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There are two ways. Both of them are a two step calculation.
The fastest, but less accurate, method uses the ``quasi-static'' approximation.
First use
`what='elastic_constants_geo'`. This option computes the elastic
constants at
*T* = 0
K for all the geometries used by
`what='mur_lc_t'` and
saves them in the directory `elastic_constants`.
In the second step, using the same input, run again
THERMO_PW with `what='mur_lc_t'` after copying in your
working directory the directory `elastic_constants` obtained
in the previous step. Elastic constants are read only when
`lmurn=.FALSE.`.
The ``quasi-static'' approximation means that the code interpolates
the
*T* = 0
K elastic constants found in the first step at the geometry that
minimizes the Helmholtz (or Gibbs at finite pressure) free energy at
temperature
*T*
.

The second method uses the ``quasi-harmonic'' approximation and
requires many phonon calculations at many geometries.
First use `what='elastic_` `constants_geo'` and set
`use_free_energy=` `.TRUE.`. This option computes the
temperature dependent elastic constants taking as unperturbed geometries
those used by `what='mur_lc_t'` and saves them in the directory
`anhar_files`.
In the second step, using the same input, run again
THERMO_PW with `what=` `'mur_lc_t'` after
copying in your
working directory the directory `anhar_files` obtained
in the previous step. Elastic constants are read only when
`lmurn=` `.FALSE.`.
The ``quasi-harmonic'' approximation means that the code interpolates
the temperature dependent elastic constants found in the first step as
second derivatives of the Helmholtz free energy (corrected to give the
stress-strain elastic constants), at the geometry that
minimizes the Helmholtz free energy.

The plot of the elastic constants will be found in a file whose default
name is `output_anhar.el_cons.ps`.
Note that when the elastic constant are available in a file,
with the option `lmurn=.FALSE.` the thermodynamic properties
are calculated also for non-cubic solids.
The main approximation of the present implementation is that the atomic
coordinates are relaxed only at
*T* = 0
K minimizing the energy and
the free energy is
not calculated as a function of the atomic positions.

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2023-05-05