Integration approach at the second-order perturbation theory: applications to ionization potential and electron...

An integration approach is developed to calculate ionization potentials (IPs), and electron affinities (EAs), which is an extension of the D-ΔMBPT(2) method [A. Beste et al., J. Chem. Phys. 2013, 138, 074101]. The latter is an extension of the single-point method of Cohen et al. [A. J. Cohen et al., J. Chem. Theory Comput. 2009, 5, 786] from the perspective of fractional charges. While relaxation effects were included only at the Hartree-Fock (HF) level in the previous methods, such effects are fully taken into account in the present method up to the second-order Møller-Plesset (MP2) level. This is made possible by deriving the full MP2 energy gradient, with respect to the orbital occupation numbers, which is solved through the coupled-perturbed HF (CP-HF) equations.