[gmx-developers] multiplicity check on proper dihedral

[David van der Spoel]

Tim Sirk wrote:
> Hello,
> 
> I've been using 3.3.3, and after updating to 4.0 RC4 I found that grompp doesn't
> accept a proper dihedral with a multiplicity of zero. grompp returns this error:
> 
> Value of 'multiplicity' in Proper Dih. is 0, which is smaller than the minimum
> of 1
> 
> I understand that mult=0 probably isn't expected, but it is part of a forcefield
> (see below) that I've used with 3.3.3 and I'd like to continue using it. Might a
> warning be sufficient, instead of a fatal error? Any input on a workaround would
> be appreciated.

A simple workaround would be to rewrite your potential as a 
ryckaert-bellemans, as you essentially do have a cosine series, please 
note that RB uses the polymer convention (adding 180 to your angle).
Indeed your usage seems to completely legitimate, and that was probably 
overlooked by the person implementing this check . In principle you 
could remove the check from grompp as well. Note however that adding a 
constant term to a potential has no effect on the dynamics and relative 
energies, and hence you might also just leave it out altogether.
> 
> ; CH1-CH1-CH2-CH2  =>  C22-C23-C24=C25
>     25   24    23    22     1  0.000       7.47     1
>     25   24    23    22     1  0.000       3.9      2
>     25   24    23    22     1  180.000     1.1      3
>     25   24    23    22     1  0.000      -2.8425   0
> ; CH1-CH1-CH2-CH2  =>  C24=C25-C26-C27
>     27   26    25    24     1  0.000       7.47     1
>     27   26    25    24     1  0.000       3.9      2
>     27   26    25    24     1  180.000     1.1      3
>     27   26    25    24     1  0.000      -2.8425   0
> 
> 
> Thanks.
> 
> Tim Sirk
> 12C Randolph Hall
> Graduate Research Assistant
> Mechanical Engineering Dept
> Virginia Tech
> 
> 
> 
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-- 
David van der Spoel, Ph.D., Professor of Biology
Molec. Biophys. group, Dept. of Cell & Molec. Biol., Uppsala University.
Box 596, 75124 Uppsala, Sweden. Phone:	+46184714205. Fax: +4618511755.
spoel at xray.bmc.uu.se	spoel at gromacs.org   http://folding.bmc.uu.se