Complete active-space self-consistent-field (CASSCF) methods are enormously important in electronic structure theory, as they allow for the description of the electronic states that are dominated by more than one electronic configuration. The application of CASSCF to large active spaces requires that one abandon a configuration-interaction-based description of the active space in favor of polynomial-scaling approaches such as the density-matrix renormalization group (DMRG) or variational two-electron reduced-density matrix (v2RDM) methods. We recently developed a v2RDM-driven CASSCF procedure that can treat active spaces with 50 electrons in 50 orbitals and the simultaneous optimization of nearly 2000 orbitals.  Analytic gradients are available for the approach, and v2RDM-CASSCF-optimized bond lengths typically agree with those from configuration-based approaches to within 0.01 Å.  Excited electronic states can also be obtained within the v2RDM-CASSCF framework using an extended random phase approximation.  However, a proper treatment of excitations from degenerate ground states requires the application of pure-state N-representability conditions.

Large-scale v2RDM-driven CASSCF methods

253rd National Meeting (2017)

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