Performance of Spin-Symmetry-Broken Electronic Structure Methods for Atomic Clusters

Strongly correlated systems demand wavefunctions of high complexity and so are challenging to study in condensed matter physics and quantum chemistry. The emergence of atomic lattices and multispin systems for many-body simulators and quantum computing motivates us to explore methods that reduce the complexity of the wave function by relaxing symmetry constraints. We study linear chains and systems with C2v or D4h symmetry including atoms such as H, Be, N, and Fe. Using as a reference standard complete active space self-consistent field, we analyze the performance of broken-symmetry density functional and wavefunction methodologies. We find that both are promising in closely describing the reference results. In general, the symmetry-breaking description improves the energetics relative to pure mean-field theory, but they alone are insufficient. Our results suggest the need for external post-processing models to enhance the results with respect to the reference simulations.