[gmx-users] FF parameter conversion factor from OPLSAA to Gromacs

[Ruhong Zhou]

Hi, 

I have noticed that the conversion factor used in gromacs from kcal/mol to 
kJ/mol is 
4.1868 instead of 4.184 when I was checking the OPLSAA torsion parameters 
in gromacs 
database. Then I found it is true for the bond stretching parameters as 
well. 
Is there a particular reason for this 4.1868 J/cal conversion factor, or 
am I missing something 
here? It is a not a huge difference, but I thought the standard conversion 
factor should be 
1kcal/mol = 4.184 kJ/mol.

Here are two examples. Gromacs converts the OPLSAA torsion parameters to 
the five-term 
Ryckaert-Bellemans form. The formular is

  C0=V0+V2+0.5(V1+V3)
  C1=0.5(3*V3-V1)
  C2=-V2
  C3=-2V3
  C4=0
  C5=0

1. Torsion CT-CT-CT-CT:

GROMACS:
CT     CT     CT     CT      3      2.93076  -1.46538   0.20934  -1.67472  
0.00000   0.00000 ; hydrocarbon 
OPLSAA (2000):
CT-CT-CT-CT                          0.000    1.300  -0.050   0.200 
hydrocarbon

A quick check C2/(-V2) = 0.20934 kJ/mol/(0.05 kcal/mol) = 4.1868 kJ/kcal 
<====== 

2. Bond CT-NT:

GROMACS:
 CT    NT      1    0.14480   319871.5   ; -idem-
OPLSAA (2000):
CT-NT     1.448    382.0

319871.5 kJ/mol/nm^2 / (2 x 382.kcal/mol/A^2)  = 4.1868    kJ/kcal 
<=========

Thanks.

Ruhong

Ruhong Zhou, PhD
Computational Biology Center 
IBM Thomas J. Watson Research Center 
Yorktown Heights,  NY 10598 
Tel: (914) 945-3591, Fax: (914) 945-4104 
http://www.research.ibm.com/people/r/ruhong 
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