[gmx-users] question about OPLS-AA force field -required bond constraints

[Martin, Erik W]

Hi, I'm new to both Gromacs and OPLS.  I have always used CHARMM or AMBER before so am a bit confused about some idiosyncrasies.

When I go to a production run on my system (a protein in a truncated octahedron box of Tip4 water) I have removed all constraints.  When I run grompp, I get an error telling me that a bond between a HG proton and OG oxygen of protein has an oscillation period of 9e-3 ps and is incompatible with my 1 fs time step.  I am used to being able to run a completely unconstrained simulation with a 1 fs time step… so I'm assuming this is a property of OPLS-AA?

Could someone please let me know what the bond constraint requirements are for OPLS-AA and GROMOS forcefields (I'm going to need to use that next).  I can't seem to find this information by searching but imagine its on a website somewhere?

Thanks a lot,
Erik

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