Optimal Tuning Perspective of Range-Separated Double Hybrid Functionals

We study the optimal tuning of the free parameters inrange-separated double hybrid functionals, based on enforcing the exactconditions of piecewise linearity and spin constancy. Wefind thatintroducing the range separation in both the exchange and the correlationterms allows for the minimization of both fractional charge and fractionalspin errors for singlet atoms. The optimal set of parameters is systemspecific, underlining the importance of the tuning procedure. We test theperformance of the resulting optimally tuned functionals for thedissociation curves of diatomic molecules. Wefind that they recoverthe correct dissociation curve for the one-electron system, H2+, andimprove the dissociation curves of many-electron molecules such as H2and Li2, but they also yield a nonphysical maximum and only converge tothe correct dissociation limit at very large distances