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
```