# I would like to use the 2c-molecular-mean-field Hamiltonian (X2Cmmf) - what do i need to consider?¶

The 2c-molecular-mean-field Hamiltonian (X2Cmmf) is at present available for the following methods:

• Coupled-Cluster
• Fock-space Coupled Cluster (including the intermediate Hamiltonian version)

The remaining modules are not supported yet but patches are planned in the course of 2013:

• 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).

As follows we will consider the necessary inputs for an X2Cmmf calculation where we perform the SCF + post-SCF calculation in one shot or two separate runs:

• one-step procedure:
• add .X2Cmmf to the **HAMILTONIAN keyword section:
**HAMILTONIAN
.X2Cmmf

that’s it. After the SCF calculation the program will proceed with the transformation to 2-components prior to the post-HF steps.

• two-step procedure (EXPERT mode!):
1. *SCF step*. Add .X2Cmmf to the **HAMILTONIAN keyword section:
**HAMILTONIAN
.X2Cmmf

and save the file DFCOEF (keyword –outcmo with our default submit script pam)

2. *post-HF step*. Remove .X2Cmmf from the **HAMILTONIAN section (non-intuitive (!) step which will be solved in Dirac12) and add the following keywords to the **MOLTRA and namelist RELCCSSD input section.

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