2007-04-20 23:38:14 +08:00
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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 heat 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 heat N eflux
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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>heat = style name of this fix command
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<LI>N = add/subtract heat every this many timesteps
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<LI>eflux = rate of heat addition or subtraction (energy/time units)
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>fix 3 qin heat 1 1.0
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fix 4 qout heat 1 -1.0
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</PRE>
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<P><B>Description:</B>
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</P>
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2007-04-21 07:59:34 +08:00
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<P>Add non-translational kinetic energy (heat) to a group of atoms such
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that their aggregate momentum is conserved. Two of these fixes can be
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used to establish a temperature gradient across a simulation domain by
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2007-04-23 22:56:08 +08:00
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adding heat to one group of atoms (hot reservoir) and subtracting heat
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2007-04-21 07:59:34 +08:00
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from another (cold reservoir). E.g. a simulation sampling from the
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McDLT ensemble. Note that the fix is applied to a group of atoms, not
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a geometric region, so that the same set of atoms is affected wherever
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they may move to.
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2007-04-20 23:38:14 +08:00
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</P>
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<P>Heat addition/subtraction is performed every N timesteps. The <I>eflux</I>
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parameter determines the change in aggregate energy of the entire
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2007-04-21 07:59:34 +08:00
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group of atoms. Since eflux is in units of energy/time, this means a
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2007-04-23 22:56:08 +08:00
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larger value of N will add/subtract a larger amount of energy each
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2007-04-21 07:59:34 +08:00
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timestep the fix is invoked. If heat is subtracted from the system
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too aggressively so that the group's kinetic energy goes to zero,
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LAMMPS halts with an error message.
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2007-04-20 23:38:14 +08:00
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</P>
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<P>Fix heat is different from a thermostat such as <A HREF = "fix_nvt.html">fix nvt</A>
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or <A HREF = "fix_temp_rescale.html">fix temp/rescale</A> in that energy is
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added/subtracted continually. Thus if there isn't another mechanism
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in place to counterbalance this effect, the entire system will heat or
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2007-04-23 22:56:08 +08:00
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cool continuously. You can use multiple heat fixes so that the net
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2007-04-20 23:38:14 +08:00
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energy change is 0.0 or use <A HREF = "fix_viscous">fix viscous</A> to drain energy
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from the system.
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</P>
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<P>This fix does not change the coordinates of its atoms; it only scales
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their velocities. Thus you must still use an integration fix
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(e.g. <A HREF = "fix_nve.html">fix nve</A>) on the affected atoms. This fix should
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not normally be used on atoms that have their temperature controlled
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by another fix - e.g. <A HREF = "fix_nvt.html">fix nvt</A> or <A HREF = "fix_langevin.html">fix
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langevin</A> fix.
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</P>
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2007-06-26 08:03:39 +08:00
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<P><B>Restart, fix_modify, thermo output, run start/stop, minimize info:</B>
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</P>
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<P>No information about this fix is written to <A HREF = "restart.html">binary restart
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files</A>. None of the <A HREF = "fix_modify.html">fix_modify</A> options
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are relevant to this fix. No quantities calculated by this fix can be
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output by the <A HREF = "thermo_style.html">thermo_style custom</A> command. No
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parameter of this fix can be used with the <I>start/stop</I> keywords of
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the <A HREF = "run.html">run</A> command. This fix is not invoked during <A HREF = "minimize.html">energy
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minimization</A>.
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</P>
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2007-04-20 23:38:14 +08:00
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<P><B>Restrictions:</B> none
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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 = "compute_temp.html">compute temp</A>, <A HREF = "compute_temp_region.html">compute
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temp/region</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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