DEVELOPMENT AND ASSESSMENT OF NEW EXCHANGE-CORRELATION FUNCTIONALS

We recently presented a new method for developing generalized gradient approximation (GGA) exchange-correlation energy functionals, using a least-squares procedure involving numerical exchange-correlation poten tials and experimental energetics and nuclear gradients. In this paper we use the same method to develop a new GGA functional, denoted HCTH, based on an expansion recently suggested by Becke [J. Chem. Phys. 107 , 8554 (1997)]. For our extensive training set, the new functional yie lds improved energetics compared to both the BLYP and B3LYP functional s [Phys. Rev. A 38, 3098 (1988); Phys. Rev. B 37, 785 (1988); J. Chem. Phys. 98, 5648 (1993); J. Phys. Chem. 98, 11623 (1994)]. The geometri es of these systems, together with those of a set of transition metal compounds, are shown to be an improvement over the BLYP functional, wh ile the reaction barriers for six hydrogen abstraction reactions are c omparable to those of B3LYP. These improvements are achieved without i ntroducing any fraction of exact orbital exchange into the new functio nal. We have also re-optimized the functional of Becke-which does invo lve exact exchange-for use in self-consistent calculations. (C) 1998 A merican Institute of Physics. [S0021-9606(98)30938-1].