[gmx-users] VDW switched off in CHARMM tip3p model

[Mark Abraham]

On Thu, Aug 15, 2013 at 12:04 AM, Christopher Neale
<chris.neale at mail.utoronto.ca> wrote:
> >From my inspection of the force field files included in gromacs-4.6.1, when you
> use the charmm ff and the charmm tip3p (tips3p.itp) you get the hydrogen type HT,
> which does have LJ on the hydrogen atoms, whereas you only get the hydrogen
> type HWT3 when you use the non-charmm tip3p (tip3p.itp).
>
> If I understand Mark's comment to be that there is no good reason to use the charmm
> tip3p water model with the charmm ff, then I disagree (See below).
>
> I have posted an image ( http://tinypic.com/r/110ak5s/5 ) of the area per lipid (APL)
> for a POPC bilayer that I built with charmm-gui (83 lipids/leaflet; 43 waters/lipid)
> and then simulated under the charmm36 lipid parameters for ~ 300 ns many times
> successively with different water models (2 repeats each, starting from different
> conformations out of charmm-gui). Note the large difference in the APL for the
> regular tip3p and the charmm tip3p.  In fact, some papers note that you get
> gel-phase bilayers with the charmm36 lipid ff and the non-charmm tip3p model when using
> certain cutoff treatments
> (Piggot and Khalid -- http://pubs.acs.org/doi/abs/10.1021/ct3003157 -- is one such paper).

Good to know, thanks!

Mark

> Chris.
>
> -- original message --
>
> You could do that, but it's extra cost for no known benefit. See
> http://dx.doi.org/10.1021/ct900549r
>
> On Tue, Aug 13, 2013 at 7:15 PM, Weilong Zhao <wz24 at zips.uakron.edu> wrote:
>> Hi,
>>
>> I noticed that for GROMACS versions of 4.5 of later, the force field for
>> TIP3P water model in CHARMM27.ff package has a following line included:
>>
>>  The following atom types are NOT part of the CHARMM distribution
>> ..
>> .
>> HWT3    1       1.008000        0.417   A       0.0     0.0     ; TIP3p H
>>
>> It seems that VDW interaction of the default hydrogen of water is switched
>> off, which makes it not the correct TIP3P ff of CHARMM. However this
>> hydrogen type HWT3, is used as default by gromacs as CHARMM water model.
>>
>> Even though the hydrogen without VDW is the original TIP3P water model, I
>> wonder is there any problem to include it in CHARMM ff set? Really
>> appreciate if anyone could share some opinion.
> --
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