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

[João T. S. Coimbra]

Dear All,
I am fairly new to the GROMACS world.
Recently I have came across 2 sections in the GROMACS manual intitled "angle-angle cross term" and "bond-angle cross term".
I was wondering if it is possible to implement these additional parameters for the commonly available force fields: AMBER, GROMACS and CHARMM (in regards to protein simulations).
I have tested the Morse potential with the AMBER ff99SB, and this only needs a keyword for selecting this potential. For the case of bond-angle and angle-angle cross terms I cannot find such "keyword-modes". Does this mean that a carefull parameterization of such cross terms is needed?
If so, can I then implement such MM terms in the previously discussed force fields?
Thanks,JC