Q. How to determine number of solution in each symmetry?

 

A. It is better to perform a preliminary calculation before the final calculations. There are several prescriptions.

1. Use of CIS result: The SD-R calculation uses CIS states for the reference state in the perturbation selection. Since CIS results provide qualitative picture, you can determine how many solutions are necessary to obtain the state of interest. In the SAC-CI keyword, “SelectCISOnly” option stops the calculation after the CIS diagonalization and the selection of operators.

2. Use of “NoSymmetry” keyword (non SAC-CI option): This option abandons use of symmetry in the calculations. Specify seemingly enough number of solutions and perform SAC-CI calculation with the “NoSymmetry” option and low-level threshold for the perturbation selection. Looking at the MOs, one can easily find symmetry of MOs and hence symmetry of the excited states. Finally, you determine number of solutions in each irreducible representation. With the use of symmetry, you can perform fine calculation with the much better threshold.