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155 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 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 T_target seed
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</PRE>
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<UL><LI>mu = dynamic viscosity (dynamic viscosity 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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<LI>T_target = desired temperature (temperature units)
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<LI>seed = random number seed (positive integer)
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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 1.3 5878598
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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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<PRE>pair_style lubricate 1.0 1 1 1 0 2.3 2.4 1.3 5878598
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pair_coeff * *
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variable mu equal ramp(1,2)
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fix 1 all adapt 1 pair lubricate mu * * v_mu
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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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their relative velocities in the presence of a background solvent with
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viscosity mu. Note that this is dynamic viscosity which has units of
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mass/distance/time, not kinematic viscosity.
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</P>
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<P>The viscosity mu can be varied in a time-dependent manner over the
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course of a simluation, in which case in which case the pair_style
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setting for mu will be overridden. See the <A HREF = "fix_adapt.html">fix adapt</A>
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command for details.
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</P>
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<P>Rc is the outer cutoff specified in the pair_style command, the
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translational velocities of the 2 particles are v1 and v2, the angular
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velocities are w1 and w2, and n is the unit vector in the direction
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from particle 1 to 2. The 4 terms represent four modes of pairwise
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interaction: squeezing, shearing, pumping, and twisting. The 4 flags
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in the pair_style command turn on or off each of these modes by
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including or excluding each term. The 4 coefficients on each term are
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functions of the separation distance of the particles and the
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viscosity. Details are given in <A HREF = "#Ball">(Ball and Melrose)</A>, including
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the forces and torques that result from taking derivatives of this
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equation with respect to velocity (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 extended spheres (same radii), so
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that 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>A Langevin thermostatting term is also added to the pairwise force,
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similar to that provided by the <A HREF = "fix_langevin.html">fix langevin</A> or
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<A HREF = "pair_dpd.html">pair_style dpd</A> commands. The target temperature for
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the thermostat is the specified <I>T_target</I>. The <I>seed</I> is used for
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the random numbers generated for the thermostat.
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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, 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 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>This pair style requires that atoms be finite-size spheres with a
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diameter, as defined by the <A HREF = "atom_style.html">atom_style sphere</A>
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command.
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</P>
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<P>Per-particle or per-type polydispersity is not yet supported by this
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pair style; all particles must have the same diameter.
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</P>
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<P>This pair style requires you to use the <A HREF = "communicate.html">communicate vel
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yes</A> option so that velocites are stored by ghost
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atoms.
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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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