Re: Kb, Ktheta values for TIP3P water model

From: Kenno Vanommeslaeghe (
Date: Tue Aug 05 2014 - 13:41:12 CDT

That doesn't mean the model wouldn't be rigid, just that the temperature
wouldn't be right. More importantly, I don't think the person who asked
the original question is planning to perform Langevin integration.
Langevin is good to mimic the effects of solvent collisions when running
implicit solvent and can also be used to improve sampling in some
instances, bit it wouldn't be very appropriate for his purpose. The gold
standard for explicit solvent simulations with CHARMM is Verlet (and
that's also what was used to parametrize the force field).

On 08/05/2014 12:52 PM, Hadi Dinpajooh wrote:
> Well, as mentioned in the link NAMD is not well developed if one uses
> rigid models and Langevin integration:
> " ... Although /NAMD/ will run with Langevin integration and rigid bonds
> both active, the equilibrium temperature of the system does not converge
> to the correct value. Different Langevin equations are needed to produce
> the correct velocity distribution, which have not yet been added to /NAMD/."
> Hadi
> Sent from my iPhone
> On Aug 5, 2014, at 7:54 AM, Kenno Vanommeslaeghe
> < <>> wrote:
>> On 08/05/2014 03:14 AM, Hadi wrote:
>>> (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.
>>> I hope it is clear that unlike MC simulations, a rigid model can actually
>>> be slightly flexible in MD simulations.
>>> Hadi
>> Umm no, provided that no mistakes were made in the set-up of SHAKE
>> (rigidBonds in NAMD), a rigid model is really rigid in MD. Except for
>> insignificant numerical deviations related to the SHAKE tolerance. More
>> info here:

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