Switching between OPTX and PBE exchange functionals

We report here a study into what causes the dramatic differences between OPBE and PBE for reaction barriers, spin-state energies, hydrogen-bonding and π − π stacking energies. It is achieved by the implementation of a new functional that switches smoothly from OPBE to PBE at a predefined point P of the reduced density gradient s. By letting the point P run as function of the reduced density gradient s, with values from s = 0.1 to s = 10, we can determine which part of the exchange functional determines its behavior for the different interactions. Based on the thus obtained results, we have created a new exchange functional that shows the good results of OPBE for reaction barriers and spin-state energies, and combines it with the good respectively reasonable results of PBE for hydrogen-bonding and π − π stacking. In summary, the new functional combines the best of OPBE with the best of PBE.