[gmx-developers] Re: nstenergy and nstcalcenergy?

[Berk Hess]

Hi all,

I want to ask your opinion on the setup of mdp energy and coupling
frequencies.

In 4.0 and before energies were calculated at every step and T and P
coupling were
also done every step (except with mdrun -nosum).

In 4.5 we try to do global summation less often (meaning calculating
energies and
T/P coupling) and PME solving is already cheaper without E and P.
In future versions the innerloops will also be cheaper.
So therefore I introduced nstcalcenergy which determines the frequency
of energy
calculation, T and P coupling.

Michael Shirts suggested to have a separate nstcouple. But it currently
never makes sense
to have that set different from nstcalcenergy except for twin-range
interactions, where the energy
and pressure can only be calculated every nstlist steps and you might
want to do T coupling
every step (but doing that every nstlist is not a major issue).
So I could add nsttcouple. nstpcouple does not make sense, because the
only reasonable
setting would be equal to nstcalcenergy.

So my question is:
Do we want only nstcalcenergy or nstcalcenergy and nsttcouple (or even a
useless nstpcouple)?

Berk


Berk Hess wrote:
> Not exactly.
>
> nstcalcenergy controls the calculation frequency of energies, which also
> affects the statistics in the "exact" averages and fluctuations in the
> energy files.
> If this would not be the case we would not need a parameter at all and
> we would
> just need a conditional with many components in the md loop.
>
> I had thought of adding nstcouple, but that is yet another parameter
> which the user
> can set and also has to be checked against several other nst...
> parameters for reasonable
> performance. Currently there would not be an advantage to having the
> freedom to
> set nstcouple different from nstenergy, except for the NVT ensemble
> where you only
> need to determine ekin and no other energies or pressures.
>
> On the other hand, having an nstcouple might be easier to understand for
> the user,
> since then is it much clearer what the function of nstcouple and
> nstcalcenergy is.
>
> Berk
>
> Shirts, Michael (mrs5pt) wrote:
>   
>> Following up on the last one, currently nstcalcenergy controls the frequency
>> at which coupling occurs. Perhaps there should be a nstcoupling, and then
>> nstcalcenergy is determined as a function of nstcoupling, nstenergy, nstlist
>> and any other variables that are affected, instead of controlling it
>> directly? 
>>
>> Best,
>> ~~~~~~~~~~~~
>> Michael Shirts
>> Assistant Professor
>> Department of Chemical Engineering
>> University of Virginia
>> michael.shirts at virginia.edu
>> (434)-243-1821
>>
>>
>>   
>>     
>>> From: "Shirts, Michael (mrs5pt)" <mrs5pt at eservices.virginia.edu>
>>> Reply-To: "michael.shirts at virginia.edu" <michael.shirts at virginia.edu>
>>> Date: Thu, 10 Jun 2010 17:41:06 -0400
>>> To: "gmx-developers at gromacs.org" <gmx-developers at gromacs.org>
>>> Subject: nstenergy and nstcalcenergy?
>>>
>>> I'm wondering about the relationship between nstenergy and nstcalcenergy.
>>> Is there a need for two variables?   This seems to lead to confusion.  For
>>> example, one issue that's appearing is that when nstenergy = 1, gromacs now
>>> only computes the energy every nstcalcenergy steps.  It seems that there
>>> should be either one variable, or they should be synchronized in some way.
>>> Or perhaps change the variable names if they are actually controlling
>>> something else.
>>>
>>> Best,
>>> ~~~~~~~~~~~~
>>> Michael Shirts
>>> Assistant Professor
>>> Department of Chemical Engineering
>>> University of Virginia
>>> michael.shirts at virginia.edu
>>> (434)-243-1821
>>>
>>>     
>>>       
>>   
>>     
>
>