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.