Time-Dependent Particle-Breaking Hartree-Fock Model for Electronically Open Molecules

We develop the time-dependent particle-breaking Hartree-Fock (TDPBHF) model to describe excited states and linear response properties of electronically open molecules. This work represents the first step toward building a wave function-based response theory framework for electronically open quantum systems equivalent to that of closed quantum systems. In the limit of particle conservation, TDPBHF reduces to standard time-dependent Hartree-Fock theory. We illustrate the TDPBHF model by computing valence absorption spectra and frequency-dependent electric dipole polarizabilities for a set of small- to medium-sized organic molecules. The particle-breaking interactions are observed to nonuniformly redshift the excitation energies and induce qualitative changes in the absorption profile. In addition, the mixing of multiple excitations in the TDPBHF wave function is observed to dampen the divergence of the response function in the vicinity of resonance energies.