dft_tools/doc/basicnotions/structure.rst

Structure of DFT Tools
======================

.. image:: images/structure.png
   :width: 700
   :align: center

The central part of :program:`dft_tools`, which is performing the
steps for the DMFT self-consistency cycle, is written following the
same philosophy as the :ref:`TRIQS <triqslibs:welcome>` toolbox. At
the user level, easy-to-use python modules are provided that allow to
write simple and short scripts performing the actual
calculation. Here, we will describe the general structure of the
package, for the details of how to use the modules, please consult the 
user guide of this :ref:`documentation`.

The interface layer
-------------------

Since the input for this DMFT part has to be provided by DMFT
calculations, there needs to be another layer that connects the
python-based modules with the DFT output. Naturally, this layer
depends on the DFT package at hand. At the moment, there is an
interface to the Wien2k band structure package, and a very light 
interface that can be used in a more general setup. Note that this
light interface layer **does not** allow full charge self-consistent
calculations. 

Wien2k interface
""""""""""""""""

This interface layer consists of two parts. First, the output from Wien2k
is taken, and localised Wannier orbitals are constructed. This is done
by the fortran program :program:`dmftproj`. The second part consist in
the conversion of the :program:`dmftproj` into the hdf5 file
format to be used for the DMFT calculation. This step is done by a
python routine called :class:`Wien2kConverter`, that reads the text output and
creates the hdf5 input file with the necessary ingredients. Quite
naturally, :program:`dft_tools` will adopt this converter concept also for future
developments for other DFT packages.

General interface
"""""""""""""""""

In addition to the specialised Wien2k interface, :program:`dft_tools`
provides also a very light-weight general interface. It basically
consists of a very simple :class:`HkConverter`. As input it requires a
hamiltonian matrix :math:`H_{mn}(\mathbf{k})` written already in
localised-orbital indices :math:`m,n`, on a :math:`\mathbf{k}`-point
grid covering the Brillouin zone, and just a few other informations
like total numer of electrons, how many correlated atoms in the unit
cell, and so on. It converts this hamiltonian into a hdf5 format and 
sets some variables to standard values, such that it can be used with
the python modules performing the DMFT calculation. How the
hamiltonian matrix :math:`H_{mn}(\mathbf{k})` is actually calculated,
is **not** part of this interace.

The DMFT calculation
--------------------

As mentioned above, there are a few python routines that allow to
perform the multi-band DMFT calculation in the context of real
materials. The major part is contained inte module
:class:`SumkDFT`. It contains routines to
       
* calculate local Greens functions
* do the upfolding and downfolding from Bloch bands to Wannier
  orbitals
* calculate the double-counting correction
* calculate the chemical potential in order to get the electron count right    
* other things like determining the structure of the local
  hamiltonian, rotating from local to global coordinate systems, etc.

At the user level, all these routines can be used to construct
situation- and problem-dependent DMFT calculations in a very efficient
way.

Full charge self consistency
----------------------------

Using the Wien2k interface, one can perform full charge
self-consistent calculations. :class:`SumkDFT` provides routines to
calculate the correlated density matrix and stores it in a format that
can be read in by the :program:`lapw2` part of the Wien2k
package. Changing a one-shot calculation in a full charge
self-consistent one is only a couple of additional lines in the code! 

Post-processing
---------------

The main result of DMFT calculation is the interacting Greens function
and the Self energy. However, one is normally interested in
quantitites like band structure, density of states, or transport
properties. In order to calculate these things, :program:`dft_tools`
provides the post-processing modules :class:`SumkDFTTools`. It
contains routines to calculate

* (projected) density of states
* partial charges
* correlated band structures (*spaghettis*)
* transport properties such as optical conductivity, resistivity,
  or thermopower.

.. warning::
   At the moment neither :ref:`TRIQS<triqslibs:welcome>` nor :program:`dft_tools`
   provides Maximum Entropy routines! You can use the Pade
   approximants implemented in the TRIQS library, or you use your own
   home-made Maximum Entropy code to do the analytic continuation from
   Matsubara to the real-frequency axis.