PW86-PW91 density functional calculation of vertical ionization potentials: Some implications for present-day functionals

A total of 181 vertical ionization potentials (VIPs) of 41 molecules were c alculated by density functional theory (DFT) employing the Perdew-Wang 1986 (PW86) exchange and Perdew-Wang 1991 (PW91) correlation functionals and us ing the aug-cc-pV5Z basis and experimental ground-state geometries. The ove rall average absolute deviation (AAD) from experiment was found to be 0.55 eV and only 0.31 eV for linear molecules but 0.86 eV for nonplanar molecule s. A number of VIPs were in error by over 2 eV. In particular, DFT performe d most poorly when ionization was from an orbital with highly varying densi ty gradients (which arise from the orbital's shape or compactness or throug h its density being distributed over a number of atoms). Indications are th at many or all present-day functionals suffer from the same failings. (C) 2 001 John Wiley & Sons, Inc.