Constrained CASSCF and Tight-Binding Calculations in Q-Chem

Learn about advanced quantum chemistry methods for studying electronic structure in open quantum environments through this 57-minute webinar presented by Xinchun Wu. Explore the constrained CASSCF (Complete Active Space Self-Consistent Field) method, which addresses the challenge of studying electron transfer in systems with fractional charges by providing smooth ground and excited potential energy surfaces while avoiding unwanted internal excitations. Discover how this method excels in nonadiabatic systems, particularly in strong coupling regions, through its innovative approach of optimizing both charge states simultaneously. Examine the algorithmic modifications implemented in Q-Chem software to enable constrained orbital optimization. Delve into the integration of tight-binding calculation features within Q-Chem, understanding how this hybrid approach combines ab initio calculations with empirical models by working with different classes of basis functions and evaluating Hamiltonian components at varying levels of accuracy. Gain insights into embedding and approximation strategies essential for simulating large open quantum systems, and understand how tight-binding models can approximate two-electron interactions effectively.