[gmx-users] charm to gromacs

francesco oteri francesco.oteri at gmail.com
Thu Mar 8 10:49:54 CET 2012

Dear gromacs users,
I am trying to convert a charmm style parameter file into gromacs. So I al
trying to understand how converting the force constants from CHARMM to
Using the CHARMM27 force field as a reference, I've noticed that to convert
the bond constant the following formula is supposed to be used:

k_gromacs = k_charmm**4184**2
In fact if I try to convert the following line from
par_all27_prot_lipid.prm charmm file:
CEL1  CEL1  440.000     1.340   ! butene, yin,adm jr., 12/95

I obtain the right result ( 440.000*4184*2. = 368192.0)
charmm27/ffbonded.itp    CEL1    CEL1    1       0.134   368192.0

But to convert the angle constants I've to use the following formula:

k_gromacs = k_charmm**4.184**2.

how demostrated by the conversion of the following line:

par_all27_prot_lipid.prm  CEL1    CEL1    CTL2   48.00     123.50   ! from
2-butene, yin,adm jr., 12/95


charmm27/ffbonded.itp   CEL1    CEL1    CTL2    5       123.50  401.664 0.0

My question is:

Why in the former case I need the constant 4184 while in the latter the
constant 4.184 is supposed to be used? Am I miss something?
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