forked from lijiext/lammps
63 lines
2.0 KiB
Plaintext
63 lines
2.0 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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third_order command :h3
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[Syntax:]
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third_order group-ID style delta args keyword value ... :pre
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group-ID = ID of group of atoms to displace :ulb,l
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style = {regular} or {eskm} :l
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delta = finite different displacement length (distance units) :l
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one or more keyword/arg pairs may be appended :l
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keyword = {file} or {binary}
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{file} name = name of output file for the third order tensor
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{binary} arg = {yes} or {no} or {gzip} :pre
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:ule
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[Examples:]
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third_order 1 regular 0.000001
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third_order 1 eskm 0.000001
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third_order 3 regular 0.00004 file third_order.dat
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third_order 5 eskm 0.00000001 file third_order.dat binary yes :pre
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[Description:]
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Calculate the third order force constant tensor by finite difference of the selected group,
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:c,image(JPG/third_order_force_constant.png))
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where Phi is the third order force constant tensor.
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The output of the command is the tensor, three elements at a time. The
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three elements correspond to the three gamma elements for a specific i/alpha/j/beta/k.
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The initial five numbers are i, alpha, j, beta, and k respectively.
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If the style eskm is selected, the tensor will be using energy units of 10 J/mol.
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These units conform to eskm style from the dynamical_matrix command, which
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will simplify operations using dynamical matrices with third order tensors.
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[Restrictions:]
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The command collects a 9 times the number of atoms in the group on every single MPI rank,
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so the memory requirements can be very significant for large systems.
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This command is part of the USER-PHONON 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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"fix phonon"_fix_phonon.html "dynamical_matrix"_dynamical_matrix.html
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[Default:]
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The default settings are file = "third_order.dat", binary = no
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