The extended Koopmans' theorem: vertical ionization potentials from natural orbital functional theory

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate prediction of molecular electronic properties is crucial in chemistry.
• The Piris natural orbital functional (PNOF5) is a method for electronic structure calculations.
• Koopmans' theorem provides a theoretical link between ionization potentials and orbital energies.

Purpose of the Study:

• To evaluate the performance of the PNOF5 method for predicting vertical ionization potentials (IPs) and electron affinities (EAs).
• To assess the accuracy of PNOF5 for a diverse set of organic and inorganic molecules and radicals.
• To investigate the impact of basis set size on the accuracy of PNOF5 predictions.

Main Methods:

• Vertical ionization potentials were calculated using the extended Koopmans' theorem with PNOF5.
• Electron affinities were estimated as the inverse of ionization potentials of anionic species.
• Calculations were performed using Dunning's cc-pVXZ and aug-cc-pVXZ basis sets to assess basis set limit effects.
• Statistical analysis, including mean absolute errors (MAEs), was used to compare calculated and experimental values.

Main Results:

• PNOF5 successfully predicted vertical ionization potentials for 30 spin-compensated molecules.
• Electron affinities were accurately estimated for 10 selected radicals.
• Basis set convergence was studied using cc-pVXZ and aug-cc-pVXZ series.
• The MAEs for calculated IPs and EAs were found to be less than 0.5 eV, indicating satisfactory agreement with experimental data.

Conclusions:

• The PNOF5 method, combined with the extended Koopmans' theorem, provides a reliable and accurate approach for predicting ionization potentials and electron affinities.
• The computational efficiency and accuracy of PNOF5 make it a valuable tool for theoretical chemistry research.
• The study confirms the satisfactory performance of PNOF5 for a wide range of molecular systems.