[gmx-users] negative dihedral cp (during cgenff conversion)
Mark.Abraham at anu.edu.au
Mon Aug 15 14:50:34 CEST 2011
On 15/08/2011 10:50 AM, Peter C. Lai wrote:
> Random noob question:
> Should the cp field (field 7) in [ dihedraltypes ] always be positive or can
> it be negative?
> In the CGenFF conversion, the CHARMM dihedral has:
> OG302 CG2O2 CG321 CG331 -0.1500 1 180.00 ! LIPID methyl proprionate
> when I convert to gromacs:
> [ dihedraltypes ]
> ; i j k l func phi0 cp mult
> OG302 CG2O2 CG321 CG331 9 180.0 -0.62760 1 ;LIPID methyl prop
> This is the only instance I have seen of a negative number for the dihedral
> term, so I wonder what the right way to handle it is...
> (I suppose I could just run it and see...)
There are a small minority of CHARMM27 dihedrals that do this. There's
no intrinsic reason why the potential should be positive or negative.
Forces and energy differences are unaffected by the sign of any
component of the energy. Probably they result from optimization of these
parameters to reproduce reference quantites in the context of other
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