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compute_temp_deform.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>
</CENTER>
<HR>
<H3>compute temp/deform command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>compute ID group-ID temp/deform
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "compute.html">compute</A> command
<LI>temp/deform = style name of this compute command
</UL>
<P><B>Examples:</B>
</P>
<PRE>compute myTemp all temp/deform
</PRE>
<P><B>Description:</B>
</P>
<P>Define a computation that calculates the temperature of a group of
atoms, after subtracting out a streaming velocity induced by the
simulation box changing size and/or shape, for example in a
non-equilibrium MD (NEMD) simulation. The size/shape change is
induced by use of the <A HREF = "fix_deform.html">fix deform</A> command. A compute
of this style is created by the <A HREF = "fix_nvt_sllod.html">fix nvt/sllod</A>
command to compute the thermal temperature of atoms for thermostatting
purposes. A compute of this style can also be used by any command
that computes a temperature, e.g. <A HREF = "thermo_modify.html">thermo_modify</A>,
<A HREF = "fix_temp_rescale.html">fix temp/rescale</A>, <A HREF = "fix_npt.html">fix npt</A>, etc.
</P>
<P>The deformation fix changes the box size and/or shape over time, so
each point in the simulation box can be thought of as having a
"streaming" velocity. For example, if the box is being sheared in x,
relative to y, then points at the bottom of the box (low y) have a
small x velocity, while points at the top of the box (hi y) have a
large x velocity. This position-dependent streaming velocity is
subtracted from each atom's actual velocity to yield a thermal
velocity which is used to compute the temperature.
</P>
<P>IMPORTANT NOTE: <A HREF = "fix_deform.html">Fix deform</A> has an option for
remapping either atom coordinates or velocities to the changing
simulation box. To use this compute, the fix should NOT remap atom
positions, but rather should let atoms respond to the changing box by
adjusting their own velocities (or let fix deform remap the atom
velocities). If the fix does remap atom positions, their velocity is
not changed, and thus they do not have the streaming velocity assumed
by this compute. LAMMPS will warn you if this setting is not
consistent.
</P>
<P>The temperature is calculated by the formula KE = dim/2 N k T, where
KE = total kinetic energy of the group of atoms (sum of 1/2 m v^2),
dim = 2 or 3 = dimensionality of the simulation, N = number of atoms
in the group, k = Boltzmann constant, and T = temperature. Note that
v in the kinetic energy formula is the atom's thermal velocity.
</P>
<P>A 6-component kinetic energy tensor is also calculated by this compute
for use in the computation of a pressure tensor. The formula for the
components of the tensor is the same as the above formula, except that
v^2 is replaced by vx * vy for the xy component, etc.
</P>
<P>The number of atoms contributing to the temperature is assumed to be
constant for the duration of the run; use the <I>dynamic</I> option of the
<A HREF = "compute_modify.html">compute_modify</A> command if this is not the case.
</P>
<P>This compute subtracts out degrees-of-freedom due to fixes that
constrain molecular motion, such as <A HREF = "fix_shake.html">fix shake</A> and
<A HREF = "fix_rigid.html">fix rigid</A>. This means the temperature of groups of
atoms that include these constraints will be computed correctly. If
needed, the subtracted degrees-of-freedom can be altered using the
<I>extra</I> option of the <A HREF = "compute_modify.html">compute_modify</A> command.
</P>
<P><B>Restrictions:</B> none
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "compute_temp_ramp.html">compute temp/ramp</A>, <A HREF = "fix_deform.html">fix
deform</A>, <A HREF = "fix_nvt_sllod.html">fix nvt/sllod</A>
</P>
<P><B>Default:</B> none
</P>
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