2.3.8. Compute, fixes, variables — LAMMPS documentation

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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 compute

- `lammps.extract_fix()`: extract value(s) from a fix

- `lammps.extract_variable()`: extract value(s) from a variable

- `lammps.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 arrays

- `lammps.numpy.extract_fix()`: extract value(s) from a fix, return arrays as numpy arrays

- `lammps.numpy.extract_variable()`: extract value(s) from a variable, return arrays as numpy arrays