forked from lijiext/lammps
205 lines
8.3 KiB
Plaintext
205 lines
8.3 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
|
|
|
|
:link(lws,http://lammps.sandia.gov)
|
|
:link(ld,Manual.html)
|
|
:link(lc,Section_commands.html#comm)
|
|
|
|
:line
|
|
|
|
pair_style meam command :h3
|
|
|
|
[Syntax:]
|
|
|
|
pair_style meam :pre
|
|
|
|
[Examples:]
|
|
|
|
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
|
|
|
|
[Description:]
|
|
|
|
Style {meam} computes pairwise interactions for a variety of materials
|
|
using modified embedded-atom method (MEAM) potentials
|
|
"(Baskes)"_#Baskes. Conceptually, it is an extension to the original
|
|
"EAM potentials"_pair_eam.html 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.
|
|
|
|
In the MEAM formulation, the total energy E of a system of atoms is
|
|
given by:
|
|
|
|
:c,image(Eqs/pair_meam.jpg)
|
|
|
|
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 "(Gullet)"_#Gullet and references therein.
|
|
|
|
The various parameters in the MEAM formulas are listed in two files
|
|
which are specified by the "pair_coeff"_pair_coeff.html 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 "metal units"_units.html.
|
|
|
|
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.
|
|
|
|
Only a single pair_coeff command is used with the {meam} 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:
|
|
|
|
MEAM library file
|
|
Elem1, Elem2, ...
|
|
MEAM parameter file
|
|
N element names = mapping of MEAM elements to atom types :ul
|
|
|
|
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:
|
|
|
|
pair_coeff * * library.meam Si C sic.meam Si Si Si C :pre
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
If a mapping value is specified as NULL, the mapping is not performed.
|
|
This can be used when a {meam} potential is used as part of the
|
|
{hybrid} pair style. The NULL values are placeholders for atom types
|
|
that will be used with other potentials.
|
|
|
|
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:
|
|
|
|
elt, lat, z, ielement, atwt, alpha, b0, b1, b2, b3, alat, esub, asub,
|
|
t0, t1, t2, t3, rozero, ibar
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
keyword = value
|
|
keyword(I) = value
|
|
keyword(I,J) = value
|
|
keyword(I,J,K) = value :pre
|
|
|
|
The recognized keywords are as follows:
|
|
|
|
Ec, alpha, rho0, delta, lattce, attrac, repuls, nn2, Cmin, Cmax, rc, delr,
|
|
augt1, gsmooth_factor, re
|
|
|
|
where
|
|
|
|
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 LAMMPS 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
|
|
|
|
Rc, delr, re are in distance units (Angstroms in the case of metal
|
|
units). Ec and delta are in energy units (eV in the case of metal
|
|
units).
|
|
|
|
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.
|
|
|
|
Thus these lines
|
|
|
|
rho0(2) = 2.25
|
|
alpha(1,2) = 4.37 :pre
|
|
|
|
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.
|
|
|
|
:line
|
|
|
|
[Restrictions:] none
|
|
|
|
[Related commands:]
|
|
|
|
"pair_coeff"_pair_coeff.html, "pair_style eam"_pair_eam.html
|
|
|
|
[Default:] none
|
|
|
|
:line
|
|
|
|
:link(Baskes)
|
|
[(Baskes)] Baskes, Phys Rev B, 46, 2727-2742 (1992).
|
|
|
|
:link(Gullet)
|
|
[(Gullet)] Gullet, Wagner, Slepoy, SANDIA Report 2003-8782 (2003).
|
|
This report may be accessed on-line via "this link"_sandreport.
|
|
|
|
:link(sandreport,http://www.prod.sandia.gov/cgi-bin/techlib/access-control.pl/2003/038782.pdf)
|