An optimal tuning perspective of range-separated double-hybrid functionals

We study the tuning of the free parameters of range-separated double hybrid functionals, based on the exact conditions of piecewise linearity and spin constancy. We find that introducing range-separation in both exchange and correlation allows for the minimization of both fractional charge and fractional spin errors for singlet atoms. The optimal set of parameters is system specific, underlining the importance of the tuning procedure. We use the tuned functionals to compute dissociation curves of diatomic molecules. We recover the correct dissociation curve for the one-electron system, H₂⁺.We also find improvement in the dissociation curves of H₂ and Li₂, but they still exhibit a nonphysical maximum before approaching the correct dissociation limit at very large distances.