Calculating core electron ionization energies with Q-Chem

There are two ways to compute ionization energies of core electrons in Q-Chem. The first approach is a simple energy difference calculation in which core ionization is computed from energy differences computed for the neutral and core-ionized state. It is illustrated in the following calculation (click for the output and input). In this job, we first compute CCSD energy of the neutral CH₄.

Note that Koopmans' IE is 11.210 Hartree = 305.03 eV.

In the second job, we do the same for core-ionized CH₄. To get the proper SCF solution, the MOM_START option and the occupied keyword are used.

Thus, ΔE(CCSD) = (40.357481 - 29.647940) = 10.709 Hartree = 291.42 eV.

In the second approach (see output and input), we use EOM-IP to compute core-ionized states. Since core states are very high in energy, we use the "frozen core" trick to eliminate valence ionized states from the calculations. That is, we reorder the MOs such that our core is the last occupied orbital and then freeze all the rest. Thus, we end up freezing all valence occupied orbitals in CC/EOM calculations. The so-computed EOM-IP energy is 245.57 eV.

From the EOM-IP amplitude, we note that this state is of Koopmans' character (dominated by single core ionization); thus, canonical HF MOs provide a good representation of the correlated Dyson orbital.

The accuracy of both calculations can be improved using triples corrections, e.g., CCSD(T) and EOM-IP-CCSD(dT). The use of a better basis that has more core functions is also recommended.