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On the choice of reference in multi-reference electronic structure theory: minimal references for bond breaking

Minimal reference space definitions are presented for complete active space self-consistent field (CASSCF) and restricted active space self-consistent field (RASSCF) descriptions of bond-breaking reactions and compared to CASSCF wavefunctions using full-valence and one-to-one active spaces. Potential energy curves for reactions breaking both single and multiple bonds are computed using multi-reference perturbation theory and multi-reference configuration interaction based upon these reference wavefunctions, and results are compared to full configuration interaction benchmark curves. This allows one to ascertain the ability of minimal reference spaces to recover the strong non-dynamical correlation effects in these systems by comparison to results with larger reference spaces and to the exact results for a given basis set. Minimal reference spaces, at a fraction of the cost, typically perform on par with much larger reference spaces in computations which also include dynamical electron correlation.