Diabatic Many-Body Expansion: Development and Application to Charge-Transfer Reactions.

We explore the convergence of the many-body expansion for a prototypical hole-transfer reaction between Zn(0) and Zn(I) in a condensed-phase environment. Poor convergence of state energies is seen when the adiabatic representation is used, which can be understood from the fragment single-point calculations at low orders of the many-body expansion incorrectly localizing charges compared to the full system, thus leading to qualitative errors in the electronic structure of the adiabatic states between fragments. Using a charge-localized representation of the electronic Hamiltonian, we introduce a diabatic many-body expansion method with quantitative accuracy for ground- and excited-state potential energy surfaces of a charge-transfer reaction. Combining with a multiconfigurational self-consistent field affords a fragmentation approach that scales quadratically with system size while retaining chemical accuracy (<1 kcal/mol) in total energies compared to full system calculations.