Re: Kb, Ktheta values for TIP3P water model

From: Kenno Vanommeslaeghe (
Date: Tue Jul 29 2014 - 23:21:58 CDT

On 07/29/2014 06:59 PM, Hadi wrote:
> Below you can find the parameters of the modified TIP3P model (CGenFF:
> Parameters for the Charmm General Force Field). This model is often used
> in NAMD.

(1) Small detail: the only thing this has to do with CGenFF is that it is
the water model on which *all* the all-atom additive CHARMM force fields
are based, the first one of which predates CGenFF by roughly 15 years. The
confusion likely arises from the fact that the NAMD version of the CHARMM
water stream file can currently be found on my CGenFF page. Instead, we
probably should put it on the main CHARMM force field page alongside the
CHARMM version.

(2) It is my understanding that the parameters you copy-pasted into your
e-mail are just "reasonable values" for in case an MD engine or
methodology is used that does not support rigid water. They won't give the
exact same properties as the true rigid TIP3P model. NAMD, however, does
support rigid water; assuming the psf is generated correctly, all one need
to do is put the following line in the NAMD input:
rigidBonds water
Though if you're using the CHARMM force field, it's actually recommended
to instead use
rigidBonds all
because that's what the force field is parametrized for.

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