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Re: [lammps-users] TIP5P support/development?
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Re: [lammps-users] TIP5P support/development?

From: Steve Plimpton <sjplimp@...24...>
Date: Wed, 2 May 2018 08:56:58 -0600

Hi Ahmed - I agree with the discussion points on this.  If someone
created a pair_tip5p style that was specific to the TIP5P model
(allowed long timesteps, etc), it would be a nice addition to LAMMPS.


On Sun, Apr 29, 2018 at 2:07 PM, Axel Kohlmeyer <akohlmey@...24...> wrote:
On Sun, Apr 29, 2018 at 2:40 PM, Ahmed E. Ismail <aeismail@...24...> wrote:
> On Sat, Apr 28, 2018 at 9:01 PM Axel Kohlmeyer <akohlmey@...24...> wrote:
>> On Sat, Apr 28, 2018 at 7:42 PM, Ahmed E. Ismail <aeismail@...24...>
>> wrote:
>> > Dear all:
>> >
>> > I know it was asked in an earlier post about TIP5P, but I was curious if
>> > anyone was currently working on trying to implement a five-point model
>> > in
>> > LAMMPS right now. One of my students is interested in working with
>> > TIP5P, so
>> > it would be good to know if anybody is currently developing such a
>> > model, or
>> > if there is sufficient interest to be worth doing it ourselves.
>> you can easily set up TIP5P using fix rigid/small. either disable the
>> test for per type mass == 0 or use something small like 1.0e-10.
>> only downside is the rather short time step required to have stable
>> time integration with rigid bodies.
> I guess the big question here is how small does that time step need to be?
> With TIPxP styles, we can use SHAKE to get away with a 2 fs time step. I'd
> rather not have to do 20 or 30 time steps to replace that; I doubt that
> would be very efficient in the long run.

how small would depend on your system and the kind of dynamics
happening. i would expect you have to use about 0.5fs. on a well
equilibrated bulk system you may get away with a larger time step.
that is still a significant cost compared with 2fs, but not as drastic
as you suspect it could be.

i view efficiency from the perspective of time to solution. these
days, it is often easier to get more CPU time, than to get your hands
on a suitably motivated student or postdoc that can program well
enough, not to count the time it will take to implement and test such
a new pair style compared to being able to start a production run
right away. while you can in principle follow the same approach as for
TIP4P, things get a little bit more complex for TIP5P, as you have to
not just displace one position, but have to do this twice, and if you
want to use long-range electrostatics, you need a matching kspace
style as well.


> —AEI
> --
> ========================
> Ahmed E. Ismail
> aei@...495...
> aei@...496...

Dr. Axel Kohlmeyer  akohlmey@...43...4...
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.

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