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Compute styles :h3

Classes that compute scalar and vector quantities like temperature
and the pressure tensor, as well as classes that compute per-atom
quantities like kinetic energy and the centro-symmetry parameter
are derived from the Compute class.  New styles can be created
to add new calculations to LAMMPS.

Compute_temp.cpp is a simple example of computing a scalar
temperature.  Compute_ke_atom.cpp is a simple example of computing
per-atom kinetic energy.

Here is a brief description of methods you define in your new derived
class.  See compute.h for details.

init: perform one time setup (required)
init_list: neighbor list setup, if needed (optional)
compute_scalar: compute a scalar quantity (optional)
compute_vector: compute a vector of quantities (optional)
compute_peratom: compute one or more quantities per atom (optional)
compute_local: compute one or more quantities per processor (optional)
pack_comm: pack a buffer with items to communicate (optional)
unpack_comm: unpack the buffer (optional)
pack_reverse: pack a buffer with items to reverse communicate (optional)
unpack_reverse: unpack the buffer (optional)
remove_bias: remove velocity bias from one atom (optional)
remove_bias_all: remove velocity bias from all atoms in group (optional)
restore_bias: restore velocity bias for one atom after remove_bias (optional)
restore_bias_all: same as before, but for all atoms in group (optional)
pair_tally_callback: callback function for {tally}-style computes (optional).
memory_usage: tally memory usage (optional) :tb(s=:)

Tally-style computes are a special case, as their computation is done
in two stages: the callback function is registered with the pair style
and then called from the Pair::ev_tally() function, which is called for
each pair after force and energy has been computed for this pair. Then
the tallied values are retrieved with the standard compute_scalar or
compute_vector or compute_peratom methods. The USER-TALLY package
provides {examples}_compute_tally.html for utilizing this mechanism.

