Practical Density Functional Theory Beyond the Zero-Sum Limit: M11plus

Conventional approximate DFT functionals are based on what exchange would be a nearly homogeneous electron gas. This model effectively uses like-spin exchange to model opposite-spin correlation, producing a "zero-sum" tradeoff in performance for some classes of problems, including bond energies vs. barrier heights or valence vs. Rydberg excitations. We argue that including new ingredients in a functional can provide beyond-zero-sum broad accuracy. We demonstrate this by adding a new ingredient to the flexible M11 long-range-corrected hybrid meta functional form. The new ingredient is an inexpensive rung-3.5 bound to the exact exchange energy density. The M11 form was reoptimized with these terms, producing a functional called M11plus. Tests for thermochemistry, kinetics, and response properties suggest M11plus is one of the most broadly accurate occupied-orbital-only DFT functionals available to date.