Computational study of vertical ionization potentials using density functional theory and Green's function methods

Over one hundred vertical ionization potentials (VIPs) were computed using density functional theory (DFT) and Green's function (GF) based methods. Th e DFT approaches include the unrestricted transition state (uTS) and unrest ricted diffuse ionization (uDI) approximations using the Becke88-Perdew86 e xchange-correlation functional. Green's function methods include the outer- valence GF (OVGF) approach, the parametrized GF2 (pGF2), and the parametriz ed GF2 times screened interaction (pGW2) approximations. DFT computations o f IPs using the uTS approximation was found to be nearly as accurate as tho se predicted using the elaborate OVGF method. The much more computationally efficient uDI approximation provides predictions of moderate accuracy and is recommended for computing IPs for larger molecules. We have observed tha t the average absolute deviations from a uDI calculation using poorer basis set (DZVP) and poorer geometry (AM1 optimization) is only slightly larger.