[gmx-users] TIP4p water in gromos force field files (OWT4 and IW)??

[Rasmus Lundsgaard]

Dear Justin (and the rest)

I have been using the TIP4p water model with OPLS-AA and TraPPE - and 
now for comparison I would like to try with the Gromos force field. From 
the TIP4p.itp file in the /shared/top directory it states:

...
#ifdef _FF_GROMACS
1       OWT4            1       SOL      OW     1       0.0
2       HW              1       SOL     HW1     1       0.52
3       HW              1       SOL     HW2     1       0.52
4       IW              1       SOL      MW     1      -1.04
#endif
#ifdef _FF_GROMOS96
1       OWT4            1       SOL      OW     1       0.0    15.9994
2       H               1       SOL     HW1     1       0.52    1.008
3       H               1       SOL     HW2     1       0.52    1.008
4       IW              1       SOL      MW     1      -1.04    0.0
#endif
...

So from this it should be implemented in the GROMACS  gromos nb and bon 
files - right?!? I mean that the OTW4 and IW are in the ffgmxnb, 
ffgmxbon (hence the ifdef _FF_GROMACS).

What I mean is that if it's very rare to use gromos with TIP4p water 
models (and maybe even not a strictly correct use of the force field), 
why is it then mentioned in the TIP4p.itp example file in the official 
GROMACS release?

Best regards
Rasmus Lundsgaard


Justin A. Lemkul wrote:
>
>
> Rasmus Lundsgaard wrote:
>> Dear all
>>
>> I would like to do some simulation with the TIP4p water model with 
>> the ffG53a6 (Gromos96) force field.
>>
>> In *TIP4p.itp* it states to use the *OWT4* atom for the oxygen and 
>> the *IW* for the charged atom (4 site water model), but these two 
>> atoms are not listed in the *ffG53a6nb.itp* and *ffG53a6bon.itp* 
>> files. This means that GROMACS can not find these atoms and how to 
>> calculate LJ and bonded forces with these atoms...
>>
>> Have I overseen something or are they suddenly lost from the gromos 
>> itp's in GROMACS??
>>
>> The OW atom in the itp's are for the SPC/E water model - so they are 
>> not use able for TIP4p (different LJ parameters)
>
> I don't know that I've seen anyone do a simulation with 
> Gromos96+TIP4P.  The original derivation of Gromos96 (and the most 
> common usage) is Gromos96+SPC or SPC/E.  If you're set on using TIP4P 
> as your water model, you'd better be ready to provide justification 
> for that choice.
>
> If you need to add atom types, you certainly can, but you may have to 
> re-name them so as not to conflict with existing SPC or SPC/E atom 
> types.  It's very simple to put these parameters in the nb.itp, 
> bon.itp, and .atp files.
>
> -Justin
>
>>
>>
>> Best regard!
>> Rasmus
>>
>> -- 
>> PhD student at "Center for Energy Resources Engineering" (CERE-DTU)
>> Department of Chemical and Biochemical Engineering, Building 229
>> Technical University of Denmark - DTU
>> DK - 2800 Lyngby, Denmark
>> Email: RAL at kt.dtu.dk
>>
>