Bridging the Gap Between Molecules and Materials in Quantum Chemistry with Localized Active Spaces

The number of materials that "bridge the gap" between single molecules and extended solids, such as metal-organic frameworks and organic semiconductors, has been increasing. Consequently, there is a growing need for modeling approaches that effectively integrate the real-space molecular perspective employed by computational chemists and the reciprocal-space dispersive perspective employed by computational physicists. Here, we propose the localized active space (LAS) approach as a promising method to successfully bridge this gap. The LAS approach extends the active space concept from multiconfigurational methods such as complete active space self-consistent field theory to multiple molecular fragments via a product-form wave function ansatz. This concept is applied naturally to solid state phenomena by treating each unit cell as a molecular fragment with different sets of local quantum numbers (e.g., charge and excitation number). State interaction between these LAS states (LASSI) thus provides a comprehensive basis for the study of charge and energy transfer. We show how the LAS approach allows for the computation of multiconfigurational band structures. Additionally, we apply the approach to several one-dimensional model systems to demonstrate its capacity to treat sophisticated phenomenon such as excitation at p-n junctions and metal-to-insulator transitions.