Now we turn to the cadmium atom in \(D_{2h}\) with the .SPINFREE Hamiltonian. The \(D_{2h}\) double group has two fermionic IRREPS (g and u) leading to XREPS=1,2. By using the .SPINFREE option a real calculation in the ordinary \(D_{2h}\) point group is performed in DIRAC leading to eight symmetries as known from the character table. Therefore we set XREPS=1,2,3,4,5,6,7,8 if we need all symmetries. The relation of the irrep name to the number can be obtained from the Dirac output and is reproduced in the ADC code as well. After the calculation the \(A_{1g}\) final states are the in SSPEC.01, the \(B_{2g}\) in SSPEC.02 asf. The complete spectrum is then the merge of SSPEC.01 ... SSPEC.08. Remember that for the plot we only need the IP and the corresponding pole strength. We therefore do a grep ‘@’ on the merged SSPEC.X files (cat SSPEC.* > SPEC.all) and obtain all lines in each symmetry. If we need only a specific range we do “sort -n SPEC.all > SPEC.range” and edit according to our needs. Then we can use gnuplot with “plot “SPEC.range” u 1:2 w i”. For ADC(3) with constant diagrams and 600 Lanczos iterations we would have an input like this:

**DIRAC
.TITLE
 input for Cd atom
.WAVE F
.4INDEX
**GENERAL
.DIRECT
 1 1 1
**INTEGRALS
*READIN
*TWOINT
.SOFOCK
.SCREEN
1.E-16
**HAMILTONIAN
.SPINFREE
**WAVE FUNCTIONS
.SCF
.RELADC
*SCF
.CLOSED
 30 18
.FCKCNV
5.0E-09
.INTFLG
1 1 1
**MOLTRA
.INTFLG
1 1 1
.CORE
1..8
1..6
.ACTIVE
9..25
7..22
**RELADC
.DOSIPS
.ADCLEVEL
 3
.SIPREPS
 8
 1,2,3,4,5,6,7,8
**LACNZOS
.SIPITER
 600
*END OF