lammps/doc/pair_style_eam.txt

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pair_style eam command :h3
pair_style eam/alloy command :h3
pair_style eam/fs command :h3

[Syntax:]

pair_style style :pre

style = {eam} or {eam/alloy} or {eam/fs} :ul

[Examples:]

pair_style eam
pair_coeff * * cuu3
pair_coeff 1*3 1*3 niu3 :pre

pair_style eam/alloy
pair_coeff * * nialhjea 1 2 1 1 :pre

pair_style eam/fs
pair_coeff * * nial.fs 1 2 1 1 :pre

[Description:]

Style {eam} computes pairwise interactions for metals and metal alloys
using embedded-atom method (EAM) potentials "(Daw)"_#Daw.  The total
energy Ei of an atom I is given by

:c,image(Eqs/pair_eam.jpg)

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.

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 "pair_coeff"_pair_coeff.html command.  These are ASCII text files
in a DYNAMO-style format which is described in the documentation for
the "pair_coeff"_pair_coeff.html command.  DYNAMO is a serial MD code
Several DYNAMO potential files for different metals are included in
the "potentials" directory of the LAMMPS distribution.

IMPORTANT NOTE: The {eam} style reads single-element EAM potentials in
the DYNAMO {funcfl} format.  Either single element or alloy systems
can be modeled with {funcfl} files and style {eam}.  For the alloy
case LAMMPS mixes the single-element potentials to produce alloy
potentials the same way that DYNAMO does.  Alternatively, DYNAMO
{setfl} files can be used by LAMMPS to model alloy systems by invoking
the {eam/alloy} style as described below.  {Setfl} files require no
mixing since they specify alloy interactions explicitly.

For style {eam}, potential values are read from a file that is in the
DYNAMO single-element {funcfl} 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.

Note that unlike for other potentials, you do not set cutoffs for EAM
potentials in the pair_style or pair_coeff command; they are specified
in the EAM potential files.

For style {eam} you must assign a potential file to each I,I pair of
atom types by using a single pair_coeff argument:

filename :ul

Thus the following command

pair_coeff *2 1*2 cuu3 :pre

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.

There are several {funcl} files in the {potentials} directory of the
LAMMPS distribution, with the suffix ".eam".  A DYNAMO single-element
{funcfl} file is formatted as follows:

line 1: comment (ignored)
line 2: atomic number, mass, lattice constant, lattice type (e.g. FCC)
line 3: Nrho, drho, Nr, dr, cutoff :ul

On line 2, all values but the mass are ignored by LAMMPS.  The mass is
in atomic mass units which is converted by LAMMPS to the appropriate
internal mass "units"_units.html.  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.

Following the 3 header lines are 3 arrays of tabulated values:

embedding function F (Nrho values)
pair potential function phi (Nr values)
density function rho (Nr values) :ul

The values for each array can be listed as multiple values per line,
so long as each array starts on a new line.  The individual values are
(for example) phi(r) for r = 0,dr,2*dr, ... (Nr-1)*dr.

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Style {eam/alloy} computes pairwise interactions using the same
formula as style {eam}.  However the associated
"pair_coeff"_pair_coeff.html command reads a DYNAMO {setfl} file
instead of a {funcfl} file.  {Setfl} files can be used to model a
single-element or alloy system.  In the alloy case, as explained
above, {setfl} files contain explicit tabulated values for alloy
interactions.  Thus they allow more generality than {funcfl} files for
modeling alloys.

For style {eam/alloy}, potential values are read from a file that is
in the DYNAMO multi-element {setfl} 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.

Only one pair_coeff command can be used (one file).  DYNAMO {setfl}
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:

filename
N values from 0 to M = mapping of {setfl} elements to atom types :ul

As an example, the nialhjea {setfl} file has tabulated EAM values for
3 elements and their alloy interactions: Ni, Al, and H.  If your
LAMMPS simulation has 4 atoms types and you want the 1st 3 to be Ni,
and the 4th to be Al, you would use the following pair_coeff command:

pair_coeff * * nialhjea 1 1 1 2 :pre

The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
The first three "1" values map LAMMPS atom types 1,2,3 to the 1st
element (Ni) in the {setfl} file.  The final "2" value maps LAMMPS
atom type 4 to the 2nd element = Al.  If a mapping value is "0", the
mapping is not performed.  This can be used when an {eam/alloy}
potential is used as part of the {hybrid} pair style.  The 0 values
are used as placeholders for atom types that will be used with other
potentials.

{Setfl} files in the {potentials} directory of the LAMMPS distribution
have a ".eam.alloy" suffix.  A DYNAMO multi-element {setfl} file is
formatted as follows:

lines 1,2,3 = comments (ignored)
line 4: Nelements = # of elements in the file
line 5: Nrho, drho, Nr, dr, cutoff :ul

The meaning of the values in line 5 is the same as for the {funcfl}
file described above.  Note that the cutoff is a global value, valid
for all pairwise interactions for all element pairings.

Following the 5 header lines are Nelements sections, one for each
element, each with the following format:

line 1 = atomic number, mass, lattice constant, lattice type (e.g. FCC)
embedding function F (Nrho values)
density function rho (Nr values) :ul

As with the {funcfl} files, only the mass is used by LAMMPS from the 1st
line.  The F and rho arrays are unique to a single element and are
formatted the same as in a {funcfl} file.

Following the Nelements sections, values for the pair potential phi
arrays 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).  The tabulated values for each phi function are listed in
{setfl} files as r*phi, rather than as phi (in {funcfl} files).

:line

Style {eam/fs} computes pairwise interactions for metals and metal
alloys using a generalized form of EAM potentials due to Finnis and
Sinclair "(Finnis)"_#Finnis.  The total energy Ei of an atom I is
given by

:c,image(Eqs/pair_eam_fs.jpg)

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.

The associated "pair_coeff"_pair_coeff.html command for style {eam/fs}
reads a DYNAMO {setfl} file that has been extended to include
additional rho_alpha_beta arrays of tabulated values.  The details are
given in the "pair_coeff"_pair_coeff.html documentation.

A discussion of how FS EAM differs from conventional EAM alloy
potentials is given in "(Ackland1)"_#Ackland1.  An example of such a
potential is the same author's Fe-P FS potential
"(Ackland2)"_#Ackland2.  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 he desires in the FS potential files.

For style {eam/fs}, the form of the pair_coeff command is exactly the
same as for style {eam/alloy}, e.g.

pair_coeff * * filename 1 1 1 2 :pre

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 {eam/alloy}.  As with {eam/alloy}, if a mapping value is
"0", the mapping is not performed.  This can be used when an {eam/fs}
potential is used as part of the {hybrid} pair style.  The 0 values
are used as placeholders for atom types that will be used with other
potentials.

FS EAM files include more information than the DYNAMO {setfl} format
files read by {eam/alloy}, so that the i,j density functionals for all
pairs of elements are included as needed by the Finnis/Sinclair
formulation of the EAM.

FS EAM files in the {potentials} directory of the LAMMPS distribution
have a ".eam.fs" suffix.  Ther are formatted as follows:

lines 1,2,3 = comments (ignored)
line 4: Nelements = # of elements in the file
line 5: Nrho, drho, Nr, dr, cutoff :ul

The 5-line header section is identical to an EAM {setfl} file.

Following the header are Nelements sections, one for each element I,
each with the following format:

line 1 = atomic number, mass, lattice constant, lattice type (e.g. FCC)
embedding function F (Nrho values)
density function rho for element I at element 1 (Nr values)
density function rho for element I at element 2
...
density function rho for element I at element Nelement :ul

Following the Nelements sections, values for the pair potential phi
arrays are listed in the same manner (r*phi) as in EAM {setfl} files.
Note that the rho arrays in Finnis/Sinclair can be asymmetric (i,j !=
j,i) so there are Nelements^2 of them listed in the file.  But the phi
arrays are still symmetric, so only phi arrays for i >= j are listed.

:line

[Restrictions:] none

[Related commands:]

"pair_coeff"_pair_coeff.html

[Default:] none

:line

:link(Ackland1)
[(Ackland1)] Ackland, Condensed Matter (2005).

:link(Ackland2)
[(Ackland2)] Ackland, Mendelev, Srolovitz, Han and Barashev, Journal 
of Physics: Condensed Matter, 16, S2629 (2004).

:link(Daw)
[(Daw)] Daw, Baskes, Phys Rev Lett, 50, 1285 (1983).  
Daw, Baskes, Phys Rev B, 29, 6443 (1984).

:link(Finnis)
[(Finnis)] Finnis, Sinclair, Philosophical Magazine A, 50, 45 (1984).