Toward reliable adiabatic connection models free from adjustable parameters

Methods rooted in the adiabatic connection theorem offer a significant improvement over standard density functional approximations. Among them, the most successful model is the so-called B3LYP functional, which include, three empirical parameters optimized by a best fit to atomization energies, We introduce a related functional, BILYP, in which the ratio between Hartree-Fock and density functional exchange is determined a priori from purely theoretical considerations and no further parameters are present, The numerical results obtained on standard molecular data jet and on some 'delicate' chemical systems indicate that this model has essentially the same performance as the B3LYP model, but within a more satisfactory theoretical framework. (C) 1997 Elsevier Science B.V.