[gmx-users] bond-bond and bond-angle cross terms

[Camilo Calderon]

Hi all!
I am new to the GROMACS-world, and I am mystified.
How do I implement the 3 body potentials advertised in sec. 4.2.8 and sec.
4.2.9 of the GROMACS 3.3 manual?
Right now I have the following:

 [ cross_cross_bond ]
;ai  aj  funct  r1e  r2e  krr
<numbers here>

 [ cross_bond_angle ]
;ai aj ak funct r1e r2e r3e krt
<more numbers here>

when I grompp the topol.top, what I get is a Error, with a complaint
"invalid directive", and then a Fatal Error, with the complaint "Incorrect
number of parameters - found 5, expected 2 or 4."  What is really tough
about this is that I've been poring over the source code, trying to decypher
what input format ought to be - there is nothing on this either in the
Manual, or in the various lists!

Thanks a ton!

Camilo
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