Examples
========

Silicon from first-principles
-----------------------------
Here a short example of how to calculate the transport
coefficients from a VASP output file (typically vasprun.xml)
is presented.

Preparation
~~~~~~~~~~~
Start with the :file:`param.yml`,
:file:`bandparam.yml` located in the `tests/13` directory.
In the same directory a sample :file:`vasprun.xml` file is
also provided.

Copy these files into an `input` directory in the directory
where you want to execute T4ME.

The general parameters
~~~~~~~~~~~~~~~~~~~~~~
Here follows a brief explanation of the general parameters
that need attention. They are already specified in the
sample :file:`param.yml` file so the user should not have to
change this for a test run. All other parameters should at
this point not need to be touched.

::

   dispersion_interpolate: False

We do not want to interpolate the input at this step.

::

   dispersion_velocities_numdiff: True

We need to calculate the group velocities of the electrons
as this is not supplied by VASP by default.

::

   transport_calc: True

Calculate the transport coefficients.

::

   transport_method: "numerick"

Integrate numerically in k-space.

::

   transport_integration_method: "trapz"

Used trapezoidal integration.

::

   transport_chempot_min: -0.4

The minimum chemical potential to sample, in eV.

::

   transport_chempot_max: 1.0

The maximum chemical potential to sample, in eV.

::

   transport_chempot_samples: 20

How many samples do you want between ``transport_chempot_min``
and ``transport_chempot_max``.
Computational time and storage may vary depending on
how this parameter is set.

::

   transport_use_analytic_scattering: False

Use density-of-states models for the electron scattering.

::

   dos_integrating_method: "trapz"

In order to calculate the scattering, it is necessary to
calculate the density-of-states. Here this is done by using
the trapezoidal integration with the delta
function approximated by a Gaussian.

::

   dos_smearing: 0.1

The smearing factor in eV used for the Gaussian approximation
of the delta function. This needs to be sufficiently big in
order for the density-of-states to converge, but also in order
for the scattering data to have a smooth onset at the carrier
energy. It is recommended that this, for calculations of
scattering properties is not set below 0.1 eV.

::

   temperature_min: 300

The minimum temperature in K.

::

   temperature_max: 300

The maximum temperature in K.

::

   temperature_steps: 1

The number of temperature steps between ``temperature_min``
and ``temperature_max``.

::

   e_fermi: True

Set the zero in energy to Fermi level supplied by the
first-principle code, here VASP. The ``transport_chempot_min``
and ``transport_chempot_max`` parameters are thus set with
reference to the shifted grid where the zero in energy is
usually at the top valence band.

::

   read: vasp

Read data from VASP output files, here :file:`vasprun.xml`.

::

   symprec: 1e-6

The symmetry cutoff used to detect symmetry. Should somewhat
match with the value used in VASP. Passed along to Spglib to
generate the irreducible to full Brillouin zone mapping.

::

   onlytotalrate: True

Only store the total concatenated relaxation time arrays.
Saves memory.

The band parameters
~~~~~~~~~~~~~~~~~~~
Here follows a brief explanation of the band parameters
that need attention. They are already specified in the
sample :file:`bandparam.yml` file so the user should not have
to change this for a test run. All other parameters should at
this point not need to be touched.

::

   Band 1-:

Tells the reader that it should apply all consecutive
parameters to all the bands in the supplied system.

::

   select_scattering: [0,0,0,0,0,0,0,0,0,0,0,1]

Only use constant scattering.

::

   tau0_c: 100

The value of the constant relaxation time in fs.

Execution
~~~~~~~~~
After all parameters have been set (should only be necessary
to copy files as stated before) the transport coefficients
can be calculated by executing

::

   python t4me.py

During execution the file :file:`info.log` in the directory `output`
can be inspected in order to assess progress
and that everything works as expected.

Output
~~~~~~
On completion the transport coefficients can be found in the
`output` directory.