Frozen natural orbitals for ionized states within equation-of-motion coupled-cluster formalism
The frozen natural orbital (FNO) approach, which has been used in ground state coupled-cluster calculations, is extended to open-shell ionized electronic states within equation-of-motion coupled-cluster (EOM-IP-CC) formalism. FNOs enable truncation of the virtual orbital space significantly reducing the computational cost with a negligible decline in accuracy. Implementation of MP2-based FNO truncation scheme within EOM-IP-CC is presented and benchmarked using ionized states of water, water dimer, nitrogen, and uracil dimer. The results show that the natural occupation threshold (i.e., percentage of the total natural occupation recovered in the truncated virtual orbital space) provides a more robust truncation criterion as compared to the percentage of virtual orbitals retained. Employing 99% - 99.5% of the natural occupation threshold, which results in the virtual space reduction by 70% - 30% in correlated calculations, yields errors below 1 kcal/mol. Moreover, the ionization energies (IEs) computed by EOM-IP-CC with singles and doubles (EOM-IP-CCSD) exhibit linear dependence as a function of the percentage of the natural occupation retained allowing extrapolation to the full virtual space values. The capabilities of the new method are demonstrated by calculation of the twelve lowest vertical IEs and the lowest adiabatic IE of guanine. In addition to IE calculations, we present the scans of potential energy surfaces (PESs) for ionized water dimer. The scans demonstrate that the FNO truncation does not introduce significant non-parallelity errors and accurately describes PESs shapes and the corresponding dissociation energies.