Koopmans' approximation applied in atoms and diatomic molecules using the diffusion quantum Monte Carlo method

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Koopmans' theorem was applied to generate trial wave functions for ions from neutral systems and further applied in the determination of vertical ionization energies using the diffusion quantum Monte Carlo (DQMC) method. The results for successive and conventional ionization energies for atoms and diatomic molecules using the Slater minimum basis set with polarization wave functions indicate that DQMC provides a significant quantity of relaxation and electronic correlation effects. The conventional ionizations for core electrons calculated with DQMC for atoms and diatomic molecules yielded an average deviation of around 2–3 eV from the experimental results, which is excellent when compared with a deviation of around 17 eV estimated from Koopmans' theorem.