs version
5.0.7) are - I have seen various sources on the OPLS units, and have mostly
confirmed that most all are already converted to a kJ basis within this
file (i.e. harmonic bond angle force constant [kJ/mol/rad^2],
Ryckaert-Bellemans constants [kJ/mol]). My main question was in the
improper angle's periodic force constant at the bottom -

i.e. in the lines:
; Z -CA-X -Y improper torsion. CA is any ring carbon
#define improper_Z_CA_X_Y 180.0 4.60240 2

Is the 4.60240 in kcal/mol or kJ/mol? I found one OPLS derived force field
file online where a different value was used for the benzene improper
dihedral. Essentially, is there any source which outlines exactly the units
within ffbonded.itp, or do you just have to back-reference the GROMACS user
manual's functional forms as they have already been converted to the proper
form before inclusion within the Gromacs distribution?

Thank you,
Matthew Howenstine

Matthew Howenstine
University of Notre Dame
Chemical Engineering
mhowenst at nd.edu

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