.. _bandgab:

==================================================
Calculating band gap using the GLLB-sc functional
==================================================

In this tutorial, we use the GLLB-sc to calculate the band gap of KTaO3 using
the XC functional GLLB-sc. This functional uses the GLLB response potential to
replace the PBEsol response potential of the exchange. [GLLB-sc]_ This has
been shown to improve the band gap description as shown in the figure below
taken from [Castelli2012]_.

.. figure:: GLLB-SC_gap.png

A GLLB-sc band gap calculation is performed as given here:

.. literalinclude:: gllbsc_band_gap.py


Spin-polarized GLLB-SC
======================

Spin-polarized GLLB-SC is currently implemented to svn trunk. However there
are some convergence issues releated to fermi smearing and the reference
energy of highest orbital. Also some parts are still untested. The code will
be improved to robust version soon, but in the meanwhile please contact
mikael.kuisma@tut.fi before using.


-------------

.. [GLLB-sc] M. Kuisma, J. Ojanen, J. Enkovaara, and T. T. Rantala1,
   PHYSICAL REVIEW B 82, 115106 (2010),
   *Kohn-Sham potential with discontinuity for band gap materials*,
   DOI: 10.1103/PhysRevB.82.115106

.. [Castelli2012] Ivano E. Castelli, Thomas Olsen, Soumendu Datta,
   David D. Landis, Søren Dahl, Kristian S. Thygesena
   and Karsten W. Jacobsen,
   Energy Environ. Sci., 2012, 5, 5814,
   *Computational screening of perovskite metal oxides for optimal solar light
   capture*,
   DOI: 10.1039/c1ee02717d
