forked from lijiext/lammps
119 lines
3.9 KiB
Plaintext
119 lines
3.9 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,Section_commands.html#comm)
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:line
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pair_style sw command :h3
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[Syntax:]
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pair_style sw :pre
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[Examples:]
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pair_style sw
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pair_coeff * * si.sw Si
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pair_coeff * * SiC.sw Si C Si :pre
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[Description:]
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The {sw} style computes a 3-body "Stillinger-Weber"_#Stillinger
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potential for the energy E of a system of atoms as
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:c,image(Eqs/pair_sw.jpg)
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where phi2 is a two-body term and phi3 is a three-body term. The
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summations in the formula are over all neighbors J and K of atom I
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within a cutoff distance = a*sigma.
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Only a single pair_coeff command is used with the {sw} style which
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specifies a Stillinger-Weber potential file with parameters for all
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needed elements. These are mapped to LAMMPS atom types by specifying
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N additional arguments after the filename in the pair_coeff command,
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where N is the number of LAMMPS atom types:
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filename
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N element names = mapping of SW elements to atom types :ul
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As an example, imagine the SiC.sw file has Stillinger-Weber values for
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Si and C. If your LAMMPS simulation has 4 atoms types and you want
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the 1st 3 to be Si, and the 4th to be C, you would use the following
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pair_coeff command:
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pair_coeff * * SiC.sw Si Si Si C :pre
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The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
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The first three Si arguments map LAMMPS atom types 1,2,3 to the Si
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element in the SW file. The final C argument maps LAMMPS atom type 4
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to the C element in the SW file. If a mapping value is specified as
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NULL, the mapping is not performed. This can be used when a {sw}
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potential is used as part of the {hybrid} pair style. The NULL values
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are placeholders for atom types that will be used with other
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potentials.
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Stillinger-Weber files in the {potentials} directory of the LAMMPS
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distribution have a ".sw" suffix. Lines that are not blank or
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comments (starting with #) define parameters for a triplet of
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elements. The parameters in a single entry correspond to the two-body
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and three-body coefficients in the formula above:
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element 1 (the center atom in a 3-body interaction)
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element 2
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element 3
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epsilon (energy units)
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sigma (distance units)
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a
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lambda
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gamma
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costheta0
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A
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B
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p
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q :ul
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The epsilon, sigma, a, A, B, p, and q parameters are for two-body
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interactions. The lambda, gamma, and costheta0 parameters are for
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three-body interactions. The non-annotated parameters are unitless.
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The Stillinger-Weber potential file must contain entries for all the
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elements listed in the pair_coeff command. It can also contain
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entries for additional elements not being used in a particular
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simulation; LAMMPS ignores those entries.
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For a single-element simulation, only a single entry is required
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(e.g. SiSiSi). For a two-element simulation, the file must contain 8
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entries (for SiSiSi, SiSiC, SiCSi, SiCC, CSiSi, CSiC, CCSi, CCC), that
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specify SW parameters for all permutations of the two elements
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interacting in three-body configurations. Thus for 3 elements, 27
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entries would be required, etc.
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As annotated above, the first element in the entry is the center atom
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in a three-body interaction. Thus an entry for SiCC means a Si atom
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with 2 C atoms as neighbors. By symmetry, three-body parameters for
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SiCSi and SiSiC entries should be the same. Two-body parameters for
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an interaction come from the entry where the 2nd element is repeated.
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Thus the two-body parameters for Si interacting with C, comes from the
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SiCC entry. Again by symmetry, the two-body parameters in the SiCC
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and CSiSi entries should thus be the same. Two-body parameters in
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entries whose 2nd and 3rd element are different (e.g. SiCSi) are
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ignored.
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[Restrictions:]
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This pair potential requires the "newton"_newton.html setting to be
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"on" for pair interactions.
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[Related commands:]
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"pair_coeff"_pair_coeff.html
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[Default:] none
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:line
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:link(Stillinger)
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[(Stillinger)] Stillinger and Weber, Phys Rev B, 31, 5262 (1985).
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