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Generate a FCIDUMP file for external DMRG calculations
======================================================

This tutorial briefly demonstrates how to generate a FCIDUMP integral file which can be used to perform relativistic DMRG
calculations with an external DMRG code. The interface and the first relativistic DMRG implementation and 
calculation are described in :cite:`Knecht2014a`. 

We will explain the basic steps to generate a FCIDUMP file by looking at the HF molecule using a small cc-pVDZ basis set
for each atom. The molecular input file **hf.xyz** is::

  2

  H      0.0000000000   0.0000000000   0.8547056701
  F      0.0000000000   0.0000000000  -0.0453403237


and the corresponding wave function input file **fci.inp** is::

  **DIRAC
  .TITLE
   FCIDUMP file tutorial
  .WAVE FUNCTION
  **HAMILTONIAN
  .DOSSSS
  **WAVE FUNCTION
  .SCF
  .KR CI
  *SCF
  .CLOSED SHELL
   10
  *KRCICALC
  .CI PROGRAM
  LUCIAREL
  .CIROOTS
   0  1
  .MAX CI
   100
  .INACTIVE
   1
  .GAS SHELLS
   1
   8 8 / 9
  **MOLECULE
  *BASIS
  .DEFAULT
  cc-pVDZ
  *END OF INPUT


So let's have a closer look at the above wave function input. The default Hamiltonian (to be specified under
\*\*Hamiltonian) in Dirac is the Dirac-Coulomb Hamiltonian and we explicitly ask to also include the class of (SS|SS)
integrals. The wave function input asks for a self-consistent-field Hartree-Fock calculation for the closed-shell HF
molecule with 10 electrons followed by a FCI/CASCI calculation for the :math:`\Omega=0` state using an active space of 
**8** electrons in **9** Kramer's pairs (== 18 spinors). The syntax (min max # of electrons in the CAS space / # of Kramer's pairs)
reads in detail as::

  .GAS SHELLS
   1
   8 8 / 9

Note that for this FCI/CASCI calculation we have frozen the 1s shell of F by setting::

  .INACTIVE
   1

To run the above example we use DIRACs pyhton script *pam* with the following command line::

  $ $path-to-dirac-build-directory/pam --inp=fci.inp --mol=hf.xyz  

A closer look at the output of the CI program at the bottom of the Dirac calculation will tell us the final FCI/CASCI
energy for the :math:`\Omega=0` state to be **-100.1956847...** Hartree. This is the reference energy we should expect to
get from the DMRG calculation on the same active space. 

To obtain now the integral file **FCIDUMP** for the above FCI/CASCI example we first need to modify the above input file
by introducing the keyword .FCIDUMP and change the CI program from *LUCIAREL* to *GASCIP* for technical reasons. Save the
file as **fcidump.inp** and run Dirac with the following command line::

  $ $path-to-dirac-build-directory/pam --inp=fcidump.inp --mol=hf.xyz --get=FCIDUMP 

The  **fcidump.inp** reads as::

  **DIRAC
  .TITLE
   FCIDUMP file tutorial
  .WAVE FUNCTION
  **HAMILTONIAN
  .DOSSSS
  **WAVE FUNCTION
  .SCF
  .KR CI
  *SCF
  .CLOSED SHELL
   10
  *KRCICALC
  .CI PROGRAM
  GASCIP
  .FCIDUMP
  .CIROOTS
   0  1
  .MAX CI
   100
  .INACTIVE
   1
  .GAS SHELLS
   1
   8 8 / 9
  **MOLECULE
  *BASIS
  .DEFAULT
  cc-pVDZ
  *END OF INPUT

Happy DMRG computing!