Improving robustness of fifth-rung functionals with linearized ladder correlation

Strong electron correlation effects due to multiconfigurational character remain a consistent challenge for modern density functional theory (DFT). Despite their prominent position atop Jacob's Ladder at rung 5, double-hybrid functionals are often even more sensitive to strong correlation than lower-rung functionals, casting doubt on their general usefulness for transition-metal complexes. Much of these problems can be traced to the second-order Møller-Plesset (MP2) correlation energy, which is the most common wavefunction-theoretic ingredient in double-hybrid functionals. Herein we introduce a novel, size-consistent double-hybrid functional that uses Coulomb-attenuated linearized hole-hole ladder coupled-cluster correlation as a robust alternative to MP2. The resultant ωB97X-L-V functional performs as well as other traditional double-hybrids on main-group chemistry, can smoothly dissociate covalent bonds, and performs well for reaction energies and bond dissociation energies of transition-metal complexes. Our work sets the stage for further improvements to ωB97X-L-V and future double-hybrid functionals that add many-body screening to the MP2 correlation energy.