forked from lijiext/lammps
112 lines
4.4 KiB
HTML
112 lines
4.4 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>fix nvt/asphere command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID nvt/asphere Tstart Tstop Tdamp keyword value ...
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>nvt/asphere = style name of this fix command
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<LI>Tstart,Tstop = desired temperature at start/end of run
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<LI>Tdamp = temperature damping parameter (time units)
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<LI>zero or more keyword/value pairs may be appended to the args
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<LI>keyword = <I>drag</I>
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<PRE> <I>drag</I> value = drag factor added to thermostat (0.0 = no drag)
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>fix 1 all nvt/asphere 300.0 300.0 100.0
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fix 1 all nvt/asphere 300.0 300.0 100.0 drag 0.2
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Perform constant NVT integration to update positions, velocities,
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and angular velocities each timestep for aspherical or ellipsoidal
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particles in the group using a Nose/Hoover temperature thermostat.
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V is volume; T is temperature. This creates a system trajectory
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consistent with the canonical ensemble.
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</P>
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<P>The desired temperature at each timestep is a ramped value during the
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run from <I>Tstart</I> to <I>Tstop</I>. The <A HREF = "run.html">run</A> command documents
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how to make the ramping take place across multiple runs. The <I>Tdamp</I>
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parameter is specified in time units and determines how rapidly the
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temperature is relaxed. For example, a value of 100.0 means to relax
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the temperature in a timespan of (roughly) 100 time units (tau or
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fmsec or psec - see the <A HREF = "units.html">units</A> command).
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</P>
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<P>In some cases (e.g. for solids) the temperature of the system can
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oscillate undesirably when a Nose/Hoover thermostat is applied. The
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optional <I>drag</I> keyword will damp these oscillations, although it
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alters the Nose/Hoover equations. A value of 0.0 (no drag) leaves the
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Nose/Hoover formalism unchanged. A non-zero value adds a drag term;
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the larger the value specified, the greater the damping effect.
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Performing a short run and monitoring the temperature is the best way
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to determine if the drag term is working. Typically a value between
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0.2 to 2.0 is sufficient to damp oscillations after a few periods.
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</P>
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<P>This fix computes a temperature each timestep. To do this, the fix
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creates its own compute of style "temp/asphere", as if this command
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had been issued:
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</P>
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<PRE>compute fix-ID_temp group-ID temp/asphere
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</PRE>
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<P>See the <A HREF = "compute_temp_asphere.html">compute temp/asphere</A> command for
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details. Note that the ID of the new compute is the fix-ID with
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underscore + "temp" appended and the group for the new compute is
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the same as the fix group.
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</P>
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<P>Note that this is NOT the compute used by thermodynamic output (see
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the <A HREF = "thermo_style.html">thermo_style</A> command) with ID = <I>thermo_temp</I>.
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This means you can change the attributes of this fix's temperature
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(e.g. its degrees-of-freedom) via the
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<A HREF = "compute_modify.html">compute_modify</A> command or print this temperature
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during thermodyanmic output via the <A HREF = "thermo_style.html">thermo_style
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custom</A> command using the appropriate compute-ID.
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It also means that changing attributes of <I>thermo_temp</I> will have no
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effect on this fix. Alternatively, you can directly assign a new
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compute (for calculating temeperature) that you have defined to this
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fix via the <A HREF = "fix_modify.html">fix_modify</A> command.
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</P>
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<P>This fix makes a contribution to the potential energy of the system
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that can be included in thermodynamic output of potential energy using
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the <A HREF = "fix_modify.html">fix_modify energy</A> option. The contribution can
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also be printed by itself via the keyword <I>f_fix-ID</I> in the
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<A HREF = "thermo_style.html">thermo_style custom</A> command.
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>Can only be used if LAMMPS was built with the "asphere" package.
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</P>
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<P>The final Tstop cannot be 0.0 since it would make the target T = 0.0
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at some timestep during the simulation which is not allowed in
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the Nose/Hoover formulation.
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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_nvt.html">fix nvt</A>, <A HREF = "fix_nve_asphere.html">fix nve_asphere</A>, <A HREF = "fix_npt_asphere.html">fix
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npt_asphere</A>, <A HREF = "fix_modify.html">fix_modify</A>
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</P>
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<P><B>Default:</B>
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</P>
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<P>The keyword defaults are drag = 0.0.
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</P>
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