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From: |
Pengyu Huang <hughh.py@...24...> |

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?

Regards,

Pengyu

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.

axel.

> ------------------------------

On Mon, Apr 16, 2018 at 9:40 AM, Pengyu Huang <hughh.py@...24...> 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 <hughh.py@...24...> 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

>> http://lammps.sandia.gov/threads/msg66029.html (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... http://goo.gl/1wk0

College of Science & Technology, Temple University, Philadelphia PA, USA

International Centre for Theoretical Physics, Trieste. Italy.

**Attachment:
P_for_r-RESPA_test.jpg**

**Attachment:
etot_for_r-RESPA_test.jpg**

**Follow-Ups**:**Re: [lammps-users] r-RESPA for pressure calculation***From:*Axel Kohlmeyer <akohlmey@...24...>

**References**:**[lammps-users] r-RESPA for pressure calculation***From:*Pengyu Huang <hughh.py@...24...>

**Re: [lammps-users] r-RESPA for pressure calculation***From:*Pengyu Huang <hughh.py@...24...>

**Re: [lammps-users] r-RESPA for pressure calculation***From:*Axel Kohlmeyer <akohlmey@...24...>

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