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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>
</CENTER>
<HR>
<H3>pair_style meam command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>pair_style mean
</PRE>
<P><B>Examples:</B>
</P>
<PRE>pair_style meam
pair_coeff * * ../potentials/library.meam Si ../potentials/si.meam Si
pair_coeff * * ../potentials/library.meam Ni Al NULL Ni Al Ni Ni
</PRE>
<P><B>Description:</B>
</P>
<P>Style <I>meam</I> computes pairwise interactions for a variety of materials
using modified embedded-atom method (MEAM) potentials
<A HREF = "#Baskes">(Baskes)</A>. Conceptually, it is an extension to the original
<A HREF = "pair_style_eam.html">EAM potentials</A> which adds angular forces. It is
thus suitable for modeling metals and alloys with fcc, bcc, hcp and
diamond cubic structures, as well as covalently bonded materials like
silicon and carbon.
</P>
<P>In the MEAM formulation, the total energy E of a system of atoms is
given by:
</P>
<CENTER><IMG SRC = "Eqs/pair_meam.jpg">
</CENTER>
<P>where F is the embedding energy which is a function of the atomic
electron density rho, and phi is a pair potential interaction. The
pair interaction is summed over all neighbors J of atom I within the
cutoff distance. As with EAM, the multi-body nature of the MEAM
potential is a result of the embedding energy term. Details of the
computation of the embedding and pair energies, as implemented in
LAMMPS, are given in <A HREF = "#Gullet">(Gullet)</A> and references therein.
</P>
<P>The various parameters in the MEAM formulas are listed in two files
which are specified by the <A HREF = "pair_coeff.html">pair_coeff</A> command.
These are ASCII text files in a format consistent with other MD codes
that implement MEAM potentials, such as the serial DYNAMO code and
Warp. Several MEAM potential files with parameters for different
materials are included in the "potentials" directory of the LAMMPS
distribution with a ",meam" suffix. All of these are parameterized in
terms of LAMMPS <A HREF = "units.html">metal units</A>.
</P>
<P>Note that unlike for other potentials, cutoffs for MEAM potentials are
not set in the pair_style or pair_coeff command; they are specified in
the MEAM potential files themselves.
</P>
<P>Only a single pair_coeff command is used with the <I>meam</I> style which
specifies two MEAM files and the element(s) to extract information
for. The MEAM elements are mapped to LAMMPS atom types by specifying
N additional arguments after the 2nd filename in the pair_coeff
command, where N is the number of LAMMPS atom types:
</P>
<UL><LI>MEAM library file
<LI>Elem1, Elem2, ...
<LI>MEAM parameter file
<LI>N element names = mapping of MEAM elements to atom types
</UL>
<P>As an example, the potentials/library.meam file has generic MEAM
settings for a variety of elements. The potentials/sic.meam file has
specific parameter settings for a Si and C alloy system. If your
LAMMPS simulation has 4 atoms types and you want the 1st 3 to be Si,
and the 4th to be C, you would use the following pair_coeff command:
</P>
<PRE>pair_coeff * * library.meam Si C sic.meam Si Si Si C
</PRE>
<P>The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
The two filenames are for the library and parameter file respectively.
The Si and C arguments (between the file names) are the two elements
for which info will be extracted from the library file. The first
three trailing Si arguments map LAMMPS atom types 1,2,3 to the MEAM Si
element. The final C argument maps LAMMPS atom type 4 to the MEAM C
element.
</P>
<P>If the 2nd filename is specified as NULL, no parameter file is read,
which simply means the generic parameters in the library file are
used. Use of the NULL specification for the parameter file is
discouraged for systems with more than a single element type
(e.g. alloys), since the parameter file is expected to set element
interaction terms that are not captured by the information in the
library file.
</P>
<P>If a mapping value is specified as NULL, the mapping is not performed.
This can be used when a <I>meam</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 MEAM library file provided with LAMMPS has the name
potentials/library.meam. It is the "meamf" file used by other MD
codes. Aside from blank and comment lines (start with #) which can
appear anywhere, it is formatted as a series of entries, each of which
has 19 parameters and can span multiple lines:
</P>
<P>elt, lat, z, ielement, atwt, alpha, b0, b1, b2, b3, alat, esub, asub,
t0, t1, t2, t3, rozero, ibar
</P>
<P>The "elt" and "lat" parameters are text strings, such as elt = Si or
Cu and lat = dia or fcc. Because the library file is used by Fortran
MD codes, these strings may be enclosed in single quotes, but this is
not required. The other numeric parameters match values in the
formulas above. The value of the "elt" string is what is used in the
pair_coeff command to identify which settings from the library file
you wish to read in. There can be multiple entries in the library
file with the same "elt" value; LAMMPS reads the 1st matching entry it
finds and ignores the rest.
</P>
<P>If used, the MEAM parameter file contains settings that override or
complement the library file settings. Examples of such parameter
files are in the potentials directory with a ".meam" suffix. Their
format is the same as is read by other Fortran MD codes. Aside from
blank and comment lines (start with #) which can appear anywhere, each
line has one of the following forms. Each line can also have a
trailing comment (starting with #) which is ignored.
</P>
<PRE>keyword = value
keyword(I) = value
keyword(I,J) = value
keyword(I,J,K) = value
</PRE>
<P>The recognized keywords are as follows:
</P>
<P>Ec, alpha, rho0, delta, lattce, attrac, repuls, nn2, Cmin, Cmax, rc, delr,
augt1, gsmooth_factor, re
</P>
<P>where
</P>
<PRE>rc = cutoff radius for cutoff function; default = 4.0
delr = length of smoothing distance for cutoff function; default = 0.1
rho0(I) = relative density for element I (overwrites value
read from meamf file)
Ec(I,J) = cohesive energy of reference structure for I-J mixture
delta(I,J) = heat of formation for I-J alloy; if Ec_IJ is input as
zero, then WARP sets Ec_IJ = (Ec_II + Ec_JJ)/2 - delta_IJ
alpha(I,J) = alpha parameter for pair potential between I and J (can
be computed from bulk modulus of reference structure
re(I,J) = equilibrium distance between I and J in the reference
structure
Cmax(I,J,K) = Cmax screening parameter when I-J pair is screened
by K (I<=J); default = 2.8
Cmin(I,J,K) = Cmin screening parameter when I-J pair is screened
by K (I<=J); default = 2.0
lattce(I,J) = lattice structure of I-J reference structure:
dia = diamond (interlaced fcc for alloy)
fcc = face centered cubic
bcc = body centered cubic
dim = dimer
B1 = rock salt (NaCl structure)
gsmooth_factor = factor determining the length of the G-function smoothing
region; only significant for ibar=0 or ibar=4.
99.0 = short smoothing region, sharp step
0.5 = long smoothing region, smooth step
default = 99.0
augt1 = integer flag for whether to augment t1 parameter by
3/5*t3 to account for old vs. new meam formulations;
0 = don't augment t1
1 = augment t1
default = 1
</PRE>
<P>Each keyword represents a quantity which is either a scalar, vector,
2d array, or 3d array and must be specified with the correct
corresponding array syntax. The indices I,J,K each run from 1 to N
where N is the number of MEAM elements being used.
</P>
<P>Thus these lines
</P>
<PRE>rho0(2) = 2.25
alpha(1,2) = 4.37
</PRE>
<P>mean set rho0 for the 2nd element to the value 2.25 and set alpha for
the alloy interaction between elements 1 and 2 to 4.37.
</P>
<HR>
<P><B>Restrictions:</B> none
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "pair_coeff.html">pair_coeff</A>, <A HREF = "pair_style_eam.html">pair_style eam</A>
</P>
<P><B>Default:</B> none
</P>
<HR>
<A NAME = "Baskes"></A>
<P><B>(Baskes)</B> Baskes, Phys Rev B, 46, 2727-2742 (1992).
</P>
<A NAME = "Gullet"></A>
<P><B>(Gullet)</B> Gullet, Wagner, Slepoy, SANDIA Report 2003-8782 (2003).
This report may be accessed on-line via <A HREF = "http://www.prod.sandia.gov/cgi-bin/techlib/access-control.pl/2003/038782.pdf">this link</A>.
</P>
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