\(\renewcommand{\AA}{\text{Å}}\)

# compute stress/mop command¶

# compute stress/mop/profile command¶

## Syntax¶

```
compute ID group-ID style dir args keywords ...
```

ID, group-ID are documented in compute command

style =

*stress/mop*or*stress/mop/profile*dir =

*x*or*y*or*z*is the direction normal to the planeargs = argument specific to the compute style

keywords =

*kin*or*conf*or*total*(one of more can be specified)

stress/mopargs = pos pos =lowerorcenterorupperor coordinate value (distance units) is the position of the planestress/mop/profileargs = origin delta origin =lowerorcenterorupperor coordinate value (distance units) is the position of the first plane delta = value (distance units) is the distance between planes

## Examples¶

```
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 (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), the method of planes is compatible with mechanical balance in heterogeneous systems and at interfaces (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). For the kinetic part, the algorithm considers that atoms have crossed the plane if their positions at times \(t-\Delta t\) and \(t\) are one on either side of the plane, and uses the velocity at time \(t-\Delta t/2\) 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 stress added by the pair_modify tail yes command, since those are contributions to the global system pressure.

NOTE 3: The local stress profile generated by compute *stress/mop/profile*
is similar to that obtained by compute
stress/cartesian.
A key difference is that compute *stress/mop/profile* considers particles
crossing a set of planes, while compute *stress/cartesian* computes averages
for a set of small volumes. More information
on the similarities and differences can be found in
(Ikeshoji).

## 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.

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

## Restrictions¶

These styles are part of the EXTRA-COMPUTE package. They are only enabled if LAMMPS is built with that package. See the 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.

## Default¶

none

**(Todd)** B. D. Todd, Denis J. Evans, and Peter J. Daivis: “Pressure tensor for inhomogeneous fluids”,
Phys. Rev. E 52, 1627 (1995).

**(Ikeshoji)** Ikeshoji, Hafskjold, Furuholt, Mol Sim, 29, 101-109, (2003).