123 lines
4.8 KiB
ReStructuredText
123 lines
4.8 KiB
ReStructuredText
.. index:: compute stress/mop
|
|
.. index:: compute stress/mop/profile
|
|
|
|
compute stress/mop command
|
|
==========================
|
|
|
|
compute stress/mop/profile command
|
|
==================================
|
|
|
|
Syntax
|
|
""""""
|
|
|
|
.. parsed-literal::
|
|
|
|
compute ID group-ID style dir args keywords ...
|
|
|
|
* ID, group-ID are documented in :doc:`compute <compute>` 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)
|
|
|
|
.. parsed-literal::
|
|
|
|
*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
|
|
|
|
.. code-block:: LAMMPS
|
|
|
|
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
|
|
|
|
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
|
|
|
|
Description
|
|
"""""""""""
|
|
|
|
Compute *stress/mop* and compute *stress/mop/profile* define computations that
|
|
calculate components of the local stress tensor using the method of
|
|
planes :ref:`(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
|
|
:doc:`compute stress/atom <compute_stress_atom>`), the method of planes is
|
|
compatible with mechanical balance in heterogeneous systems and at
|
|
interfaces :ref:`(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
|
|
:ref:`(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 :doc:`pair_modify tail yes <pair_modify>` 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 :doc:`units <units>`.
|
|
|
|
The values produced by this compute can be accessed by various :doc:`output commands <Howto_output>`. For instance, the results can be written to a file using the :doc:`fix ave/time <fix_ave_time>` 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 :doc:`Build package <Build_package>`
|
|
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
|
|
""""""""""""""""
|
|
|
|
:doc:`compute stress/atom <compute_stress_atom>`
|
|
|
|
Default
|
|
"""""""
|
|
|
|
none
|
|
|
|
----------
|
|
|
|
.. _mop-todd:
|
|
|
|
**(Todd)** B. D. Todd, Denis J. Evans, and Peter J. Daivis: "Pressure tensor for inhomogeneous fluids",
|
|
Phys. Rev. E 52, 1627 (1995).
|