Reaching for Transition-Metal Chemical Accuracy by Adding Ladders to Fifth-Rung Density Functionals

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 use adiabatic connection to define a size-consistent, non-empirical double-hybrid functional that uses Coulomb-attenuated linearized hole-hole ladder coupled-cluster correlation as a robust alternative to MP2. The resultant ωB97L-V functional consistently outperforms other non-empirical double-hybrids on main-group chemistry, can smoothly dissociate covalent bonds, and performs exceptionally well for reaction energies of transition-metal complexes. Furthermore, ωB97L-V is the first functional to achieve sub-kcal/mol accuracy for the metal-organic reaction barrier heights in the MOBH28 data set. The results of our simple parameterization hold implications for functional design, suggesting that short-range correlation may be a widely-overlooked ingredient for improving the performance of double-hybrid DFT on transition-metal complexes. Our work sets the stage for further improvements to ωB97L-V and future double-hybrid functionals that add many-body screening to the MP2 correlation energy.