Abstract
Using Roothaan's open-shell procedure lowest triplet-state excitation energies are computed for several polyacenes. The results indicate that single determinant wavefunctions constructed from ground-state (closed-shell) SCF orbitals give poor descriptions of the triplet states, leading to errors of up to ½ eV in the energy. The triplet-state orbitals are given. Comparison of these calculations with Pariser's CI computations suggests that significant correlation has been introduced into the CI excited state description, but not into the ground state.
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