# 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 **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.