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

Justin Lemkul jalemkul at vt.edu
Wed Apr 25 21:08:00 CEST 2018

On 4/25/18 2:23 PM, Matthew Howenstine wrote:
> All,
> 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?

Everything is in Table 5.5 of the manual, which also links to sections 
describing those functional forms.



Justin A. Lemkul, Ph.D.
Assistant Professor
Virginia Tech Department of Biochemistry

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340 West Campus Dr.
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