compute stress/mop command¶
compute stress/mop/profile command¶
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 plane
args = argument specific to the compute style
keywords = kin or conf or total (one of more can be specified)
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
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 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
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).
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.
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.
(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).