forked from lijiext/lammps
80 lines
2.5 KiB
Plaintext
80 lines
2.5 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Commands_all.html)
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:line
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compute msd/nongauss command :h3
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[Syntax:]
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compute ID group-ID msd/nongauss keyword values ... :pre
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ID, group-ID are documented in "compute"_compute.html command :ulb,l
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msd/nongauss = style name of this compute command :l
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zero or more keyword/value pairs may be appended :l
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keyword = {com} :l
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{com} value = {yes} or {no} :pre
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:ule
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[Examples:]
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compute 1 all msd/nongauss
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compute 1 upper msd/nongauss com yes :pre
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[Description:]
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Define a computation that calculates the mean-squared displacement
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(MSD) and non-Gaussian parameter (NGP) of the group of atoms,
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including all effects due to atoms passing through periodic boundaries.
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A vector of three quantities is calculated by this compute. The first
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element of the vector is the total squared dx,dy,dz displacements
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drsquared = (dx*dx + dy*dy + dz*dz) of atoms, and the second is the
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fourth power of these displacements drfourth = (dx*dx + dy*dy +
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dz*dz)*(dx*dx + dy*dy + dz*dz), summed and averaged over atoms in the
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group. The 3rd component is the nonGaussian diffusion parameter NGP =
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3*drfourth/(5*drsquared*drsquared), i.e.
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:c,image(Eqs/compute_msd_nongauss.jpg)
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The NGP is a commonly used quantity in studies of dynamical
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heterogeneity. Its minimum theoretical value (-0.4) occurs when all
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atoms have the same displacement magnitude. NGP=0 for Brownian
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diffusion, while NGP > 0 when some mobile atoms move faster than
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others.
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If the {com} option is set to {yes} then the effect of any drift in
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the center-of-mass of the group of atoms is subtracted out before the
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displacement of each atom is calculated.
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See the "compute msd"_compute_msd.html doc page for further important
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NOTEs, which also apply to this compute.
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[Output info:]
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This compute calculates a global vector of length 3, which can be
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accessed by indices 1-3 by any command that uses global vector values
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from a compute as input. See the "Howto output"_Howto_output.html doc
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page for an overview of LAMMPS output options.
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The vector values are "intensive". The first vector value will be in
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distance^2 "units"_units.html, the second is in distance^4 units, and
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the 3rd is dimensionless.
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[Restrictions:]
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This compute is part of the MISC package. It is only enabled if
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LAMMPS was built with that package. See the "Build
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package"_Build_package.html doc page for more info.
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[Related commands:]
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"compute msd"_compute_msd.html
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[Default:]
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The option default is com = no.
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