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175 lines
6.3 KiB
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<HTML>
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<H3>pair_style edip command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>pair_style edip
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</PRE>
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<PRE>pair_style edip/omp
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</PRE>
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<P><B>Examples:</B>
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</P>
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<P>pair_style edip
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pair_coeff * * Si.edip Si
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</P>
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<P><B>Description:</B>
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</P>
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<P>The <I>edip</I> style computes a 3-body <A HREF = "#EDIP">EDIP</A> potential which is
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popular for modeling silicon materials where it can have advantages
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over other models such as the <A HREF = "pair_sw.html">Stillinger-Weber</A> or
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<A HREF = "pair_tersoff.html">Tersoff</A> potentials. In EDIP, the energy E of a
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system of atoms is
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</P>
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<CENTER><IMG SRC = "Eqs/pair_edip.jpg">
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</CENTER>
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<P>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.
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Both terms depend on the local environment of atom I through its
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effective coordination number defined by Z, which is unity for a
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cutoff distance < c and gently goes to 0 at distance = a.
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</P>
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<P>Only a single pair_coeff command is used with the <I>edip</I> style which
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specifies a EDIP 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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</P>
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<UL><LI>filename
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<LI>N element names = mapping of EDIP elements to atom types
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</UL>
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<P>See the <A HREF = "pair_coeff.html">pair_coeff</A> doc page for alternate ways
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to specify the path for the potential file.
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</P>
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<P>As an example, imagine a file Si.edip has EDIP values for Si.
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</P>
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<P>EDIP files in the <I>potentials</I> directory of the LAMMPS
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distribution have a ".edip" 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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</P>
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<UL><LI>element 1 (the center atom in a 3-body interaction)
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<LI>element 2
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<LI>element 3
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<LI>A (energy units)
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<LI>B (distance units)
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<LI>cutoffA (distance units)
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<LI>cutoffC (distance units)
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<LI>alpha
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<LI>beta
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<LI>eta
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<LI>gamma (distance units)
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<LI>lambda (energy units)
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<LI>mu
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<LI>tho
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<LI>sigma (distance units)
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<LI>Q0
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<LI>u1
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<LI>u2
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<LI>u3
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<LI>u4
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</UL>
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<P>The A, B, beta, sigma parameters are used only for two-body interactions.
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The eta, gamma, lambda, mu, Q0 and all u1 to u4 parameters are used only
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for three-body interactions. The alpha and cutoffC parameters are used
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for the coordination environment function only.
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</P>
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<P>The EDIP 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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</P>
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<P>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 EDIP 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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</P>
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<P>At the moment, only a single element parametrization is
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implemented. However, the author is not aware of other
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multi-element EDIP parametrizations. If you know any and
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you are interest in that, please contact the author of
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the EDIP package.
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</P>
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<HR>
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<P>Styles with a <I>cuda</I>, <I>gpu</I>, <I>intel</I>, <I>kk</I>, <I>omp</I>, or <I>opt</I> suffix are
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functionally the same as the corresponding style without the suffix.
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They have been optimized to run faster, depending on your available
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hardware, as discussed in <A HREF = "Section_accelerate.html">Section_accelerate</A>
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of the manual. The accelerated styles take the same arguments and
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should produce the same results, except for round-off and precision
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issues.
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</P>
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<P>These accelerated styles are part of the USER-CUDA, GPU, USER-INTEL,
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KOKKOS, USER-OMP and OPT packages, respectively. They are only
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enabled if LAMMPS was built with those packages. See the <A HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P>You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the <A HREF = "Section_start.html#start_7">-suffix command-line
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switch</A> when you invoke LAMMPS, or you can
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use the <A HREF = "suffix.html">suffix</A> command in your input script.
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</P>
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<P>See <A HREF = "Section_accelerate.html">Section_accelerate</A> of the manual for
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more instructions on how to use the accelerated styles effectively.
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</P>
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<HR>
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<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
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</P>
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<P>This pair style does not support the <A HREF = "pair_modify.html">pair_modify</A>
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shift, table, and tail options.
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</P>
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<P>This pair style does not write its information to <A HREF = "restart.html">binary restart
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files</A>, since it is stored in potential files. Thus, you
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need to re-specify the pair_style and pair_coeff commands in an input
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script that reads a restart file.
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</P>
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<P>This pair style can only be used via the <I>pair</I> keyword of the
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<A HREF = "run_style.html">run_style respa</A> command. It does not support the
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<I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
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</P>
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<HR>
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<P><B>Restrictions:</B>
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</P>
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<P>This angle style can only be used if LAMMPS was built with the
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USER-MISC package. See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A>
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section for more info on packages.
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</P>
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<P>This pair style requires the <A HREF = "newton.html">newton</A> setting to be "on"
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for pair interactions.
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</P>
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<P>The EDIP potential files provided with LAMMPS (see the potentials directory)
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are parameterized for metal <A HREF = "units.html">units</A>.
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You can use the SW potential with any LAMMPS units, but you would need
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to create your own EDIP potential file with coefficients listed in the
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appropriate units if your simulation doesn't use "metal" units.
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "pair_coeff.html">pair_coeff</A>
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</P>
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<P><B>Default:</B> none
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</P>
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<HR>
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<A NAME = "EDIP"></A>
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<P><B>(EDIP)</B> J. F. Justo et al., Phys. Rev. B 58, 2539 (1998).
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</P>
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