Quantum chemical calculations of electron affinities of alkaline earth metal atoms (Ca, Sr, Ba, and Ra).

This article focuses on high-level ab initio quantum chemical calculations to accurately determine the electron affinities (EAs) of alkaline earth (AE) metal atoms—calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra)—which are known to have notably small EAs. The study demonstrates that the commonly used coupled-cluster singles and doubles with perturbative triples [CCSD(T)] method is insufficient for precise EA calculations of these atoms, necessitating inclusion of higher-order excitations via the CCSDT(2)Q method, along with spin–orbit coupling (SOC) and core–valence correlation effects. Incorporating SOC corrections calculated through a Dirac–Hartree–Fock CCSD(T) approach and additional diffuse basis functions yields EA values for Ca, Sr, and Ba that closely match experimental data within chemical accuracy (±4 kJ/mol). For Ra, lacking reliable experimental data, the study proposes an EA of 9.88 kJ/mol based on these advanced computational methods. The findings provide a comprehensive theoretical benchmark for AE metal atom EAs and encourage further experimental investigations, particularly for Ra.