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<title>pair_style eim command — LAMMPS 15 May 2015 version documentation</title>
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<li class="toctree-l1"><a class="reference internal" href="Section_intro.html">1. Introduction</a></li>
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<div class="section" id="pair-style-eim-command">
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<span id="index-0"></span><h1>pair_style eim command<a class="headerlink" href="#pair-style-eim-command" title="Permalink to this headline">¶</a></h1>
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</div>
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<div class="section" id="pair-style-eim-omp-command">
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<h1>pair_style eim/omp command<a class="headerlink" href="#pair-style-eim-omp-command" title="Permalink to this headline">¶</a></h1>
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<div class="section" id="syntax">
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<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>pair_style style
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</pre></div>
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</div>
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<ul class="simple">
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<li>style = <em>eim</em></li>
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</ul>
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</div>
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<div class="section" id="examples">
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<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>pair_style eim
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pair_coeff * * Na Cl ../potentials/ffield.eim Na Cl
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pair_coeff * * Na Cl ffield.eim Na Na Na Cl
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pair_coeff * * Na Cl ../potentials/ffield.eim Cl NULL Na
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</pre></div>
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</div>
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</div>
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<div class="section" id="description">
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<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
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<p>Style <em>eim</em> computes pairwise interactions for ionic compounds
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using embedded-ion method (EIM) potentials <a class="reference internal" href="pair_polymorphic.html#zhou"><span>(Zhou)</span></a>. The
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energy of the system E is given by</p>
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<img alt="_images/pair_eim1.jpg" class="align-center" src="_images/pair_eim1.jpg" />
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<p>The first term is a double pairwise sum over the J neighbors of all I
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atoms, where phi_ij is a pair potential. The second term sums over
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the embedding energy E_i of atom I, which is a function of its charge
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q_i and the electrical potential sigma_i at its location. E_i, q_i,
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and sigma_i are calculated as</p>
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<img alt="_images/pair_eim2.jpg" class="align-center" src="_images/pair_eim2.jpg" />
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<p>where eta_ji is a pairwise function describing electron flow from atom
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I to atom J, and psi_ij is another pairwise function. The multi-body
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nature of the EIM potential is a result of the embedding energy term.
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A complete list of all the pair functions used in EIM is summarized
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below</p>
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<img alt="_images/pair_eim3.jpg" class="align-center" src="_images/pair_eim3.jpg" />
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<p>Here E_b, r_e, r_(c,phi), alpha, beta, A_(psi), zeta, r_(s,psi),
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r_(c,psi), A_(eta), r_(s,eta), r_(c,eta), chi, and pair function type
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p are parameters, with subscripts ij indicating the two species of
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atoms in the atomic pair.</p>
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<div class="admonition warning">
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<p class="first admonition-title">Warning</p>
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<p class="last">Even though the EIM potential is treating atoms as
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charged ions, you should not use a LAMMPS <a class="reference internal" href="atom_style.html"><em>atom_style</em></a>
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that stores a charge on each atom and thus requires you to assign a
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charge to each atom, e.g. the <em>charge</em> or <em>full</em> atom styles. This is
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because the EIM potential infers the charge on an atom from the
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equation above for q_i; you do not assign charges explicitly.</p>
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</div>
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<hr class="docutils" />
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<p>All the EIM parameters are listed in a potential file which is
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specified by the <a class="reference internal" href="pair_coeff.html"><em>pair_coeff</em></a> command. This is an
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ASCII text file in a format described below. The “ffield.eim” file
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included in the “potentials” directory of the LAMMPS distribution
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currently includes nine elements Li, Na, K, Rb, Cs, F, Cl, Br, and I.
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A system with any combination of these elements can be modeled. This
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file is parameterized in terms of LAMMPS <a class="reference internal" href="units.html"><em>metal units</em></a>.</p>
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<p>Note that unlike other potentials, cutoffs for EIM potentials are not
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set in the pair_style or pair_coeff command; they are specified in the
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EIM potential file itself. Likewise, the EIM potential file lists
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atomic masses; thus you do not need to use the <a class="reference internal" href="mass.html"><em>mass</em></a>
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command to specify them.</p>
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<p>Only a single pair_coeff command is used with the <em>eim</em> style which
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specifies an EIM potential file and the element(s) to extract
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information for. The EIM elements are mapped to LAMMPS atom types by
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specifying N additional arguments after the filename in the pair_coeff
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command, where N is the number of LAMMPS atom types:</p>
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<ul class="simple">
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<li>Elem1, Elem2, ...</li>
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<li>EIM potential file</li>
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<li>N element names = mapping of EIM elements to atom types</li>
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</ul>
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<p>See the <a class="reference internal" href="pair_coeff.html"><em>pair_coeff</em></a> doc page for alternate ways
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to specify the path for the potential file.</p>
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<p>As an example like one of those above, suppose you want to model a
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system with Na and Cl atoms. If your LAMMPS simulation has 4 atoms
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types and you want the 1st 3 to be Na, and the 4th to be Cl, you would
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use the following pair_coeff command:</p>
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<div class="highlight-python"><div class="highlight"><pre>pair_coeff * * Na Cl ffield.eim Na Na Na Cl
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</pre></div>
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</div>
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<p>The 1st 2 arguments must be * * so as to span all LAMMPS atom types.
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The filename is the EIM potential file. The Na and Cl arguments
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(before the file name) are the two elements for which info will be
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extracted from the potentail file. The first three trailing Na
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arguments map LAMMPS atom types 1,2,3 to the EIM Na element. The
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final Cl argument maps LAMMPS atom type 4 to the EIM Cl element.</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 an <em>eim</em> potential is used as part of the
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<em>hybrid</em> pair style. The NULL values are placeholders for atom types
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that will be used with other potentials.</p>
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<p>The ffield.eim file in the <em>potentials</em> directory of the LAMMPS
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distribution is formated as follows:</p>
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<p>Lines starting with # are comments and are ignored by LAMMPS. Lines
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starting with “global:” include three global values. The first value
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divides the cations from anions, i.e., any elements with
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electronegativity above this value are viewed as anions, and any
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elements with electronegativity below this value are viewed as
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cations. The second and third values are related to the cutoff
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function - i.e. the 0.510204, 1.64498, and 0.010204 shown in the above
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equation can be derived from these values.</p>
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<p>Lines starting with “element:” are formatted as follows: name of
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element, atomic number, atomic mass, electronic negativity, atomic
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radius (LAMMPS ignores it), ionic radius (LAMMPS ignores it), cohesive
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energy (LAMMPS ignores it), and q0 (must be 0).</p>
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<p>Lines starting with “pair:” are entered as: element 1, element 2,
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r_(c,phi), r_(c,phi) (redundant for historical reasons), E_b, r_e,
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alpha, beta, r_(c,eta), A_(eta), r_(s,eta), r_(c,psi), A_(psi), zeta,
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r_(s,psi), and p.</p>
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<p>The lines in the file can be in any order; LAMMPS extracts the info it
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needs.</p>
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<hr class="docutils" />
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<p>Styles with a <em>cuda</em>, <em>gpu</em>, <em>intel</em>, <em>kk</em>, <em>omp</em>, or <em>opt</em> suffix are
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functionally the same as the corresponding style without the suffix.
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They have been optimized to run faster, depending on your available
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hardware, as discussed in <a class="reference internal" href="Section_accelerate.html"><em>Section_accelerate</em></a>
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of the manual. The accelerated styles take the same arguments and
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should produce the same results, except for round-off and precision
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issues.</p>
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<p>These accelerated styles are part of the USER-CUDA, GPU, USER-INTEL,
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KOKKOS, USER-OMP and OPT packages, respectively. They are only
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enabled if LAMMPS was built with those packages. See the <a class="reference internal" href="Section_start.html#start-3"><span>Making LAMMPS</span></a> section for more info.</p>
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<p>You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the <a class="reference internal" href="Section_start.html#start-7"><span>-suffix command-line switch</span></a> when you invoke LAMMPS, or you can
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use the <a class="reference internal" href="suffix.html"><em>suffix</em></a> command in your input script.</p>
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<p>See <a class="reference internal" href="Section_accelerate.html"><em>Section_accelerate</em></a> of the manual for
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more instructions on how to use the accelerated styles effectively.</p>
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</div>
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<hr class="docutils" />
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<div class="section" id="restrictions">
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<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
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<p>This style is part of the MANYBODY package. It is only enabled if
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LAMMPS was built with that package (which it is by default).</p>
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</div>
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<div class="section" id="related-commands">
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<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline">¶</a></h2>
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<p><a class="reference internal" href="pair_coeff.html"><em>pair_coeff</em></a></p>
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<p><strong>Default:</strong> none</p>
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<hr class="docutils" />
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<p id="zhou"><strong>(Zhou)</strong> Zhou, submitted for publication (2010). Please contact
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Xiaowang Zhou (Sandia) for details via email at xzhou at sandia.gov.</p>
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