Nucleus–electron correlation revising molecular bonding fingerprints from the exact wavefunction factorization

Abstract

We present a novel theory and implementation for computing coupled electronic and quantal nuclear subsystems on a single potential energy surface, moving beyond the standard Born–Oppenheimer (BO) separation of nuclei and electrons. We formulate an exact self-consistent nucleus–electron embedding potential from the single product molecular wavefunction and demonstrate that the fundamental behavior of the correlated nucleus–electron can be computed for mean-field electrons that are responsive to a quantal anharmonic vibration of selected nuclei in a discrete variable representation. Geometric gauge choices are discussed and necessary for formulating energy invariant biorthogonal electronic equations. Our method is further applied to characterize vibrationally averaged molecular bonding properties of molecular energetics, bond lengths, and protonic and electron densities. Moreover, post-Hartree–Fock electron correlation can be conveniently computed on the basis of nucleus–electron coupled molecular orbitals, as demonstrated for correlated models of second-order Møllet–Plesset perturbation and full configuration interaction theories. Our approach not only accurately quantifies non-classical nucleus–electron couplings for revising molecular bonding properties but also provides an alternative time-independent approach for deploying non-BO molecular quantum chemistry.