lammps/doc/kspace_modify.txt

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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 (nozforce} or {compute} or {diff} :l
  {mesh} value = x y z
    x,y,z = grid size in each dimension for long-range Coulombics
  {mesh/disp} value = x y z
    x,y,z = grid size in each dimension for 1/r^6 dispersion
  {order} value = N
    N = gridextent of Gaussian for PPPM or MSM mapping of charge to grid
  {order/disp} value = N
    N = extent of Gaussian for PPPM mapping of dispersion term to grid
  {order/split} value = N
    N = order of Taylor series used to split the potential between different MSM levels
  {force} value = accuracy (force units)
  {gewald} value = rinv (1/distance units)
    rinv = G-ewald parameter for Coulombics
  {gewald/disp} value = rinv (1/distance units)
    rinv = G-ewald parameter for dispersion
  {slab} value = volfactor or {nozforce}
    volfactor = ratio of the total extended volume used in the
      2d approximation compared with the volume of the simulation domain
  {nozforce} turns off kspace forces in the z direction
  {compute} value = {yes} or {no} 
  {diff} value = {ik} or {ad} :pre
:ule

[Examples:]

kspace_modify mesh 24 24 30 order 6 splitorder 3
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 grid size for kspace style {pppm} or
{msm}.  In the case of PPPM, this is the FFT mesh, and each dimension
must be factorizable into powers of 2, 3, and 5.  In the case of MSM,
this is the finest scale real-space mesh, and each dimension must be
factorizable into powers of 2.  When this option is not set, the PPPM
or MSM 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 {mesh/disp} keyword sets the grid size for kspace style
{pppm/disp}.  This is the FFT mesh for long-range dispersion and ach
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 grid in kspace style {pppm} or {msm}.  
The default for this parameter is 5 for PPPM and 4 for MSM, which means 
each charge spans 5 or 4 grid cells in each dimension, respectively.  
For the LAMMPS implementation of MSM, the order can range from 4 to 10
and must be even. For PPPM, the minimum allowed setting is 2 and the 
maximum allowed setting is 7.  The larger the value of this parameter, 
the smaller the grid will need to be to achieve the requested accuracy.  
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 or MSM direct sum, but a larger order parameter
will increase the cost of interpolating charge/fields to/from the grid.

The {order/disp} keyword determines how many grid spacings an atom's
dispersion term extends when it is mapped to the grid in kspace style
{pppm/disp}.  It has the same meaning as the {order} setting for
Coulombics.

The {order/split} keyword determines the order of the Taylor series
used to split the potential between different MSM grid levels, and can
range from 2 and 6. "(Hardy)"_#Hardy recommends that the {order/split}
be roughly half of the order parameter.  For example, the default MSM
order is 4 and the default {splitorder} is 2.  For higher accuracy in
MSM, one can use order 10 and {order/split} 5 or 6, though this will
increase the interpolation cost as described above.

The PPPM order parameter may be reset by LAMMPS when it sets up the 
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. Automatic reduction of order is not currently
implemented in MSM, so an error (instead of a warning) will be generated.

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}, the FFT grid size for style
{pppm}, or the real space grid size for style {msm}.

The {gewald} keyword sets the value of the Ewald or PPPM G-ewald
parameter for charge 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.  MSM does not
use the {gewald} parameter.

The {gewald/disp} keyword sets the value of the Ewald or PPPM G-ewald
parameter for dispersion as {rinv} in reciprocal distance units.  It
has the same meaning as the {gewald} setting for Coulombics.

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.  An alternative slab
option can be invoked with the {nozforce} keyword in lieu of the
volfactor. This turns off all kspace forces in the z direction. The
{slab} and {nozforce} options are not allowed for MSM.

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 for
PPPM.  It performs differentiation in Kspace, but uses 3 FFTs to
transfer the computed fields back to real space (total of 4 FFTs per
timestep). The analytic differentiation, or {ad} approach uses only 1
FFT to transfer the computed fields back to real space (total of 2
FFTs per timestep), but requires a somewhat larger PPPM mesh to
achieve the same accuracy as the {ik} approach. Analogous approaches
have been implemented in MSM and can be specified using the same
keywords. The {ad} approach is the default for MSM.

IMPORTANT NOTE: Currently, not all {pppm} styles support the {ad}
option.  Support for those {pppm} variants will be added later.

[Restrictions:] none

[Related commands:]

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

[Default:]

The option defaults are mesh = mesh/disp = 0 0 0, order = order/disp =
5 (PPPM), order = 4 (MSM), order/split = 2 (MSM), force = -1.0, gewald
= gewald/disp = 0.0, slab = 1.0, compute = yes, and diff = ik (PPPM),
diff = ad (MSM).

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

:link(Hardy)
[(Hardy)] David, Multilevel Summation for the Fast Evaluation of 
Forces for the Simulation of Biomolecules, University of Illinois 
at Urbana-Champaign, (2006).