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135 lines
5.1 KiB
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135 lines
5.1 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 lubricate 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 lubricate mu squeeze shear pump twist cutinner cutoff
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</PRE>
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<UL><LI>mu = viscosity (mass/distance/time units)
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<LI>squeeze = 0/1 for squeeze force off/on
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<LI>shear = 0/1 for shear force off/on
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<LI>pump = 0/1 for pump force off/on
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<LI>twist = 0/1 for twist force off/on
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<LI>cutinner = (distance units)
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<LI>cutoff = outer cutoff for interactions (distance units)
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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 lubricate 1.5 1 1 1 0 2.3 2.4
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pair_coeff 1 1 1.8 2.0
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pair_coeff * *
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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>lubricate</I> computes pairwise interactions between mono-disperse
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spherical particles via this formula from <A HREF = "#Ball">(Ball and Melrose)</A>
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</P>
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<CENTER><IMG SRC = "Eqs/pair_lubricate.jpg">
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</CENTER>
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<P>which represents the dissipation W between two nearby particles due to
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the background solvent. Rc is the outer cutoff specified in the
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pair_style command, the translational velocities of the 2 particles
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are v1 and v2, the angular velocities are w1 and w2, and n is the unit
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vector in the direction from particle 1 to 2. The 4 terms represent
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four modes of pairwise interaction: squeezing, shearing, pumping, and
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twisting. The 4 flags in the pair_style command turn on or off each
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of these modes by including or excluding each term. The 4
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coefficients on each term are functions of the separation distance of
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the particles. Details are given in <A HREF = "#Ball">(Ball and Melrose)</A>,
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including the forces and torques that result from taking derivatives
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of this equation (see Appendix A).
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</P>
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<P>Unlike most pair potentials, the two specified cutoffs (cutinner and
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cutoff) refer to the surface-to-surface separation between two
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particles, not center-to-center distance. Currently, this pair style
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can only be used for mono-disperse spheres (same radii), so that
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separation is r_ij - 2*radius, where r_ij is the center-to-center
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distance between the particles. Within the inner cutoff <I>cutinner</I>,
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the forces and torques are evaluated at a separation of cutinner. The
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outer <I>cutoff</I> is the separation distance beyond which the pair-wise
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forces are zero.
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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 examples
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above, or in the data file or restart files read by the
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<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A>
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commands, or by mixing as described below:
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</P>
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<UL><LI>cutinner (distance units)
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<LI>cutoff (distance units)
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</UL>
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<P>The two coefficients are optional. If neither is specified, the two
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cutoffs specified in the pair_style command are used. Otherwise both
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must be specified.
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</P>
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<HR>
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<P><B>Mixing, shift, table, tail correction, per-atom energy/stress,
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restart, rRESPA info</B>:
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</P>
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<P>For atom type pairs I,J and I != J, the two cutoff distances for this
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pair style can be mixed. The default mix value is <I>geometric</I>. See
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the "pair_modify" command for details.
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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 option for the energy of the pair interaction.
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</P>
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<P>The <A HREF = "pair_modify.html">pair_modify</A> table option is not relevant
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for this pair 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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tail option for adding long-range tail corrections to energy and
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pressure.
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</P>
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<P>This pair style does not calculate per-atom energy and stress, as used
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by the <A HREF = "compute_epair_atom.html">compute epair/atom</A>, <A HREF = "compute_stress_atom.html">compute
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stress/atom</A>, and <A HREF = "dump.html">dump custom</A>
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commands.
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</P>
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<P>This pair style writes its information to <A HREF = "restart.html">binary restart
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files</A>, so pair_style and pair_coeff commands do not need
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to be specified in an input script that reads a restart file.
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</P>
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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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<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 "colloid" package. It is only enabled if
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LAMMPS was built with that package. See the <A HREF = "Section_start.html#2_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P>Because this poential computes forces and torques on particles, the
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atom style must support particles whose size is set via the
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<A HREF = "shape.html">shape</A> command. This is <A HREF = "atom_style.html">atom_style</A>
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ellipsoid and dipole. Since only spherical mono-disperse particles
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are currently allowed for pair_style lubricate, this means the 3 shape
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radii for all particle types must be the same.
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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 = "Ball"></A>
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<P><B>(Ball)</B> Ball and Melrose, Physica A, 247, 444-472 (1997).
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</P>
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</HTML>
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