Systematic design of active spaces for multi-reference calculations of singlet–triplet gaps of organic diradicals, with benchmarks against doubly electron-attached coupled-cluster data

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S. J. Stoneburner, J. Shen, A. O. Ajala, P. Piecuch, D. G. Truhlar, and L. Gagliardi, Systematic design of active spaces for multi-reference calculations of singlet–triplet gaps of organic diradicals, with benchmarks against doubly electron-attached coupled-cluster data, J. Chem. Phys., 2017, 147, 164120. DOI: 10.1063/1.4998256