Untangling Multireference Electronic Structure Problems using the STP-DAS Framework

Multireference methods are used to describe the electronic structure of atoms and molecules that cannot be captured by a single electronic configuration. The small tensor product distributed active space (STP-DAS) configuration interaction (CI) framework allows for the development of methods to treat multireference problems with greatly reduced computational and storage requirements. In this work, we will highlight modern developments using the STP-DAS framework including complete active space self-consistent field (CASSCF) and multireference CI (MRCI) methods. We explore these methods in the context of relativistic quantum methods using the exact two-component (X2C) transformation. We will also discuss extensions of this work to the time domain and to capture missing dynamic correlation. These contributions allows for the accurate description of large relativistic quantum-mechanical systems in which static correlation plays a significant role.