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Re: [lammps-users] Question about using r-respa integrator
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Re: [lammps-users] Question about using r-respa integrator


From: Axel Kohlmeyer <akohlmey@...24...>
Date: Wed, 11 Apr 2018 13:11:04 -0400

On Wed, Apr 11, 2018 at 1:04 PM, Pengyu Huang <hughh.py@...24...> wrote:
> Dear all,
>
> I would like to model flexible water, co2 and solid surfaces in my system
> with LAMMPS, which requires a very small time step (for flexible water),
> i.e. dt = 0.25 fs. I think r-respa may be one way to speed up the
> simulation. According to the LAMMPS document, it suggests using the
> following setup, so that the innermost time step is 0.5 fs:
>
> timestep  4.0
> run_style respa 4 2 2 2 inner 2 4.5 6.0 middle 3 8.0 10.0 outer 4
>
> I will try this set up in my simulations with timestep = 2 fs so that the
> innermost timestep is 0.25 fs (for bonds and angles) . My question is: apart
> from checking the energy conservation, is there any "pitfalls" about using
> the r-respa method in molecular dynamics simulations that I should pay
> attention to? It will be great if anyone who has had experiences with the
> r-respa method can provide any comments, suggestions or hints. All comments
> are welcome.

you are confusing multi-cutoff separation with bond/angle/pair separation.
you are not going to save much time with your setup from above, since
you are computing a lot of interactions multiple times.

you probably want something like this:

timestep 4.0

run_style respa 3 4 2 dihedral 1 pair 2 kspace 3

however, the biggest problem is not the flexible water, but the co2.
for that you would need a rigid integrator and those will require a
short time step anyway to run stable.
so using r-RESPA may not be as helpful as you expect.

axel.


>
> Thank you for your time.
>
> Regards,
> Pengyu
>
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-- 
Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.