Valence Excited States in Large Molecules via Local Multireference Singles and Doubles Configuration Interaction.

We demonstrate that valence excited states in large molecules can be treated using local multireference singles and doubles configuration interaction (LMRSDCI). The interior eigenvalues corresponding to the excited states of interest are transformed and shifted to the extrema of the spectrum by way of oblique projections and a matrix shift within a modified Davidson diagonalization scheme. In this way, the approximate wave function associated with the excited state of interest can be isolated independently of the lower lying roots, and residual minimization is used for final convergence to the target eigenstate. We find that vertical excitation energies calculated using LMRSDCI are mostly within 0.2 eV of nonlocal MRSDCI values.