[DFTB-Plus-User] Hubbard correction in DFTB

[Bálint Aradi]

Dear Gergely,

There are two different 'U' values in DFTB:

* The U values in the SK-files (usually called Hubbard Us), which
determine the r->0 limit of the SCC-interactions. They are calculated
from ab-initio atom calculations as second derivative of the atom energy
with respect of the occupation of the highest occupied orbital (chemical
hardness of the atom). They are not user tunable parameters and should
left unchanged.

* Additionally, you may add a +U type term to your Hamiltonian as in
LDA+U. It adds orbital-dependant local contributions (when using the
fully localised implementation) to the Hamiltonian based on the on-site
block of the dual-density matrix. The coupling constant (U-J) is user
tunable, and is often used (similar to ab initio programs) to achieve a
certain property (right band gap, linearity of the energy with respect
of occupation, etc.)

The relevant keyword is OrbitalPotential, and the DFTB+ manual contains
a trivial example and some references about the method.

  Best regards,

  Bálint

-- 
Dr. Bálint Aradi
Bremen Center for Computational Materials Science, University of Bremen
http://www.bccms.uni-bremen.de/cms/people/b-ar

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