forked from lijiext/lammps
134 lines
4.9 KiB
HTML
134 lines
4.9 KiB
HTML
<HTML>
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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 thole 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 thole damp cutoff
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</PRE>
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<UL><LI>thole = style name
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<LI>damp = global damping parameter
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<LI>cutoff = global cutoff
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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 hybrid/overlay ... thole 2.6 12.0
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pair_coeff 1 1 thole 1.0
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pair_coeff 1 2 thole 1.0 2.6 10.0
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pair_coeff * 2 thole 1.0 2.6
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>The <I>thole</I> pair style is meant to be used with force fields that
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include explicit polarization through Drude dipoles. This link
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describes how to use the <A HREF = "tutorial_drude.html">thermalized Drude oscillator
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model</A> in LAMMPS and polarizable models in LAMMPS
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are discussed in <A HREF = "Section_howto.html#howto_25">this Section</A>.
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</P>
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<P>The <I>thole</I> pair style should be used as a sub-style within in the
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<A HREF = "pair_hybrid.html">pair_hybrid/overlay</A> command, in conjunction with a
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main pair style including Coulomb interactions, i.e. any pair style
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containing <I>coul/cut</I> or <I>coul/long</I> in its style name.
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</P>
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<P>The <I>thole</I> pair style computes the Coulomb interaction damped at
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short distances by a function
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</P>
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<P>\begin{equation} T_{ij}(r_{ij}) = 1 - \left( 1 +
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\frac{s_{ij} r_{ij} }{2} \right)
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\exp \left( - s_{ij} r_{ij} \right) \end{equation}
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</P>
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<P>This function results from an adaptation to point charges
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<A HREF = "#Noskov">(Noskov)</A> of the dipole screening scheme originally proposed
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by <A HREF = "#Thole">Thole</A>. The scaling coefficient \(s_{ij} \) is determined
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by the polarizability of the atoms, \( \alpha_i \), and by a Thole
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damping parameter \( a \). This Thole damping parameter usually takes
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a value of 2.6, but in certain force fields the value can depend upon
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the atom types. The mixing rule for Thole damping parameters is the
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arithmetic average, and for polarizabilities the geometric average
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between the atom-specific values.
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</P>
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<P>\begin{equation} s_{ij} = \frac{ a_{ij} }{
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(\alpha_{ij})^{1/3} } = \frac{ (a_i + a_j)/2 }{
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[(\alpha_i\alpha_j)^{1/2}]^{1/3} } \end{equation}
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</P>
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<P>The damping function is only applied to the interactions between the
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point charges representing the induced dipoles on polarizable sites,
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that is, charges on Drude particles, \( q_{D,i} \), and opposite
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charges, \( -q_{D,i} \), located on the respective core particles
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(to which each Drude particle is bonded). Therefore, Thole screening
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is not applied to the full charge of the core particle \( q_i \), but
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only to the \( -q_{D,i} \) part of it.
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</P>
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<P>The interactions between core charges are subject to the weighting
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factors set by the <A HREF = "special_bonds.html">special_bonds</A> command. The
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interactions between Drude particles and core charges or
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non-polarizable atoms are also subject to these weighting factors. The
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Drude particles inherit the 1-2, 1-3 and 1-4 neighbor relations from
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their respective cores.
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</P>
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<P>The following coefficients must be defined for each pair of atoms
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types via the <A HREF = "pair_coeff.html">pair_coeff</A> command as in the example
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above.
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</P>
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<UL><LI>alpha (distance units^3)
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<LI>damp
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<LI>cutoff (distance units)
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</UL>
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<P>The last two coefficients are optional. If not specified the global
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Thole damping parameter or global cutoff specified in the pair_style
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command are used. In order to specify a cutoff (third argument) a damp
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parameter (second argument) must also be specified.
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</P>
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<P><B>Mixing</B>:
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</P>
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<P>The <I>thole</I> pair style does not support mixing. Thus, coefficients
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for all I,J pairs must be specified explicitly.
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>These pair styles are part of the USER-DRUDE package. They are only
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enabled if LAMMPS was built with that package. See the <A HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P>This pair_style should currently not be used with the <A HREF = "dihedral_charmm.html">charmm dihedral
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style</A> if the latter non-zero 1-4 weighting
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factors. This is because the <I>thole</I> pair style does not know which
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pairs are 1-4 partners of which dihedrals.
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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 = "fix_drude.html">fix drude</A>, <A HREF = "fix_langevin_drude.html">fix
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langevin/drude</A>, <A HREF = "fix_drude_transform.html">fix
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drude/transform</A>, <A HREF = "compute_temp_drude.html">compute
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temp/drude</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 = "Noskov"></A>
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<P><B>(Noskov)</B> Noskov, Lamoureux and Roux, J Phys Chem B, 109, 6705 (2005).
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</P>
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<A NAME = "Thole"></A>
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<P><B>(Thole)</B> Chem Phys, 59, 341 (1981).
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</P>
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</HTML>
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