Q. How can we understand the SAC-CI wave function in the output?

 

A. The SAC-CI operators are defined as excitations, ionizations, and electron attachments from the closed-shell SAC reference function. In singlet case, the output looks like

 

Single excitation         5   6     0.70

Double excitation        5   6    4   7   0.31             5   7    4   6   -0.32

                                  5   6    5   7   0.11      

 

The output shows that a single excitation from the 5th to 6th MO has coefficient of 0.70. The double excitation from the 4th and 5th MOs to 6th and 7th MOs is also a main configuration.. In this case, the electrons in 5th and 6th MOs have a singlet-coupling, and the electrons in 4th and 7th MOs make another spin-coupling. The excitation operator “5   7    4   6” and “5   6    4   7” has the same spatial occupation, but different spin-coupling. The two configurations span complete spin space. See the SAC-CI guide for the details.

In the CationDoublet case, the output looks like

 

###  1-st  ###

Single excitation           5    0.90

Double excitation          5   6    4   0.17       4   6    5     -0.15     

###  2-nd  ###

Single excitation           4    0.86

Double excitation          5   6    4   0.27       4   6    4     -0.12       

 

In the first solution, the single excitation “5” means the ionization from 5th MO. The double excitation “5   6    4” stands for the singlet excitation from 5th to 6th MO and the ionization from 4th MO.