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