[gmx-users] unit conversion

Justin Lemkul jalemkul at vt.edu
Mon Jul 1 00:41:21 CEST 2013

On 6/30/13 12:11 PM, Hyunjin Kim wrote:
> Hi,
> When I convert FF parameters from OPLS (in Gromacs) to Charmm, I divide
> bond and angle force constants, i.e., Kb and Ktheta, by 836.8 and 8.368.
> However, parameters for tip3p water do not convert properly in this ways.
> For example, Kb = 502416.0 kJ/mol/nm^2 for OW-HW in Gromacs and 450.0
> kcal/mol/A^2 in Charmm.
> 502416.0/836.8 = 600.4 instead of 450.0.
> I wonder what I did is incorrect.

CHARMM has its own special variation of TIP3P that corresponds to the values you 
are trying to produce.  In tip3.itp, it's the #ifdef CHARMM_TIP3P block.  The 
other parameters are for standard TIP3P.



Justin A. Lemkul, Ph.D.
Postdoctoral Associate

Department of Pharmaceutical Sciences
School of Pharmacy
Health Sciences Facility II, Room 601
University of Maryland, Baltimore
20 Penn St.
Baltimore, MD 21201

jalemkul at outerbanks.umaryland.edu | (410) 706-7441


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