2007-01-30 07:28:45 +08:00
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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>
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<HR>
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<H3>pair_style meam command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>pair_style meam
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</PRE>
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<P><B>Examples:</B>
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</P>
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<PRE>pair_style meam
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pair_coeff * * ../potentials/library.meam Si ../potentials/si.meam Si
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pair_coeff * * ../potentials/library.meam Ni Al NULL Ni Al Ni Ni
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Style <I>meam</I> computes pairwise interactions for a variety of materials
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using modified embedded-atom method (MEAM) potentials
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<A HREF = "#Baskes">(Baskes)</A>. Conceptually, it is an extension to the original
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<A HREF = "pair_eam.html">EAM potentials</A> which adds angular forces. It is
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2007-01-30 07:28:45 +08:00
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thus suitable for modeling metals and alloys with fcc, bcc, hcp and
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diamond cubic structures, as well as covalently bonded materials like
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silicon and carbon.
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</P>
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<P>In the MEAM formulation, the total energy E of a system of atoms is
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given by:
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</P>
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<CENTER><IMG SRC = "Eqs/pair_meam.jpg">
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</CENTER>
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<P>where F is the embedding energy which is a function of the atomic
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electron density rho, and phi is a pair potential interaction. The
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pair interaction is summed over all neighbors J of atom I within the
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cutoff distance. As with EAM, the multi-body nature of the MEAM
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potential is a result of the embedding energy term. Details of the
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computation of the embedding and pair energies, as implemented in
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LAMMPS, are given in <A HREF = "#Gullet">(Gullet)</A> and references therein.
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</P>
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<P>The various parameters in the MEAM formulas are listed in two files
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which are specified by the <A HREF = "pair_coeff.html">pair_coeff</A> command.
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These are ASCII text files in a format consistent with other MD codes
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that implement MEAM potentials, such as the serial DYNAMO code and
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Warp. Several MEAM potential files with parameters for different
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materials are included in the "potentials" directory of the LAMMPS
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distribution with a ".meam" suffix. All of these are parameterized in
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terms of LAMMPS <A HREF = "units.html">metal units</A>.
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</P>
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<P>Note that unlike for other potentials, cutoffs for MEAM potentials are
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not set in the pair_style or pair_coeff command; they are specified in
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the MEAM potential files themselves.
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</P>
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<P>Only a single pair_coeff command is used with the <I>meam</I> style which
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specifies two MEAM files and the element(s) to extract information
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for. The MEAM elements are mapped to LAMMPS atom types by specifying
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N additional arguments after the 2nd filename in the pair_coeff
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command, where N is the number of LAMMPS atom types:
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</P>
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<UL><LI>MEAM library file
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<LI>Elem1, Elem2, ...
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<LI>MEAM parameter file
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<LI>N element names = mapping of MEAM elements to atom types
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</UL>
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<P>As an example, the potentials/library.meam file has generic MEAM
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settings for a variety of elements. The potentials/sic.meam file has
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specific parameter settings for a Si and C alloy system. If your
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LAMMPS simulation has 4 atoms types and you want the 1st 3 to be Si,
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and the 4th to be C, you would use the following pair_coeff command:
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</P>
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<PRE>pair_coeff * * library.meam Si C sic.meam Si Si Si C
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</PRE>
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<P>The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
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The two filenames are for the library and parameter file respectively.
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The Si and C arguments (between the file names) are the two elements
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for which info will be extracted from the library file. The first
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three trailing Si arguments map LAMMPS atom types 1,2,3 to the MEAM Si
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element. The final C argument maps LAMMPS atom type 4 to the MEAM C
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element.
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</P>
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<P>If the 2nd filename is specified as NULL, no parameter file is read,
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which simply means the generic parameters in the library file are
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used. Use of the NULL specification for the parameter file is
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discouraged for systems with more than a single element type
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(e.g. alloys), since the parameter file is expected to set element
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interaction terms that are not captured by the information in the
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library file.
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</P>
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<P>If a mapping value is specified as NULL, the mapping is not performed.
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This can be used when a <I>meam</I> potential is used as part of the
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<I>hybrid</I> pair style. The NULL values are placeholders for atom types
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that will be used with other potentials.
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</P>
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<P>The MEAM library file provided with LAMMPS has the name
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potentials/library.meam. It is the "meamf" file used by other MD
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codes. Aside from blank and comment lines (start with #) which can
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appear anywhere, it is formatted as a series of entries, each of which
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has 19 parameters and can span multiple lines:
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</P>
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<P>elt, lat, z, ielement, atwt, alpha, b0, b1, b2, b3, alat, esub, asub,
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t0, t1, t2, t3, rozero, ibar
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</P>
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<P>The "elt" and "lat" parameters are text strings, such as elt = Si or
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Cu and lat = dia or fcc. Because the library file is used by Fortran
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MD codes, these strings may be enclosed in single quotes, but this is
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not required. The other numeric parameters match values in the
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formulas above. The value of the "elt" string is what is used in the
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pair_coeff command to identify which settings from the library file
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you wish to read in. There can be multiple entries in the library
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file with the same "elt" value; LAMMPS reads the 1st matching entry it
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finds and ignores the rest.
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</P>
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<P>If used, the MEAM parameter file contains settings that override or
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complement the library file settings. Examples of such parameter
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files are in the potentials directory with a ".meam" suffix. Their
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format is the same as is read by other Fortran MD codes. Aside from
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blank and comment lines (start with #) which can appear anywhere, each
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line has one of the following forms. Each line can also have a
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trailing comment (starting with #) which is ignored.
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</P>
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<PRE>keyword = value
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keyword(I) = value
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keyword(I,J) = value
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keyword(I,J,K) = value
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</PRE>
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<P>The recognized keywords are as follows:
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</P>
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<P>Ec, alpha, rho0, delta, lattce, attrac, repuls, nn2, Cmin, Cmax, rc, delr,
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augt1, gsmooth_factor, re
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</P>
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<P>where
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</P>
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<PRE>rc = cutoff radius for cutoff function; default = 4.0
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delr = length of smoothing distance for cutoff function; default = 0.1
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rho0(I) = relative density for element I (overwrites value
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read from meamf file)
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Ec(I,J) = cohesive energy of reference structure for I-J mixture
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delta(I,J) = heat of formation for I-J alloy; if Ec_IJ is input as
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zero, then LAMMPS sets Ec_IJ = (Ec_II + Ec_JJ)/2 - delta_IJ
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alpha(I,J) = alpha parameter for pair potential between I and J (can
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be computed from bulk modulus of reference structure
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re(I,J) = equilibrium distance between I and J in the reference
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structure
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Cmax(I,J,K) = Cmax screening parameter when I-J pair is screened
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by K (I<=J); default = 2.8
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Cmin(I,J,K) = Cmin screening parameter when I-J pair is screened
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by K (I<=J); default = 2.0
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lattce(I,J) = lattice structure of I-J reference structure:
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dia = diamond (interlaced fcc for alloy)
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fcc = face centered cubic
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bcc = body centered cubic
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dim = dimer
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b1 = rock salt (NaCl structure)
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nn2(I,J) = turn on second-nearest neighbor MEAM formulation for
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I-J pair (see for example <A HREF = "#Lee">(Lee)</A>). Only valid for I=J.
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0 = second-nearest neighbor formulation off
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1 = second-nearest neighbor formulation on
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default = 0
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attrac(I,J) = additional cubic attraction term in Rose energy I-J pair potential
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default = 0
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repuls(I,J) = additional cubic repulsive term in Rose energy I-J pair potential
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default = 0
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gsmooth_factor = factor determining the length of the G-function smoothing
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region; only significant for ibar=0 or ibar=4.
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99.0 = short smoothing region, sharp step
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0.5 = long smoothing region, smooth step
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default = 99.0
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augt1 = integer flag for whether to augment t1 parameter by
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3/5*t3 to account for old vs. new meam formulations;
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0 = don't augment t1
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1 = augment t1
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default = 1
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</PRE>
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<P>Rc, delr, re are in distance units (Angstroms in the case of metal
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units). Ec and delta are in energy units (eV in the case of metal
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units).
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</P>
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<P>Each keyword represents a quantity which is either a scalar, vector,
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2d array, or 3d array and must be specified with the correct
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corresponding array syntax. The indices I,J,K each run from 1 to N
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where N is the number of MEAM elements being used.
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</P>
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<P>Thus these lines
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</P>
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<PRE>rho0(2) = 2.25
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alpha(1,2) = 4.37
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</PRE>
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2007-02-10 05:40:32 +08:00
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<P>set rho0 for the 2nd element to the value 2.25 and set alpha for the
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alloy interaction between elements 1 and 2 to 4.37.
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</P>
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2009-02-25 02:37:34 +08:00
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<P>The augt1 parameter is related to modifications in the MEAM
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formulation of the partial electron density function. In recent
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literature, an extra term is included in the expression for the
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third-order density in order to make the densities orthogonal (see for
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example <A HREF = "#Wang">(Wang)</A>, equation 3d); this term is included in the
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MEAM implementation in lammps. However, in earlier published work
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this term was not included when deriving parameters, including most of
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those provided in the library.meam file included with lammps, and to
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account for this difference the parameter t1 must be augmented by
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3/5*t3. If augt1=1, the default, this augmentation is done
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automatically. When parameter values are fit using the modified
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density function, as in more recent literature, augt1 should be set to
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0.
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</P>
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<P>The parameters attrac and repuls can be used to modify the Rose energy
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function used to compute the pair potential. This function gives the
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energy of the reference state as a function of interatomic spacing.
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The form of this function is:
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</P>
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<PRE>astar = alpha * (r/re - 1.d0)
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erose = -Ec*(1+astar+a3*(astar**3)/(r/re))*exp(-astar)
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a3 = repuls, astar < 0
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a3 = attrac, astar >= 0
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</PRE>
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2009-03-25 04:05:27 +08:00
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<P>Most published MEAM parameter sets use the default values attrac=repulse=0.
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Setting repuls=attrac=delta corresponds to the form used in several
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recent published MEAM parameter sets, such as <A HREF = "#Vallone">(Vallone)</A>
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</P>
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<HR>
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2008-01-22 02:12:34 +08:00
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<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
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</P>
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<P>For atom type pairs I,J and I != J, where types I and J correspond to
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two different element types, mixing is performed by LAMMPS with
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user-specifiable parameters as described above. You never need to
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specify a pair_coeff command with I != J arguments for this style.
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</P>
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<P>This pair style does not support the <A HREF = "pair_modify.html">pair_modify</A>
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shift, table, and tail options.
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</P>
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<P>This pair style does not write its information to <A HREF = "restart.html">binary restart
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files</A>, since it is stored in potential files. Thus, you
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need to re-specify the pair_style and pair_coeff commands in an input
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script that reads a restart file.
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</P>
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2007-10-04 08:21:14 +08:00
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<P>This pair style can only be used via the <I>pair</I> keyword of the
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<A HREF = "run_style.html">run_style respa</A> command. It does not support the
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2007-10-12 07:09:49 +08:00
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<I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
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</P>
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<HR>
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<P><B>Restrictions:</B>
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</P>
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<P>This style is part of the "meam" package. It is only enabled if
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LAMMPS was built with that package, which also requires the MEAM
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library be built and linked with LAMMPS. See the <A HREF = "Section_start.html#2_3">Making
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LAMMPS</A> section for more info.
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2007-01-30 07:28:45 +08:00
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</P>
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<P><B>Related commands:</B>
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</P>
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2007-02-10 05:40:32 +08:00
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<P><A HREF = "pair_coeff.html">pair_coeff</A>, <A HREF = "pair_eam.html">pair_style eam</A>
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</P>
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<P><B>Default:</B> none
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</P>
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<HR>
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<A NAME = "Baskes"></A>
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<P><B>(Baskes)</B> Baskes, Phys Rev B, 46, 2727-2742 (1992).
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</P>
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<A NAME = "Gullet"></A>
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This report may be accessed on-line via <A HREF = "http://infoserve.sandia.gov/sand_doc/2003/038782.pdf">this link</A>.
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2007-01-30 07:28:45 +08:00
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2009-02-25 02:37:34 +08:00
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<A NAME = "Lee"></A>
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<P><B>(Wang)</B> Wang, Van Hove, Ross, Baskes, J. Chem. Phys., 121, 5410 (2004).
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2009-03-25 04:05:27 +08:00
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<A NAME = "Valone"></A>
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<P><B>(Valone)</B> Valone, Baskes, Martin, Phys. Rev. B, 73, 214209 (2006).
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2007-01-30 07:28:45 +08:00
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