[gmx-users] constant term in dihedral potential function

[Kukol, Andreas]

Dear all,

I included the following dihedral potential function in a topology:

V(phi2) = -5.865 + 7.470 [1+cos(phi2-180°)] + 3.99 [1+cos(2 phi2)] + 1.1 [1+cos(3 phi2-180°)]

The problem is about representing the constant term -5.865. Using equation 4.61 (Gromacs 4.0 manual):

V(phi) = k * (1 + cos (n phi - phis) )

I specified:

V = -5.865 * (1 + cos (0 - 90) ) = -5.865 with multiplicity n=0

That leads to the entry in the topology file:
 22    23    24    25     1    90     -5.685     0

This worked fine in Gromacs 3.3, but in Gromacs 4.05 I get the error message (from Grompp):

Fatal error:
Value of 'multiplicity' in Proper Dih. is 0, which is smaller than the minimum of 1

I wonder, if this is a bug in Gromacs 4.05, or if there is any better way to specify a constant term in a dihedral potential function.

Many thanks
Andreas