lammps/doc/compute_temp_sphere.txt

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compute temp/sphere command :h3

[Syntax:]

compute ID group-ID temp/sphere bias-ID :pre

ID, group-ID are documented in "compute"_compute.html command
temp/sphere = style name of this compute command
bias-ID = ID of a temperature compute that removes a velocity bias (optional) :ul

[Examples:]

compute 1 all temp/sphere
compute myTemp mobile temp/sphere tempCOM :pre

[Description:]

Define a computation that calculates the temperature of a group of
spherical particles, including a contribution from both their
translational and rotational kinetic energy.  This differs from the
usual "compute temp"_compute_temp.html command, which assumes point
particles with only translational kinetic energy.

Both point and finite-size particles can be included in the group.
Point particles do not rotate, so they have only translational degrees
of freedom.  For 3d spherical particles, each has 6 degrees of freedom
(3 translational, 3 rotational).  For 2d spherical particles, each has
3 degrees of freedom (2 translational, 1 rotational).

IMPORTANT NOTE: This choice for degrees of freedom (dof) assumes that
all finite-size spherical particles in your model will freely rotate,
sampling all their rotational dof.  It is possible to use a
combination of interaction potentials and fixes that induce no torque
or otherwise constrain some of all of your particles so that this is
not the case.  Then there are less dof and you should use the
"compute_modify extra"_compute_modify.html command to adjust the dof
accordingly.

The translational kinetic energy is computed the same as is described
by the "compute temp"_compute_temp.html command.  The rotational
kinetic energy is computed as 1/2 I w^2, where I is the moment of
inertia for a sphere and w is the particle's angular velocity.

IMPORTANT NOTE: For "2d models"_dimension.html, particles are treated
as spheres, not disks, meaning their moment of inertia will be the
same as in 3d.

A kinetic energy tensor, stored as a 6-element vector, is also
calculated by this compute.  The formula for the components of the
tensor is the same as the above formulas, except that v^2 and w^2 are
replaced by vx*vy and wx*wy for the xy component.  The 6 components of
the vector are ordered xx, yy, zz, xy, xz, yz.

The number of atoms contributing to the temperature is assumed to be
constant for the duration of the run; use the {dynamic} option of the
"compute_modify"_compute_modify.html command if this is not the case.

If a {bias-ID} is specified it must be the ID of a temperature compute
that removes a "bias" velocity from each atom.  This allows compute
temp/sphere to compute its thermal temperature after the translational
kinetic energy components have been altered in a prescribed way,
e.g. to remove a velocity profile.  Thermostats that use this compute
will work with this bias term.  See the doc pages for individual
computes that calculate a temperature and the doc pages for fixes that
perform thermostatting for more details.

This compute subtracts out translational degrees-of-freedom due to
fixes that constrain molecular motion, such as "fix
shake"_fix_shake.html and "fix rigid"_fix_rigid.html.  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 {extra} option of the
"compute_modify"_compute_modify.html command.

See "this howto section"_Section_howto.html#4_16 of the manual for a
discussion of different ways to compute temperature and perform
thermostatting.

[Output info:]

This compute calculates a global scalar (the temperature) and a global
vector of length 6 (KE tensor), which can be accessed by indices 1-6.
These values can be used by any command that uses global scalar or
vector values from a compute as input.  See "this
section"_Section_howto.html#4_15 for an overview of LAMMPS output
options.

The scalar value calculated by this compute is "intensive".  The
vector values are "extensive".

The scalar value will be in temperature "units"_units.html.  The
vector values will be in energy "units"_units.html.

[Restrictions:]

This compute requires that particles be represented as extended
spheres and not point particles.  This means they will have an angular
velocity and a diameter which is determined either by the
"shape"_shape.html command or by each particle being assigned an
individual radius, e.g. for "atom_style granular"_atom_style.html.

[Related commands:]

"compute temp"_compute_temp.html, "compute
temp/asphere"_compute_temp.html

[Default:] none