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Tracking Excited States in Wave Function Optimization Using Density Matrices and Variational Principles
journal contribution
posted on 2019-08-23, 10:13 authored by Lan Nguyen Tran, Jacqueline A. R. Shea, Eric NeuscammanWe
present a method for finding individual excited states’
energy stationary points in complete active space self-consistent
field theory that is compatible with standard optimization methods
and highly effective at overcoming difficulties due to root flipping
and near-degeneracies. Inspired by both the maximum overlap method
and recent progress in excited-state variational principles, our approach
combines these ideas in order to track individual excited states throughout
the orbital optimization process. In a series of tests involving root
flipping, near-degeneracies, charge transfers, and double excitations,
we show that this approach is more effective for state-specific optimization
than either the naive selection of roots on the basis of energy ordering
or a more direct generalization of the maximum overlap method. We
provide evidence that this state-specific approach improves the performance
of complete active space perturbation theory for vertical excitation
energies. Furthermore, we find that the state-specific optimization
can help avoid state-averaging-induced discontinuities on potential
energy surfaces. With a simple implementation, a low cost, and compatibility
with large active space methods, the approach is designed to be useful
in a wide range of excited-state investigations.