[gmx-users] Proper Dih. multiplicity

Alberto Malvezzi malvezzi at iq.usp.br
Mon Aug 15 17:34:40 CEST 2005

I'm having the same problem described bellow, which is the error message  

Fatal error: [ file "topology_A.itp", line 8909 ]:
             Proper Dih. multiplicity can not be perturbed 6.000000!=3.000000

I have read the answer given earlier, and I seem to understand in which file I should define this B topology.
Any help would be appreciated.

Hi all.
/>/ I'm trying to simulate a sugar molecule and I have some problems whith proper
/>/ dihedrals (ffgmx) when I try to run grompp.
/>/ I need to define dihedral angles with different multiplicity for the same
/>/ group of atoms. (I don't want to do FEP)
/>/     1     3     6     9     1       0.0         5.858             3
/>/     1     3     6     7     1       0.0         0.418             2
/>/ The program displays an error message:
/>/ Fatal error: [ file "./hial.itp", line 205 ]:
/>/              Proper Dih. multiplicity can not be perturbed 3.000000!=2.000000
This is caused by a dihedral type definition in the forcefield where
a multiplicity is specified. In your topology you specify a different
multiplicity, which is OK, except that Gromacs also *always* assigns
a 'B' topology (which is used for FEP), even if you don't use FEP.
The parameters for the 'B' topology come from the dihedral type
definition and not from your topoligy definition. So the solution is
to explicityly define the 'B' topology so it will be identical to
the 'A', like:

   1     3     6     7     1       0.0      0.418     2     0.0     
0.418     2


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