[gmx-users] constant term in dihedral potential function

[Mark Abraham]

Kukol, Andreas wrote:
> 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.

This is only relevant if you're ever using the magnitude of the 
potential energy for something. d/dr of such a term is zero, of course, 
so there's no force contribution and thus it does not affect the 
dynamics at all.

Mark