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### compute temp/sphere command

Syntax:

    compute ID group-ID temp/sphere keyword value ...

- ID, group-ID are documented in compute command
- temp/sphere = style name of this compute command
- zero or more keyword/value pairs may be appended
- keyword = bias or dof

        bias value = bias-IDuniform or gaussian
          bias-ID = ID of a temperature compute that removes a velocity bias
        dof value = all or rotate
          all = compute temperature of translational and rotational degrees of freedom
          rotate = compute temperature of just rotational degrees of freedom

Examples:

    compute 1 all temp/sphere
    compute myTemp mobile temp/sphere bias tempCOM
    compute myTemp mobile temp/sphere dof rotate

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 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 3 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 command to adjust the dof accordingly.

The translational kinetic energy is computed the same as is described by the compute temp 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, 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 command if this is not the case.

This compute subtracts out translational degrees-of-freedom due to fixes that constrain molecular motion, such as fix shake and fix rigid. 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 command.

See this howto section of the manual for a discussion of different ways to compute temperature and perform thermostatting.

The keyword/value option pairs are used in the following ways.

For the bias keyword, bias-ID refers to 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.

For the dof keyword, a setting of all calculates a temperature that includes both translational and rotational degrees of freedom. A setting of rotate calculates a temperature that includes only rotational degrees of freedom.

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 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. The vector values will be in energy units.

Restrictions:

This fix requires that atoms store torque and angular velocity (omega) and a radius as defined by the atom_style sphere command.

All particles in the group must be finite-size spheres, or point particles with radius = 0.0.

Related commands:

compute temp, compute temp/asphere

Default:

The option defaults are no bias and dof = all.