lammps/doc/kspace_modify.txt

"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

:line

kspace_modify command :h3

[Syntax:]

kspace_modify keyword value ... :pre

one or more keyword/value pairs may be listed :ulb,l
keyword = {mesh} or {order} or {gewald} or {slab} or {compute} or {diff} :l
  {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
  {force} value = accuracy (force units)
  {gewald} value = rinv (1/distance units)
    rinv = PPPM G-ewald parameter
  {slab} value = volfactor
    volfactor = ratio of the total extended volume used in the
      2d approximation compared with the volume of the simulation domain
  {compute} value = {yes} or {no} 
  {diff} value = {ik} or {ad} :pre
:ule

[Examples:]

kspace_modify mesh 24 24 30 order 6
kspace_modify slab 3.0 :pre

[Description:]

Set parameters used by the kspace solvers defined by the
"kspace_style"_kspace_style.html 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 minimum allowed setting is 2 and the
maximum allowed setting is 7.  The larger the value of this parameter,
the smaller the FFT grid will need to be to achieve the requested
precision.  Conversely, the smaller the order value, the larger the
grid will be.  Note that there is an inherent trade-off involved: a
small grid will lower the cost of FFTs, but a large order parameter
will increase the cost of intepolating charge/fields to/from the grid.
And vice versa.

The order parameter may be reset by LAMMPS when it sets up the PPPM
FFT grid if the implied grid stencil extends beyond the grid cells
owned by neighboring processors.  Typically this will only occur when
small problems are run on large numbers of processors.  A warning will
be generated indicating the order parameter is being reduced to allow
LAMMPS to run the problem.

The {force} keyword overrides the relative accuracy parameter set by
the "kspace_style"_kspace_style.html command with an absolute
accuracy.  The accuracy determines the RMS error in per-atom forces
calculated by the long-range solver and is thus specified in force
units.  A negative value for the accuracy setting means to use the
relative accuracy parameter.  The accuracy setting is used in
conjunction with the pairwise cutoff to determine the number of
K-space vectors for style {ewald} or the FFT grid size for style
{pppm}.

The {gewald} keyword sets the value of the Ewald or PPPM G-ewald
parameter as {rinv} in reciprocal distance units.  Without this
setting, LAMMPS chooses the parameter automatically as a function of
cutoff, precision, grid spacing, etc.  This means it can vary from one
simulation to the next which may not be desirable for matching a
KSpace solver to a pre-tabulated pairwise potential.  This setting can
also be useful if Ewald or PPPM fails to choose a good grid spacing
and G-ewald parameter automatically.  If the value is set to 0.0,
LAMMPS will choose the G-ewald parameter automatically.

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"_boundary.html 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.  The methodology behind the {slab}
option is explained in the paper by "(Yeh)"_#Yeh.

The {compute} keyword allows Kspace computations to be turned off,
even though a "kspace_style"_kspace_style.html is defined.  This is
not useful for running a real simulation, but can be useful for
debugging purposes or for computing only partial forces that do not
include the Kspace contribution.  You can also do this by simply not
defining a "kspace_style"_kspace_style.html, but a Kspace-compatible
"pair_style"_pair_style.html requires a kspace_style to be defined.
This keyword gives you that option.

The {diff} keyword specifies the differentiation scheme used by the
PPPM method to compute forces on particles given electrostatic 
potentials on the PPPM mesh. The ik approach is the default that 
performs differentiation in kspace, but requires 3 FFTs to transfer
the computed fields back to real space. The analytic differentiation,
or ad approach requires only 1 FFT, but requires a somewhat bigger
PPPM mesh to achieve the same accuracy as the ik approach.

[Restrictions:] none

[Related commands:]

"kspace_style"_kspace_style.html, "boundary"_boundary.html

[Default:]

The option defaults are mesh = 0 0 0, order = 5, force = -1.0, gewald
= 0.0, slab = 1.0, compute = yes, and diff = ik.

:line

:link(Yeh)
[(Yeh)] Yeh and Berkowitz, J Chem Phys, 111, 3155 (1999).