Reducing the propensity for unphysical wavefunction symmetry breaking in multireference calculations of the excited...

Unphysical spatial symmetry breaking in multiconfigurational self-consistent field calculations can lead to undesirable artifacts in the potential energy surfaces and electronic properties of molecules. Herein, we report several examples of such symmetry breaking in calculations of the excited states of small semiconductor clusters and related molecules at the state-averaged complete active space self-consistent field (SA-CASSCF) level of theory. A multireference approach is proposed to reduce its incidence: the singly excited active space complete active space configuration interaction (SEAS-CASCI) method. In SEAS-CASCI, the orbitals are determined by variationally minimizing an energy expression that does not depend on the off-diagonal Hamiltonian matrix elements which drive symmetry breaking at the SA-CASSCF level of theory. By application to several highly symmetric molecules, SEAS-CASCI is demonstrated to reduce the propensity for unphysical spatial symmetry breaking and eliminate resulting errors in the potential energy surfaces and molecular properties relative to the SA-CASSCF description. The SEAS method is also found to eliminate unphysical wavefunction distortion in asymmetric molecules. Finally, SEAS-CASCI is demonstrated to accurately describe the biradicaloid region of the potential energy surface of ethylene.