Syntax:
kspace_modify keyword value ...
mesh value = x y z x,y,z = PPPM FFT grid size in each dimension order value = N N = grid extent of Gaussian for PPPM mapping of each charge slab value = volfactor volfactor = ratio of the total extended volume used in the 2d approximation compared with the volume of the simulation domain
Examples:
kspace_modify mesh 24 24 30 order 6 kspace_modify slab 3.0
Description:
Set parameters used by the kspace solvers defined by the kspace_style command. Not all parameters are relevant to all kspace styles.
The mesh keyword sets the 3d FFT grid size for kspace style pppm. Each dimension must be factorizable into powers of 2, 3, and 5. When this option is not set, the PPPM solver chooses its own grid size, consistent with the user-specified accuracy and pairwise cutoff. Values for x,y,z of 0,0,0 unset the option.
The order keyword determines how many grid spacings an atom's charge extends when it is mapped to the FFT grid in kspace style pppm. The default for this parameter is 5, which means each charge spans 5 grid cells in each dimension.
The slab keyword allows an Ewald or PPPM solver to be used for a systems that are periodic in x,y but non-periodic in z - a boundary setting of "boundary p p f". This is done by treating the system as if it were periodic in z, but inserting empty volume between atom slabs and removing dipole inter-slab interactions so that slab-slab interactions are effectively turned off. The volfactor value sets the ratio of the extended dimension in z divided by the actual dimension in z. The recommended value is 3.0. A larger value is inefficient; a smaller value introduces unwanted slab-slab interactions. The use of fixed boundaries in z means that the user must prevent particle migration beyond the initial z-bounds, typically by providing a wall-style fix.
Restrictions: none
Related commands:
Default:
The option defaults are mesh = 0 0 0, order 5, and slab = 1.0.