.. _compute-thermal:

Compute Phonon Linewidth and Thermal Conductivity
=================================================

With the irreducible derivatives at second and third order computed with the methods described in the chapter :ref:`compute-id`,
we can predict the phonon linewidth and thermal conductivity using perturbation theory.


Phonon Linewidth
----------------

Here we continue the NaCl example from :ref:`compute-id`.

First, we need to prepare a configuration file ``config.yml`` for the ``pm-conductivity`` command:

.. code-block:: yaml
  :caption: config.yml

  structure: structure.yml
  load_fi2: fi_order2.hdf5
  load_fi3: fi_order3.hdf5
  mesh: [11, 11, 11]
  band:
    - ['0 0 0', '1/2 1/2 0']
    - ['-1/2 1/2 0', '0 0 0']
    - ['0 0 0', '1/2 1/2 1/2']
  band_label:
    - $\Gamma$
    - X
    - $\Gamma$
    - L

where:

* ``structure.yml`` contains structure of NaCl;

* ``fi_order2.hdf5`` is the HDF5 data file for the FourierInterpolation class at second order;

* ``fi_order3.hdf5`` is the HDF5 data file for the FourierInterpolation class at third order.

In this example, we use the FourierInterpolation object of a :math:`4\hat{\mathbf{1}}` FTG (:math:`4 \times 4 \times 4`) at second order and 
the FourierInterpolation object of a :math:`2\hat{\mathbf{1}}` FTG (:math:`2 \times 2 \times 2`) at third order. We interpolate the phonons and phonon interactions
to the mesh of :math:`11\hat{\mathbf{1}}` (:math:`11 \times 11 \times 11`), and compute the linewidth along the K-path :math:`\Gamma \to X \to \Gamma \to L`.

Execute the following command the compute the phonon linewidth::

  pm-conductivity --config config.yml --save-gamma


Thermal Conductivity
--------------------

Using the same NaCl example, we can prepare a configuration file ``config.yml`` similar to the one above:

.. code-block:: yaml
  :caption: config.yml

  structure: structure.yml
  load_fi2: fi_order2.hdf5
  load_fi3: fi_order3.hdf5
  mesh: [11, 11, 11]

Execute the following command the compute the thermal conductivity using relaxation time approximation (RTA)::

  pm-conductivity --config config.yml --save-kappa --mode RTA