24 How do I calculate the elastic constants as a function of temperature?

There are two ways. Both of them are a two step calculation. The fastest method uses the ``quasi-static'' approximation but it is less accurate. First use what='elastic_constants_t'. This option computes the elastic constants at T = 0 for all the geometries used by what='mur_lc_t' and saves them in the directory elastic_constants. In the second step use the same input for pw.x and 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 elastic constants at T = 0 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_t' 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 use the same input for pw.x and 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 output_anhar.el_cons.ps. Note that these approaches used with the option lmurn=.FALSE. can calculate the thermodynamic properties of non-cubic solids, but the second method is still experimental and should be limited to cubic solids.