An Orthogonal Electronic State View on Charge Delocalization and Transfer

We present a configuration interaction (CI) framework in which the electronic Hamiltonian is expressed in a basis of charge-localized determinants. This is used to independently generate adiabatic CI states and charge-localized CI states, both of which are unambiguously defined through a diagonalization procedure. The CI framework offers a simple interpretation of adiabatic states as resonance hybrids of different electron distributions, providing a simple picture for discussing charge delocalization in chemical bonding. The charge-localized states serve as a convenient orthogonal representation of initial and final states in electron transfer processes and provide a definition of their electronic coupling. These two models enable an analysis of the water dimer hydrogen bond. While there has been a longstanding debate on the amount of charge transfer in a water dimer, the pertinent question is the importance of ionic contributions to the wave function. We demonstrate that although the overall charge transfer is small (on the millielectron scale), the occurrence of particular ionic contributions is crucial to get the correct potential energy surface.