Projected-interacting configuration interaction: DFT-inspired wave function theory for huge active spaces

Entangled atoms, exemplified by the two singlet-coupled atoms in a dissociating nitrogen molecule, are key to the unique properties of qubits, quantum sensors, and other quantum materials. Simulating entangled atoms and strong electron correlation is a prerequisite for modeling quantum materials and remains an outstanding challenge for theory. I introduce a correlated wave function approach capable of simulating strong electron correlation in large active spaces. Projected-interacting full configuration interaction plus density functional theory PiFCI+DFT combines near-exact correlated wave functions of multiple partially interacting model systems, each corrected by a formally exact density functional. This approach can access large active spaces and visualize entanglement and strong correlation while maintaining competitive accuracy for molecular properties. I demonstrate PiFCI+DFT for general main-group chemistry, benchmark dissociation curves, and a model for a large entangled-atom system: the spin-singlet ground state of a square lattice of 100 nitrogen atoms. This system’s 300-electron active space is out of reach for existing ab initio multireference methods. PiFCI+DFT accurately recovers the strongly correlated dissociation limit at a modest computational cost, motivating further development for modeling quantum materials.