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 simple, recommended, one-step appraoch to a X2Cmmf calculation 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 step than in the 4-index/CC calculation. In order to proceed run the SCF first while adding .X2Cmmf to the **HAMILTONIAN keyword section:

**HAMILTONIAN
.X2Cmmf

Save the file DFCOEF (keyword –outcmo with our default submit script pam). Before restarting calculation to proceed with the 4-index transformation (MOLTRA) and the (MR)CC run, remove the keyword .X2Cmmf from the **HAMILTONIAN section (seems non-intuitive (!)) and add the following keywords to the **MOLTRA and either the namelist RELCCSSD input:

**HAMILTONIAN
**MOLTRA
.INTFL2
1 0 0 0
.INTFL4
1 0 0 0
...
...
&CCSORT USEOE=T, NORECMP=T &END

or the new RELCCSD deck input:

**MOLTRA
.INTFL2
1 0 0 0
.INTFL4
1 0 0 0
**RELCC
...
...
*CCSORT
.NORCMP

Dirac will automatically read the two-component coefficients from the file DFCOEF (to add it to the run use –incmo on the pam command line) and proceed in two-component mode with the 4-index integral transformation and (MR)CC calculation, respectively.