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2.3.8. Compute, fixes, variables
This section documents accessing or modifying data from objects like
computes, fixes, or variables in LAMMPS using the lammps
module.
For lammps.extract_compute()
and
lammps.extract_fix()
, the global, per-atom,
or local data calculated by the compute or fix can be accessed. What is returned
depends on whether the compute or fix calculates a scalar or vector or array.
For a scalar, a single double value is returned. If the compute or fix calculates
a vector or array, a pointer to the internal LAMMPS data is returned, which you can
use via normal Python subscripting.
The one exception is that for a fix that calculates a global vector or array, a single double value from the vector or array is returned, indexed by I (vector) or I and J (array). I,J are zero-based indices. See the Howto output page for a discussion of global, per-atom, and local data, and of scalar, vector, and array data types. See the doc pages for individual computes and fixes for a description of what they calculate and store.
For lammps.extract_variable()
,
an equal-style or atom-style variable is evaluated and
its result returned.
For equal-style variables a single c_double
value is returned and the
group argument is ignored. For atom-style variables, a vector of
c_double
is returned, one value per atom, which you can use via normal
Python subscripting. The values will be zero for atoms not in the
specified group.
lammps.numpy.extract_compute()
,
lammps.numpy.extract_fix()
, and
lammps.numpy.extract_variable()
are
equivalent NumPy implementations that return NumPy arrays instead of ctypes
pointers.
The lammps.set_variable()
method sets an
existing string-style variable to a new string value, so that subsequent LAMMPS
commands can access the variable.
Methods:
lammps.extract_compute()
: extract value(s) from a computelammps.extract_fix()
: extract value(s) from a fixlammps.extract_variable()
: extract value(s) from a variablelammps.set_variable()
: set existing named string-style variable to value
NumPy Methods:
lammps.numpy.extract_compute()
: extract value(s) from a compute, return arrays as numpy arrayslammps.numpy.extract_fix()
: extract value(s) from a fix, return arrays as numpy arrayslammps.numpy.extract_variable()
: extract value(s) from a variable, return arrays as numpy arrays