Comparative study of unscreened and screened molecular static linear polarizability in the Hartree-Fock, hybrid-density functional, and density functional models

The sum-over-states (SOS) polarizabilities are calculated within approximate mean-field electron theories such as the Hartree-Fock approximation and density functional models using the eigenvalues and orbitals obtained from the self-consistent solution of the single-particle equations. The SOS polarizabilities are then compared with those calculated using the finite-field (FF) method. Three widely used mean-field models are as follows: (1) the Hartree-Fock (HF) method, (2) the three parameter hybrid generalized gradient approximation (GGA) (B3LYP), and (3) the parameter-free generalized gradient approximation due to Perdew-Burke-Ernzerhof (PBE). The comparison is carried out for polarizabilities of 142 molecules calculated using the 6-311++G(d,p) basis set at the geometries optimized at the B3LYP/6-311G** level. The results show that the SOS method almost always overestimates the FF polarizabilities in the PBE and B3LYP models. This trend is reversed in the HF method. A few exceptions to these trends are found. The mean absolute errors (MAE) in the screened (FF) and unscreened (SOS) polarizability are 0.78, 1.87, and 3.44 angstrom(3) for the HF, B3LYP, and PBE-GGA methods, respectively. Finally, a simple scheme is devised to obtain FF quality polarizability from the SOS polarizability. (C) 2007 Wiley Periodicals, Inc.