  A non-empirical exchange functional based on an interpolation between two
limits of electron density: slowly varying limit and asymptotic limit, is
proposed. In the slowly varying limit, we follow the study by Kleinman in 1984
which considered the response of a free-electron gas to an external periodic
potential, but further assume that the perturbing potential also induces Bragg
diffraction of the Fermi electrons. The interpolation function is motivated by
the exact exchange functional of a hydrogen atom. Combined with our recently
proposed correlation functional, tests on 56 small molecules show that, for the
first-row molecules, the exchange-correlation combo predicts the total energies
four times more accurate than presently available Quantum Monte Carlo results.
For the second-row molecules, errors of the core electrons exchange energies
can be corrected, leading to the most accurate molecular total energy
predictions to date despite minimal computational efforts. The calculated bond
energies, zero point energies, and dipole moments are also presented.
