Accurate Core-Level Ionization Energies from an Affordable Second-Order Approach

An approach is proposed for the accurate calculation
of core-level
ionization potentials (IPs) using second-order methods. The assessed
theoretical frameworks are based on the iterative second-order algebraic-diagrammatic
construction [ADC(2)] and configuration interaction singles with perturbative
second-order correction [CIS(D)] methods. Here, our efficient implementations
of IP-ADC(2) and IP-CIS(D) [Mester, D.; Kállay, M. J. Chem. Theory Comput. 2023, 19, 3982–3995] are combined with the core–valence separation (CVS) approximation,
thereby enabling ionization from the core region. The approaches exhibit
highly favorable scaling behavior: the computational cost is practically
cubic, with only a mild prefactor that depends on the number of active
core orbitals. Furthermore, the resulting wave function-based methods
are combined with spin-scaling techniques and successfully extended
to double-hybrid (DH) functionals without any necessary modifications
in the implementation of the second-order corrections. The performance
of the proposed methods was thoroughly assessed in benchmark calculations.
Our results demonstrate that the iterative treatment of double excitations
is essential, underscoring the necessity of the more advanced DH ansatz.
Moreover, the SOS0-PBE0-2/CVS-IP-ADC(2) approach is highly competitive
with more expensive higher-level coupled-cluster methods. Finally,
the second-order correction introduces only a negligible overhead
in the overall computational timeonly about 1 min for a 61-atom
azafullerene molecule using triple-ζ basis setsthereby
enabling accurate calculations for extended molecular systems.