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Hi!
Does OpenMM have (or plan to have) an implementation of a flexible water model like SPC/Fw that is compatible with AMBER, etc. force fields for biomolecules? I am considering doing some flexible water MD but I'm not sure I want to switch to something as expensive as AMOEBA right away.
Thank you!

If you specify "constraints=None, rigidWater=False" in your call to createSystem(), you'll get flexible water. I just checked a couple of the Amber14 water models and verified that they include bond and angle terms, so presumably they were designed to support that.

I see. But doing that wouldn't result in any of the "recognized" flexible models without changing charge parameters, etc. in the SPC/E or TIP?P model. Simply letting go of the rigidity seems like a quick-and-dirty way to get flexible water, but I anticipate problems justifying the model...What do you think?

You could create your own XML file defining the water model you want. Be aware though that Amber parametrizes the ions for a specific water model, so different water models include different ion parameters. Anything you do will be hard to justify in one way or another. Amber was parametrized with rigid water.

Oh ok, I understand. It seems like AMOEBA is a better choice for protein containing systems with flexible water, but AMBER + flexible water may be useful for trial runs, etc.