"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)

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compute stress/mop command :h3
compute stress/mop/profile command :h3


[Syntax:]

compute ID group-ID style dir args keywords ... :pre

ID, group-ID are documented in "compute"_compute.html command
style = {stress/mop} or {stress/mop/profile}
dir = {x} or {y} or {z} is the direction normal to the plane
args = argument specific to the compute style
keywords = {kin} or {conf} or {total} (one of more can be specified) :ul
  {stress/mop} args = pos
    pos = {lower} or {center} or {upper} or coordinate value (distance units) is the position of the plane
  {stress/mop/profile} args = origin delta
    origin = {lower} or {center} or {upper} or coordinate value (distance units) is the position of the first plane
    delta = value (distance units) is the distance between planes :pre

compute 1 all stress/mop x lower total
compute 1 liquid stress/mop z 0.0 kin conf
fix 1 all ave/time 10 1000 10000 c_1\[*\] file mop.time
fix 1 all ave/time 10 1000 10000 c_1\[2\] file mop.time :pre

compute 1 all stress/mop/profile x lower 0.1 total
compute 1 liquid stress/mop/profile z 0.0 0.25 kin conf
fix 1 all ave/time 500 20 10000 c_1\[*\] ave running overwrite file mopp.time mode vector :pre


[Description:]

Compute {stress/mop} and compute {stress/mop/profile} define computations that
calculate components of the local stress tensor using the method of
planes "(Todd)"_#mop-todd.  Specifically in compute {stress/mop} calculates 3
components are computed in directions {dir},{x}; {dir},{y}; and
{dir},{z}; where {dir} is the direction normal to the plane, while
in compute {stress/mop/profile} the profile of the stress is computed.

Contrary to methods based on histograms of atomic stress (i.e. using
"compute stress/atom"_compute_stress_atom.html), the method of planes is
compatible with mechanical balance in heterogeneous systems and at
interfaces "(Todd)"_#mop-todd.

The stress tensor is the sum of a kinetic term and a configurational
term, which are given respectively by Eq. (21) and Eq. (16) in
"(Todd)"_#mop-todd. For the kinetic part, the algorithm considers that
atoms have crossed the plane if their positions at times t-dt and t are
one on either side of the plane, and uses the velocity at time t-dt/2
given by the velocity-Verlet algorithm.

Between one and three keywords can be used to indicate which
contributions to the stress must be computed: kinetic stress (kin),
configurational stress (conf), and/or total stress (total).

NOTE 1: The configurational stress is computed considering all pairs of atoms where at least one atom belongs to group group-ID.

NOTE 2: The local stress does not include any Lennard-Jones tail
corrections to the pressure added by the "pair_modify tail
yes"_pair_modify.html command, since those are contributions to the global system pressure.

[Output info:]

Compute {stress/mop} calculates a global vector (indices starting at 1), with 3
values for each declared keyword (in the order the keywords have been
declared). For each keyword, the stress tensor components are ordered as
follows: stress_dir,x, stress_dir,y, and stress_dir,z.

Compute {stress/mop/profile} instead calculates a global array, with 1 column
giving the position of the planes where the stress tensor was computed,
and with 3 columns of values for each declared keyword (in the order the
keywords have been declared). For each keyword, the profiles of stress
tensor components are ordered as follows: stress_dir,x; stress_dir,y;
and stress_dir,z.

The values are in pressure "units"_units.html.

The values produced by this compute can be accessed by various "output commands"_Howto_output.html. For instance, the results can be written to a file using the "fix ave/time"_fix_ave_time.html command. Please see the example in the examples/USER/mop folder.

[Restrictions:]

These styles are part of the USER-MISC package. They are only enabled if
LAMMPS is built with that package. See the "Build package"_Build_package.html
doc page on for more info.

The method is only implemented for 3d orthogonal simulation boxes whose
size does not change in time, and axis-aligned planes.

The method only works with two-body pair interactions, because it
requires the class method pair->single() to be implemented. In
particular, it does not work with more than two-body pair interactions,
intra-molecular interactions, and long range (kspace) interactions.

[Related commands:]

"compute stress/atom"_compute_stress_atom.html

[Default:] none

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:link(mop-todd)
[(Todd)] B. D. Todd, Denis J. Evans, and Peter J. Daivis: "Pressure tensor for inhomogeneous fluids",
Phys. Rev. E 52, 1627 (1995).
