CASSCF for large active spaces using the STP-DAS framework

The complete active space self-consistent field (CASSCF) method is instrumental to the electronic structure theory of molecules dominated by static correlation. However, accurate CASSCF calculations are limited by an exponential increase in computational and storage requirements as the size of the active space grows. This study introduces the distributed active space self-consistent field method, which uses the small tensor product algorithm (STP) to reduce storage demands drastically. We explore this method in the context of relativistic quantum methods using the exact two-component (X2C) transformation. This contribution allows for the accurate description of large relativistic quantum-mechanical systems in which static correlation plays a significant role.