Towards Chemical Accuracy in Atomic Ionization Energies: The Case of H, C, N, O, F, P, and S Atoms

Accurate ionization energies are essential for understanding electronic structures of atoms and molecules, benchmarking quantum-chemical methods, and modeling ioni-zation processes in chemical and biological systems. In this work, we report calculated ionization energies of the H, C, N, O, P, and S atoms using a range of quan-tum-chemical approaches, aiming at reproducing the experimental values within the chemical accuracy. The methods include the electron propagator approximations OVGF and P3+, the coupled-cluster methods CCSD(T), CCSDT, and IP-EOM-CCSD, and the composite methods G3 and CBS-QB3. The CCSD(T), CCSDT, G3, and CBS-QB3 methods, together with the DFT method with B2PLYP density functional and several post-Hartree-Fock methods, were used in conjunction with the energy-difference (ΔSCF) approach. The coupled-cluster calculations were combined with the aug-cc-pVXZ-DK, aug-cc-pVXZ, and ANO-RCC basis sets, all-electron correlation, DKH2 scalar relativistic corrections, and extrapolation to the complete basis set (CBS) limit. The OVGF and P3+ methods do not reach chemical accuracy on average, while CCSD(T) and CCSDT combined with the aug-cc-pVXZ-DK basis set and CBS extrapolation achieve chemical accuracy for all atoms. CCSD(T)/aug-cc-pVXZ-DK with CBS extrapolation provides the best compromise between accuracy and computational cost, and can be used as a reference for these atomic ionization energies.