Orthogonally Constrained Orbital Optimization: Assessing Changes of Optimal Orbitals for Orthogonal Multireference States.

The choice of molecular orbitals is decisive in configuration interaction calculations. In this letter, a democratic description of the ground and excited states follows an orthogonally constrained orbital optimization to produce state-specific orbitals. The approach faithfully recovers the excitation energy of a four-electron Hubbard trimer, whereas state-average calculations can miss the value by a factor 2.5. The method emphasizes the need for orbital optimization to reduce configuration interaction expansions and to reach spectroscopic accuracy.