[gmx-users] k-space settings in GROMACS

[Anh Vo]

Hi everyone,

I have asked about the settings in GROMACS that would do the same job as
*kspace_style* and *kspace_modify* in LAMMPS, and got reply from Mark a few
days ago, but there are some points I'm still not very clear. Please help
me with this again.

In details, these 2 commands in LAMMPS are:
*  - kspace_style    pppm  1e-5   *(kspace_style defines a long-range
solver, pppm is P3M algorithm, 1.0e-5 means that the RMS error will be a
factor of 100,000 smaller than the reference force)
*  - kspace_modify   diff  ad    *(kspace_modify sets the parameters used
by the kspace solvers, *diff*  keyword specifies the differentiation scheme
used by the PPPM method to compute forces on particles,  *ad*  keyword
means approach uses only 1 FFT to transfer information back to real space
for a total of 2 FFTs per timestep).

I know that  coulombtype *= *P3M-AD can set the pppm algorithm for long
range interaction. However, I'm not sure about the *1e-5 *part in*
'kspace_style
 pppm  1e-5'. *Would the command   'ewald-rtol   =  1e-05'  do the same
job?


I'm not sure because the 1e-5 in  'kspace_style    pppm  1e-5'  defines how
much the RMS error is smaller than the reference force, while 1e-05 in
'ewald-rtol   = 1e-05'  defines the relative strength of the Ewald-shifted
direct potential.

What values should I set for ewald-rtol to get the same meaning as '
*kspace_style  **1e-5'* ?


Also, would there be much difference (e.g. in the energy of the molecules,
in the output pressure, etc.) when PME is used instead of P3M? (I have
tried with some simple systems and got similar results when PME is used
instead of P3M-AD)

Thank you very much for your help. I really appreciate it.

Best regards,
Anh Vo