In <i>ab initio </i>electronic structure calculations, the spin symmetry must be broken when treating systems with relativistic effect or some magnetic properties. Such systems often require a reliable description of nondynamical correlation effects at the same time. The variational two-electron reduced-density-matrix (v2RDM) driven methods provide a computationally efficient alternative to the steeply scaled configuration interaction (CI)-based approaches. We have developed a complex implementation of the v2RDM approach with broken spin symmetry for v2RDM computations. Our investigations indicate that the complex spin-broken v2RDM approach has some surprising numerical properties. For example, in atomic systems, constraints on the expectation values of the square of the total orbital angular momentum and its projection on the z-axis yield quite different results for a given L and different M<sub>L</sub> values. It appears that v2RDM computations corresponding to the maximal orbital angular momentum projection provides the best agreement with CI-based computations.

Complex v2RDM driven methods with broken spin symmetries

ACS Fall 2021

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