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Re: [lammps-users] r-RESPA for pressure calculation
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Re: [lammps-users] r-RESPA for pressure calculation

From: Pengyu Huang <>
Date: Tue, 17 Apr 2018 12:38:23 +1000

Thank you, Axel, for your reply. 

Sorry that I should have provided more details and further tests. The LAMMPS version I am using is the previous stable version 11Aug2017. I also did a quick check, it seems like the r-RESPA has not been changed. 

I did a quick test with different integrators for the flexible SPC running in the NVT ensemble with "fix nvt" at T = 333.15K, restarting from a pre-equilibrated water box (which has 8287 SPC molecules) in the NPT ensemble at a targeted pressure of 20 MPa and T = 333.15 K. I have compared three integration methods: 1. r-RESPA with timestep = 0.25 fs and only one level for all forces calculations; 2 Verlet with timestep =  0.25 fs; 3 r-RESPA with timestep = 2 fs, with bond and angles calculated at 0.25 fs, pair at 1 fs and kspace at 2 fs (run_style respa 3 4 2 bond 1 angle 1 dihedral 1 improper 1 pair 2 kspace 3). In all cases, the Tdamp was 100 times of the timestep (or outermost timestep).

I have attached the plots for the total energy and the pressure against with time. The simulation was run 25 ps for the Cases 1 and 2, and 50 ps for the Case 3. It may be a bit short, but I think it already captures the problem I described. The Case 1 and 2 had similar results, while the Case 3 had higher total energy and pressure than the Cases 1 and 2. The pressure for Cases 1 and 2 were around 16 -18 MPa, respectively, while the Case 3 had an average pressure of 120 MPa. All cases had similar degrees of fluctuations, given the standard deviations were around 35-40 MPa. If there is no problem with the r-RESPA, would this means that I cannot use it for flexible water to get the correct pressure?




On Tue, Apr 17, 2018 at 2:21 AM, Axel Kohlmeyer <akohlmey@...12...24...> wrote:
you also forgot to mention the LAMMPS version you are using and
whether you get the same issue, when you run r-RESPA with 0.25fs time
step and only one level or a multiplier of 1 (so that all levels run
with the same size timestep).

finally, you mention a multi-component system so, you should make
tests with a homogeneous system first.


On Mon, Apr 16, 2018 at 9:40 AM, Pengyu Huang <> wrote:
> Just forgot to mention in my previous email, the outermost timestep is 2 fs
> in r-RESPA, so that the bond and angle forces are calculated every 0.25 fs,
> pair forces at 1 fs and kspace at 2 fs.
> Regards,
> Pengyu
> On Mon, Apr 16, 2018 at 11:37 PM, Pengyu Huang <> wrote:
>> Dear LAMMPS developers,
>> I am running the simulation for a system with flexible spc and flexible
>> co2, and silica surfaces. I found the different pressures in the bulk water
>> region between the Verlet integrator and r-RESPA integrator. I used a
>> timestep of 0.25 fs for Verlet and the following set up for r-RESPA:
>> run_style respa 3 4 2 bond 1 angle 1 dihedral 1 improper 1 pair 2 kspace
>> 3,
>> so that the bond and angle forces are calculated every 0.25 fs, pair
>> forces at 1 fs and kspace at 2 fs. For kspace calculation (pair_style
>> lj/cut/coul/long), pppm with accuracy 1e-6 was used.
>> The pressure in bulk water region (far away from the interfaces) with
>> Verlet is around 12 MPa, while one with r-RESPA gives 115 MPa, which is 10x
>> larger. The pressures in bulk co2 region from the two systems are
>> statistically similar. I am aware when using large timestep for flexible
>> water with Verlet integrator, the calculation of pressure would be
>> incorrect, as I have tested something similar in
>> (Although I got correct
>> pressures for rigid spc at 1 fs...). So, would this means that the way of
>> the pressure calculation in r-RESPA (at a larger timestep?) cause the
>> problem?
>> Thank you for your time.
>> Regards,
>> Pengyu
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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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