:orphan:

.. _mmf_X2C: 

Molecular mean-field X2C
========================

The molecular-mean-field X2C Hamiltonian (X2Cmmf) is **available** for the following methods:

-  Coupled Cluster (CC): module RELCCSD
-  multi-reference (MR) CC: Fock-space CC (including the intermediate Hamiltonian IHFSCC version): module RELCCSD

The remaining post-SCF modules are **not supported** yet:

-  KR-MCSCF
-  KR-CI
-  TD-DFT

The X2Cmmf Hamiltonian scheme can be used for the Dirac-Coulomb as well
as the Dirac-Coulomb-Gaunt Hamiltonian (add .GAUNT to \*\*HAMILTONIAN).

One-step X2Cmmf
---------------

In order to perform a two-component (MR)CC after a four-component SCF calculation add the keyword .X2Cmmf 
to the :ref:`**HAMILTONIAN` keyword section:

::

    **HAMILTONIAN
    .X2Cmmf

After the SCF calculation Dirac will carry out the transformation to two-component prior to the 4-index transformation of
the two-electron integrals and the CC run.

Two-step X2Cmmf
---------------

Besides the one-step X2Cmmf protocol as described above one may also proceed in seperate steps. 
This may be useful if one would like to use less/more MPI processes in the SCF+decoupling step than in the 4-index/CC calculation. 
The first step (SCF+decoupling) simply requires the .X2Cmmf keyword under the \*\*HAMILTONIAN input deck (same as above):

::

    **HAMILTONIAN
    .X2Cmmf

Save the files DFCOEF, AOMOMAT and X2CMAT, for example using the *pam* script::

    ./pam ... --outcmo --get="AOMOMAT X2CMAT"

To restart now any further calculation, for example the 4-index transformation (MOLTRA) and/or the (MR)CC run copy the above files to
your scratch directory::

    ./pam ... --incmo --put="AOMOMAT X2CMAT"

and set the keyword combination as indicated below under the \*\*HAMILTONIAN input deck::

    **HAMILTONIAN
    .X2C
    *X2C
    .mmf-restart

In order to run a (MR)CC calculation add also the following keyword(s) to the namelist RELCCSSD input::

    *CCSORT 
    .USEOE
    .NORECMP
    END


or the new RELCCSD deck input::

    *CCSORT
    .NORECMP

Dirac will automatically read the two-component coefficients from the file DFCOEF and proceed in two-component mode for
any wave function analysis or post-HF step required for a (MR)CC calculation.