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 includes three empirical parameters optimized by a best fit to atomization energies. We introduce a related functional, B1LYP, 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 a standard molecular data set 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.