[gmx-users] convert an itp from gmx to olps?

[Erik Lindahl]

nicolis at guideo.fr wrote:
> Hello,
> I have an .itp topology of a small drug for the gmx forcefield and I want to 
> convert it to use the opls.
> What sections should I change in addition of changing the atom types?
> I know there are different definitions for the planarities (no impropers in opls 
> but dihedrals with a periodicity of 2?) but what about the bond and angle 
> constants?
> Thank you,
> Ioannis

That depends on how you created your itp file; if you haven't added any 
explicit parameters (i.e. they are looked up based on the atom types) it 
might suffice to:

1. Add the hydrogens
2. Change atom types to OPLS ones
3. Remove chiral-center improper dihedrals
4. Change proper dihedrals to type 3.

You might also want to change planar group impropers to OPLS dihedrals 
with a periodicity of 2, but I can't imagine that will affect things too 
much....

Why not post the topology - then I can probably recommend what to do 
with it :-)

Cheers,

Erik