Next Generation, Explicitly Correlated Electronic Structure Methods
All common wave function methods of electronic structure theory have a fundamental and troublesome flaw: the inability to correctly describe the exact mathematical cusp behavior of many-electron wave functions in the vicinity of coalescence points (Coulomb singularities), and hence to fully account for instantaneous, short-range correlation among electrons. To achieve high thermochemical accuracy, next-generation methodologies were needed that solve the electron cusp problem by explicitly incorporating interelectronic variables (r 12 ) into molecular wave functions. We pursued a long-term research program to develop practical explicitly-correlated methods, to create attendant, freely available, state-of-the-art computer codes, and to undertake chemical applications of unprecedented size. Our work allows conventional CCSD(T) computations to be conjoined with explicitly-correlated second-order perturbation methods within a focal point extrapolation scheme for pinpointing the ab initio limit of electronic structure theory.