forked from lijiext/lammps
154 lines
6.0 KiB
HTML
154 lines
6.0 KiB
HTML
<HTML>
|
|
<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>
|
|
</CENTER>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<HR>
|
|
|
|
<H3>pair_style eim command
|
|
</H3>
|
|
<P><B>Syntax:</B>
|
|
</P>
|
|
<PRE>pair_style style
|
|
</PRE>
|
|
<UL><LI>style = <I>eim</I>
|
|
</UL>
|
|
<P><B>Examples:</B>
|
|
</P>
|
|
<PRE>pair_style eim
|
|
pair_coeff * * Na Cl ../potentials/ffield.eim Na Cl
|
|
pair_coeff * * Na Cl ffield.eim Na Na Na Cl
|
|
pair_coeff * * Na Cl ../potentials/ffield.eim Cl NULL Na
|
|
</PRE>
|
|
<P><B>Description:</B>
|
|
</P>
|
|
<P>Style <I>eim</I> computes pairwise interactions for ionic compounds
|
|
using embedded-ion method (EIM) potentials <A HREF = "#Zhou">(Zhou)</A>. The
|
|
energy of the system E is given by
|
|
</P>
|
|
<CENTER><IMG SRC = "Eqs/pair_eim1.jpg">
|
|
</CENTER>
|
|
<P>The first term is a double pairwise sum over the J neighbors of all I
|
|
atoms, where phi_ij is a pair potential. The second term sums over
|
|
the embedding energy E_i of atom I, which is a function of its charge
|
|
q_i and the electrical potential sigma_i at its location. E_i, q_i,
|
|
and sigma_i are calculated as
|
|
</P>
|
|
<CENTER><IMG SRC = "Eqs/pair_eim2.jpg">
|
|
</CENTER>
|
|
<P>where eta_ji is a pairwise function describing electron flow from atom
|
|
I to atom J, and psi_ij is another pairwise function. The multi-body
|
|
nature of the EIM potential is a result of the embedding energy term.
|
|
A complete list of all the pair functions used in EIM is summarized
|
|
below
|
|
</P>
|
|
<CENTER><IMG SRC = "Eqs/pair_eim3.jpg">
|
|
</CENTER>
|
|
<P>Here E_b, r_e, r_(c,phi), alpha, beta, A_(psi), zeta, r_(s,psi),
|
|
r_(c,psi), A_(eta), r_(s,eta), r_(c,eta), chi, and pair function type
|
|
p are parameters, with subscripts ij indicating the two species of
|
|
atoms in the atomic pair.
|
|
</P>
|
|
<P>IMPORTANT NOTE: Even though the EIM potential is treating atoms as
|
|
charged ions, you should not use a LAMMPS <A HREF = "atom_style.html">atom_style</A>
|
|
that stores a charge on each atom and thus requires you to assign a
|
|
charge to each atom, e.g. the <I>charge</I> or <I>full</I> atom styles. This is
|
|
because the EIM potential infers the charge on an atom from the
|
|
equation above for q_i; you do not assign charges explicitly.
|
|
</P>
|
|
<HR>
|
|
|
|
<P>All the EIM parameters are listed in a potential file which is
|
|
specified by the <A HREF = "pair_coeff.html">pair_coeff</A> command. This is an
|
|
ASCII text file in a format described below. The "ffield.eim" file
|
|
included in the "potentials" directory of the LAMMPS distribution
|
|
currently includes nine elements Li, Na, K, Rb, Cs, F, Cl, Br, and I.
|
|
A system with any combination of these elements can be modeled. This
|
|
file is parameterized in terms of LAMMPS <A HREF = "units.html">metal units</A>.
|
|
</P>
|
|
<P>Note that unlike other potentials, cutoffs for EIM potentials are not
|
|
set in the pair_style or pair_coeff command; they are specified in the
|
|
EIM potential file itself. Likewise, the EIM potential file lists
|
|
atomic masses; thus you do not need to use the <A HREF = "mass.html">mass</A>
|
|
command to specify them.
|
|
</P>
|
|
<P>Only a single pair_coeff command is used with the <I>eim</I> style which
|
|
specifies an EIM potential file and the element(s) to extract
|
|
information for. The EIM elements are mapped to LAMMPS atom types by
|
|
specifying N additional arguments after the filename in the pair_coeff
|
|
command, where N is the number of LAMMPS atom types:
|
|
</P>
|
|
<UL><LI>Elem1, Elem2, ...
|
|
<LI>EIM potential file
|
|
<LI>N element names = mapping of EIM elements to atom types
|
|
</UL>
|
|
<P>As an example like one of those above, suppose you want to model a
|
|
system with Na and Cl atoms. If your LAMMPS simulation has 4 atoms
|
|
types and you want the 1st 3 to be Na, and the 4th to be Cl, you would
|
|
use the following pair_coeff command:
|
|
</P>
|
|
<PRE>pair_coeff * * Na Cl ffield.eim Na Na Na Cl
|
|
</PRE>
|
|
<P>The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
|
|
The filename is the EIM potential file. The Na and Cl arguments
|
|
(before the file name) are the two elements for which info will be
|
|
extracted from the potentail file. The first three trailing Na
|
|
arguments map LAMMPS atom types 1,2,3 to the EIM Na element. The
|
|
final Cl argument maps LAMMPS atom type 4 to the EIM Cl element.
|
|
</P>
|
|
<P>If a mapping value is specified as NULL, the mapping is not performed.
|
|
This can be used when an <I>eim</I> potential is used as part of the
|
|
<I>hybrid</I> pair style. The NULL values are placeholders for atom types
|
|
that will be used with other potentials.
|
|
</P>
|
|
<P>The ffield.eim file in the <I>potentials</I> directory of the LAMMPS
|
|
distribution is formated as follows:
|
|
</P>
|
|
<P>Lines starting with # are comments and are ignored by LAMMPS. Lines
|
|
starting with "global:" include three global values. The first value
|
|
divides the cations from anions, i.e., any elements with
|
|
electronegativity above this value are viewed as anions, and any
|
|
elements with electronegativity below this value are viewed as
|
|
cations. The second and third values are related to the cutoff
|
|
function - i.e. the 0.510204, 1.64498, and 0.010204 shown in the above
|
|
equation can be derived from these values.
|
|
</P>
|
|
<P>Lines starting with "element:" are formatted as follows: name of
|
|
element, atomic number, atomic mass, electronic negativity, atomic
|
|
radius (LAMMPS ignores it), ionic radius (LAMMPS ignores it), cohesive
|
|
energy (LAMMPS ignores it), and q0 (must be 0).
|
|
</P>
|
|
<P>Lines starting with "pair:" are entered as: element 1, element 2,
|
|
r_(c,phi), r_(c,phi) (redundant for historical reasons), E_b, r_e,
|
|
alpha, beta, r_(c,eta), A_(eta), r_(s,eta), r_(c,psi), A_(psi), zeta,
|
|
r_(s,psi), and p.
|
|
</P>
|
|
<P>The lines in the file can be in any order; LAMMPS extracts the info it
|
|
needs.
|
|
</P>
|
|
<HR>
|
|
|
|
<P><B>Restrictions:</B>
|
|
</P>
|
|
<P>This style is part of the "manybody" package. It is only enabled if
|
|
LAMMPS was built with that package (which it is by default).
|
|
</P>
|
|
<P><B>Related commands:</B>
|
|
</P>
|
|
<P><A HREF = "pair_coeff.html">pair_coeff</A>
|
|
</P>
|
|
<P><B>Default:</B> none
|
|
</P>
|
|
<HR>
|
|
|
|
<A NAME = "Zhou"></A>
|
|
|
|
<P><B>(Zhou)</B> Zhou, submitted for publication (2010). Please contact
|
|
Xiaowang Zhou (Sandia) for details via email at xzhou at sandia.gov.
|
|
</P>
|
|
</HTML>
|