[gmx-users] POPC 53a6 topology (dihedral multiplicity)

[Alex Smolyanitsky]

Hello everyone,

I am trying to include a POPC 53a6 topology from LipidsForGro96_53a6.zip (
http://www.gromacs.org/index.php?title=Download_%26_Installation/User_contributions/Molecule_topologies)
for a simple lipid in water sim (gromacs version 4.0.5).

I am using the lipid coordinates from the same archive and here's my
topology:
***
#include "ffG53a6.itp"
#include "popc_53a6.itp"
#include "spc.itp"

[ system ]
;Name
test

[ molecules ]
POPC      128
SOL       3585
***

grompp throws a fatal error: " Value of 'multiplicity' in Proper Dih. is 0,
which is smaller than the minimum of 1" without any prior warnings.
The error has to do with two dihedral angles with zero multiplicity:

...
22   23   24   25   1   90   -5.685   0
...
24   25   26   27   1   90   -5.685   0
...

So I looked at an ancient POPC topology for GROMOS87 and these dihedrals
have zero equilibrium angle and a multiplicity of 3:

 ...
22   23   24   25   1   0.0   5.858   3
...
24   25   26   27   1   0.0   5.858   3
...

Did the author of the 53a6 topology want to simply remove these dihedrals by
setting a multiplicity of zero? Or, should these two be set according to the
older topology? I know similar questions have been asked here in the past,
but I really want to make sure my topology is sane. Thanks a lot.

Alex
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