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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 <hughh.py@...24...>
Date: Mon, 16 Apr 2018 23:40:02 +1000

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