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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 eam command
</H3>
<H3>pair_style eam/alloy command
</H3>
<H3>pair_style eam/fs command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>pair_style style
</PRE>
<UL><LI>style = <I>eam</I> or <I>eam/alloy</I> or <I>eam/fs</I>
</UL>
<P><B>Examples:</B>
</P>
<PRE>pair_style eam
pair_coeff * * cuu3
pair_coeff 1*3 1*3 niu3.eam
</PRE>
<PRE>pair_style eam/alloy
pair_coeff * * ../potentials/nialhjea.eam.alloy Ni Al Ni Ni
</PRE>
<PRE>pair_style eam/fs
pair_coeff * * nialhjea.eam.fs Ni Al Ni Ni
</PRE>
<P><B>Description:</B>
</P>
<P>Style <I>eam</I> computes pairwise interactions for metals and metal alloys
using embedded-atom method (EAM) potentials <A HREF = "#Daw">(Daw)</A>. The total
energy Ei of an atom I is given by
</P>
<CENTER><IMG SRC = "Eqs/pair_eam.jpg">
</CENTER>
<P>where F is the embedding energy which is a function of the atomic
electron density rho, phi is a pair potential interaction, and alpha
and beta are the element types of atoms I and J. The multi-body
nature of the EAM potential is a result of the embedding energy term.
Both summations in the formula are over all neighbors J of atom I
within the cutoff distance.
</P>
<P>The cutoff distance and the tabulated values of the functionals F,
rho, and phi are listed in one or more files which are specified by
the <A HREF = "pair_coeff.html">pair_coeff</A> command. These are ASCII text files
in a DYNAMO-style format which is described in the documentation for
the <A HREF = "pair_coeff.html">pair_coeff</A> command. DYNAMO is a serial MD code
Several DYNAMO potential files for different metals are included in
the "potentials" directory of the LAMMPS distribution.
</P>
<P>IMPORTANT NOTE: The <I>eam</I> style reads single-element EAM potentials in
the DYNAMO <I>funcfl</I> format. Either single element or alloy systems
can be modeled using multiple <I>funcfl</I> files and style <I>eam</I>. For the
alloy case LAMMPS mixes the single-element potentials to produce alloy
potentials, the same way that DYNAMO does. Alternatively, a single
DYNAMO <I>setfl</I> file of Finnis/Sinclair EAM file can be used by LAMMPS
to model alloy systems by invoking the <I>eam/alloy</I> or <I>eam/fs</I> styles
as described below. These files require no mixing since they specify
alloy interactions explicitly.
</P>
<P>For style <I>eam</I>, potential values are read from a file that is in the
DYNAMO single-element <I>funcfl</I> format. If the DYNAMO file was created
by a Fortran program, it cannot have "D" values in it for exponents.
C only recognizes "e" or "E" for scientific notation.
</P>
<P>Note that unlike for other potentials, cutoffs for EAM potentials are
not set in the pair_style or pair_coeff command; they are specified in
the EAM potential files themselves.
</P>
<P>For style <I>eam</I> a potential file must be assigned to each I,I pair of
atom types by using one or more pair_coeff commands, each with a
single argument:
</P>
<UL><LI>filename
</UL>
<P>Thus the following command
</P>
<PRE>pair_coeff *2 1*2 cuu3.eam
</PRE>
<P>will read the cuu3 potential file and use the tabulated Cu values for
F, phi, rho that it contains for type pairs 1,1 and 2,2 (type pairs
1,2 and 2,1 are ignored). In effect, this makes atom types 1 and 2 in
LAMMPS be Cu atoms. Different single-element files can be assigned to
different atom types to model an alloy system. The mixing to create
alloy potentials for type pairs with I != J is done automatically the
same way that the serial DYANMO code originally did it; you do not
need to specify coefficients for these type pairs.
</P>
<P>There are several <I>funcl</I> files in the <I>potentials</I> directory of the
LAMMPS distribution, with the suffix ".eam". A DYNAMO single-element
<I>funcfl</I> file is formatted as follows:
</P>
<UL><LI>line 1: comment (ignored)
<LI>line 2: atomic number, mass, lattice constant, lattice type (e.g. FCC)
<LI>line 3: Nrho, drho, Nr, dr, cutoff
</UL>
<P>On line 2, all values but the mass are ignored by LAMMPS. The mass is
in atomic mass units (g/cm^3) which is converted by LAMMPS to the
appropriate internal mass <A HREF = "units.html">units</A>. The cubic lattice
constant is in Angstroms. On line 3, Nrho and Nr are the number of
tabulated values in the subsequent arrays, drho and dr are the spacing
in density and distance space for the values in those arrays, and the
specified cutoff becomes the pairwise cutoff used by LAMMPS for the
potential. The units of dr are Angstroms; I'm not sure of the units
for drho - some measure of electron density.
</P>
<P>Following the 3 header lines are 3 arrays of tabulated values:
</P>
<UL><LI>embedding function F(rho) (Nrho values)
<LI>effective charge function Z(r) (Nr values)
<LI>density function rho(r) (Nr values)
</UL>
<P>The values for each array can be listed as multiple values per line,
so long as each array starts on a new line. For example, the
individual Z(r) values are for r = 0,dr,2*dr, ... (Nr-1)*dr.
</P>
<P>The units for the embedding function F are eV. The units for the
density function rho are the same as for drho (see above, electron
density). The units for the effective charge Z are "atomic charge" or
sqrt(Hartree * Bohr-radii). For 2 interacting atoms i,j this is used
by LAMMPS to compute the pair potential term in the EAM energy
expression as r*phi, in units of eV-Angstroms, via the formula
</P>
<PRE>r*phi = 27.2 * 0.529 * Zi * Zj
</PRE>
<P>where 1 Hartree = 27.2 eV and 1 Bohr = 0.529 Angstroms.
</P>
<HR>
<P>Style <I>eam/alloy</I> computes pairwise interactions using the same
formula as style <I>eam</I>. However the associated
<A HREF = "pair_coeff.html">pair_coeff</A> command reads a DYNAMO <I>setfl</I> file
instead of a <I>funcfl</I> file. <I>Setfl</I> files can be used to model a
single-element or alloy system. In the alloy case, as explained
above, <I>setfl</I> files contain explicit tabulated values for alloy
interactions. Thus they allow more generality than <I>funcfl</I> files for
modeling alloys.
</P>
<P>For style <I>eam/alloy</I>, potential values are read from a file that is
in the DYNAMO multi-element <I>setfl</I> format, except that element names
(Ni, Cu, etc) are added to one of the lines in the file. If the
DYNAMO file was created by a Fortran program, it cannot have "D"
values in it for exponents. C only recognizes "e" or "E" for
scientific notation.
</P>
<P>Only one pair_coeff command can be used which specifies a single
<I>setfl</I> file. DYNAMO <I>setfl</I> files contain information for M
elements. These are mapped to LAMMPS atom types by specifying N
additional arguments after the filename, where N is the number of
LAMMPS atom types:
</P>
<UL><LI>filename
<LI>N element names = mapping of <I>setfl</I> elements to atom types
</UL>
<P>As an example, the nialhjea <I>setfl</I> file has tabulated EAM values for
3 elements and their alloy interactions: Ni, Al, and H. If your
LAMMPS simulation had 4 atoms types and you wanted the 1st 3 to be Ni,
and the 4th to be Al, you would use the following pair_coeff command:
</P>
<PRE>pair_coeff * * nialhjea.eam.alloy Ni Ni Ni Al
</PRE>
<P>The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
The first three Ni arguments map LAMMPS atom types 1,2,3 to the Ni
element in the <I>setfl</I> file. The final Al argument maps LAMMPS atom
type 4 to the Al element in the <I>setfl</I> file. If a mapping value is
specified as NULL, the mapping is not performed. This can be used
when an <I>eam/alloy</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><I>Setfl</I> files in the <I>potentials</I> directory of the LAMMPS distribution
have a ".eam.alloy" suffix. A DYNAMO multi-element <I>setfl</I> file is
formatted as follows:
</P>
<UL><LI>lines 1,2,3 = comments (ignored)
<LI>line 4: Nelements Element1 Element2 ... ElementN
<LI>line 5: Nrho, drho, Nr, dr, cutoff
</UL>
<P>In a DYNAMO <I>setfl</I> file, line 4 only lists Nelements = the # of
elements in the <I>setfl</I> file. For LAMMPS, the element name (Ni, Cu,
etc) of each element must be added to the line, in the order the
elements appear in the file.
</P>
<P>The meaning and units of the values in line 5 is the same as for the
<I>funcfl</I> file described above. Note that the cutoff (in Angstroms) is
a global value, valid for all pairwise interactions for all element
pairings.
</P>
<P>Following the 5 header lines are Nelements sections, one for each
element, each with the following format:
</P>
<UL><LI>line 1 = atomic number, mass, lattice constant, lattice type (e.g. FCC)
<LI>embedding function F(rho) (Nrho values)
<LI>density function rho(r) (Nr values)
</UL>
<P>As with the <I>funcfl</I> files, only the mass (g/cm^3) is used by LAMMPS
from the 1st line. The cubic lattice constant is in Angstroms. The F
and rho arrays are unique to a single element and have the same format
and units as in a <I>funcfl</I> file.
</P>
<P>Following the Nelements sections, Nr values for each pair potential
phi(r) array are listed for all i,j element pairs in the same format
as other arrays. Since these interactions are symmetric (i,j = j,i)
only phi arrays with i >= j are listed, in the following order: i,j =
(1,1), (2,1), (2,2), (3,1), (3,2), (3,3), (4,1), ..., (Nelements,
Nelements). Unlike the effective charge array Z(r) in <I>funcfl</I> files,
the tabulated values for each phi function are listed in <I>setfl</I> files
directly as r*phi (in units of eV-Angstroms), since they are for atom
pairs.
</P>
<HR>
<P>Style <I>eam/fs</I> computes pairwise interactions for metals and metal
alloys using a generalized form of EAM potentials due to Finnis and
Sinclair <A HREF = "#Finnis">(Finnis)</A>. The total energy Ei of an atom I is
given by
</P>
<CENTER><IMG SRC = "Eqs/pair_eam_fs.jpg">
</CENTER>
<P>This has the same form as the EAM formula above, except that rho is
now a functional specific to the atomic types of both atoms I and J,
so that different elements can contribute differently to the total
electron density at an atomic site depending on the identity of the
element at that atomic site.
</P>
<P>The associated <A HREF = "pair_coeff.html">pair_coeff</A> command for style <I>eam/fs</I>
reads a DYNAMO <I>setfl</I> file that has been extended to include
additional rho_alpha_beta arrays of tabulated values. The details are
given in the <A HREF = "pair_coeff.html">pair_coeff</A> documentation.
</P>
<P>A discussion of how FS EAM differs from conventional EAM alloy
potentials is given in <A HREF = "#Ackland1">(Ackland1)</A>. An example of such a
potential is the same author's Fe-P FS potential
<A HREF = "#Ackland2">(Ackland2)</A>. Note that while FS potentials always specify
the embedding energy with a square root dependence on the total
density, the implementation in LAMMPS does not require that; the user
can tabulate any functional form desired in the FS potential files.
</P>
<P>For style <I>eam/fs</I>, the form of the pair_coeff command is exactly the
same as for style <I>eam/alloy</I>, e.g.
</P>
<PRE>pair_coeff * * nialhjea.eam.fs Ni Ni Ni Al
</PRE>
<P>where there are N additional arguments after the filename, where N is
the number of LAMMPS atom types. The N values determine the mapping
of LAMMPS atom types to EAM elements in the file, as described above
for style <I>eam/alloy</I>. As with <I>eam/alloy</I>, if a mapping value is
NULL, the mapping is not performed. This can be used when an <I>eam/fs</I>
potential is used as part of the <I>hybrid</I> pair style. The NULL values
are used as placeholders for atom types that will be used with other
potentials.
</P>
<P>FS EAM files include more information than the DYNAMO <I>setfl</I> format
files read by <I>eam/alloy</I>, in that i,j density functionals for all
pairs of elements are included as needed by the Finnis/Sinclair
formulation of the EAM.
</P>
<P>FS EAM files in the <I>potentials</I> directory of the LAMMPS distribution
have an ".eam.fs" suffix. They are formatted as follows:
</P>
<UL><LI>lines 1,2,3 = comments (ignored)
<LI>line 4: Nelements Element1 Element2 ... ElementN
<LI>line 5: Nrho, drho, Nr, dr, cutoff
</UL>
<P>The 5-line header section is identical to an EAM <I>setfl</I> file.
</P>
<P>Following the header are Nelements sections, one for each element I,
each with the following format:
</P>
<UL><LI>line 1 = atomic number, mass, lattice constant, lattice type (e.g. FCC)
<LI>embedding function F(rho) (Nrho values)
<LI>density function rho(r) for element I at element 1 (Nr values)
<LI>density function rho(r) for element I at element 2
<LI>...
<LI>density function rho(r) for element I at element Nelement
</UL>
<P>The units of these quantities in line 1 are the same as for <I>setfl</I>
files. Note that the rho(r) arrays in Finnis/Sinclair can be
asymmetric (i,j != j,i) so there are Nelements^2 of them listed in the
file.
</P>
<P>Following the Nelements sections, Nr values for each pair potential
phi(r) array are listed in the same manner (r*phi, units of
eV-Angstroms) as in EAM <I>setfl</I> files. Note that in Finnis/Sinclair,
the phi(r) arrays are still symmetric, so only phi arrays for i >= j
are listed.
</P>
<HR>
<P><B>Restrictions:</B> none
</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 = "Ackland1"></A>
<P><B>(Ackland1)</B> Ackland, Condensed Matter (2005).
</P>
<A NAME = "Ackland2"></A>
<P><B>(Ackland2)</B> Ackland, Mendelev, Srolovitz, Han and Barashev, Journal
of Physics: Condensed Matter, 16, S2629 (2004).
</P>
<A NAME = "Daw"></A>
<P><B>(Daw)</B> Daw, Baskes, Phys Rev Lett, 50, 1285 (1983).
Daw, Baskes, Phys Rev B, 29, 6443 (1984).
</P>
<A NAME = "Finnis"></A>
<P><B>(Finnis)</B> Finnis, Sinclair, Philosophical Magazine A, 50, 45 (1984).
</P>
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