forked from lijiext/lammps
130 lines
5.8 KiB
HTML
130 lines
5.8 KiB
HTML
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<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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<HR>
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<H3>fix nph/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 nph/asphere args 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>nph/asphere = style name of this fix command
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<LI>additional barostat related keyword/value pairs from the <A HREF = "fix_nh.html">fix nph</A> command can be appended
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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 nph/asphere iso 0.0 0.0 1000.0
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fix 2 all nph/asphere x 5.0 5.0 1000.0
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fix 2 all nph/asphere x 5.0 5.0 1000.0 drag 0.2
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fix 2 water nph/asphere aniso 0.0 0.0 1000.0 dilate partial
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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 NPH integration to update position, velocity,
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orientation, and angular velocity each timestep for aspherical or
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ellipsoidal particles in the group using a Nose/Hoover pressure
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barostat. P is pressure; H is enthalpy. This creates a system
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trajectory consistent with the isenthalpic ensemble.
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</P>
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<P>This fix differs from the <A HREF = "fix_nh.html">fix nph</A> command, which assumes
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point particles and only updates their position and velocity.
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</P>
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<P>Additional parameters affecting the barostat are specified by keywords
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and values documented with the <A HREF = "fix_nh.html">fix nph</A> command. See,
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for example, discussion of the <I>aniso</I>, and <I>dilate</I> keywords.
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</P>
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<P>The particles in the fix group are the only ones whose velocities and
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positions are updated by the velocity/position update portion of the
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NPH integration.
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</P>
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<P>Regardless of what particles are in the fix group, a global pressure is
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computed for all particles. Similarly, when the size of the simulation
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box is changed, all particles are re-scaled to new positions, unless the
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keyword <I>dilate</I> is specified with a value of <I>partial</I>, in which case
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only the particles in the fix group are re-scaled. The latter can be
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useful for leaving the coordinates of particles in a solid substrate
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unchanged and controlling the pressure of a surrounding fluid.
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</P>
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<HR>
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<P>This fix computes a temperature and pressure each timestep. To do
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this, the fix creates its own computes of style "temp/asphere" and
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"pressure", as if these commands had been issued:
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</P>
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<PRE>compute fix-ID_temp all temp/asphere
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compute fix-ID_press all pressure fix-ID_temp
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</PRE>
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<P>See the <A HREF = "compute_temp_asphere.html">compute temp/asphere</A> and <A HREF = "compute_pressure.html">compute
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pressure</A> commands for details. Note that the
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IDs of the new computes are the fix-ID + underscore + "temp" or fix_ID
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+ underscore + "press", and the group for the new computes is "all"
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since pressure is computed for the entire system.
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</P>
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<P>Note that these are NOT the computes 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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and <I>thermo_press</I>. This means you can change the attributes of this
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fix's temperature or pressure via the
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<A HREF = "compute_modify.html">compute_modify</A> command or print this temperature
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or pressure during thermodynamic 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> or
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<I>thermo_press</I> will have no effect on this fix.
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</P>
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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</P>
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<P>This fix writes the state of the Nose/Hoover barostat to <A HREF = "restart.html">binary
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restart files</A>. See the <A HREF = "read_restart.html">read_restart</A>
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command for info on how to re-specify a fix in an input script that
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reads a restart file, so that the operation of the fix continues in an
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uninterrupted fashion.
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</P>
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<P>The <A HREF = "fix_modify.html">fix_modify</A> <I>temp</I> and <I>press</I> options are
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supported by this fix. You can use them to assign a
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<A HREF = "compute.html">compute</A> you have defined to this fix which will be used
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in its thermostatting or barostatting procedure. If you do this, note
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that the kinetic energy derived from the compute temperature should be
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consistent with the virial term computed using all atoms for the
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pressure. LAMMPS will warn you if you choose to compute temperature
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on a subset of atoms.
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</P>
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<P>The <A HREF = "fix_modify.html">fix_modify</A> <I>energy</I> option is supported by this
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fix to add the energy change induced by Nose/Hoover barostatting to
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the system's potential energy as part of <A HREF = "thermo_style.html">thermodynamic
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output</A>.
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</P>
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<P>This fix computes the same global scalar and global vector of
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quantities as does the <A HREF = "fix_nh.html">fix nph</A> command.
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</P>
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<P>This fix can ramp its target pressure over multiple runs, using the
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<I>start</I> and <I>stop</I> keywords of the <A HREF = "run.html">run</A> command. See the
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<A HREF = "run.html">run</A> command for details of how to do this.
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</P>
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<P>This fix is not invoked during <A HREF = "minimize.html">energy minimization</A>.
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>This fix requires that atoms store torque and angular velocity (omega)
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as defined by the <A HREF = "atom_style.html">atom_style</A>. It also require they
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store either a per-particle diameter or per-type <A HREF = "shape.html">shape</A>.
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</P>
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<P>All particles in the group must be finite-size. They cannot be point
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particles, but they can be aspherical or spherical.
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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_nh.html">fix nph</A>, <A HREF = "fix_nve_asphere.html">fix nve_asphere</A>, <A HREF = "fix_nvt_asphere.html">fix
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nvt_asphere</A>, <A HREF = "fix_npt_asphere.html">fix npt_asphere</A>,
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<A HREF = "fix_modify.html">fix_modify</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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