I am interested in calculating IPs (cationic states) corresponding to ionization out of deeply lying orbitals, in particular L-shell orbitals, i.e. 2s of C, N or O, for a large system (about 30 atoms). Specifying very large (for instance 70) number of states to be calculated doesn’t help, as the 2s ionized states lie deeper, and further increasing the number of states leads to a crash of the calculation. Specifying the initial guess also does not help much. I wonder if there is some way/trick to reach those states? Can one use CVS-IP-EOM in this case?
Many thanks for your reply! I did try to use CVS-IP-EOM-CCSD to get ionization out of a C 2s orbital in benzene, having used example 7.66 in section 7.10.8 in the on-line manual, as a test. I first changed
CVS_IP_STATES = [2,1,0,0,0,0,1,2]
to
CVS_IP_STATES = [3,1,0,0,0,0,1,2]
hoping to get ionization out of the lowest C 2s of Ag symmetry. However, what I got was a C 1s-ionized satellite state (corresponding to ionization out of a C1s plus excitation involving C1s core orbitals) rather than ionization out of a C 2s orbital. If I understand it right, to get ionization out of a C2s orbital one should include C 2s orbitals in the “frozen core” set, that is, increase the number of “n_frozen_core”, which is 6 by default. I tried n_frozen_core=8 to include two C 2s orbitals. This indeed gave me the states with ionization out of a C 2s orbitals. However, the increased number of frozen core orbitals also affected the CCSD (ground state) calculation, in a way, that the lowest C 2s orbitals were not included in the correlation for the CCSD ground state, which is not good.
I wonder if it is possible to keep the number of core-frozen orbitals for the CCSD calculation by default, i.e. n_frozen_core=fc, but then increase it for the CVS-IP-EOM-CCSD calculation?