Benchmark of Electronically Excited States for Semiempirical Methods: MNDO, AM1, PM3, OM1, OM2, OM3, INDO/S, and INDO/S2.

Semiempirical configuration interaction (CI) calculations with eight different Hamiltonians are reported for a recently proposed benchmark set of 28 medium-sized organic molecules. Vertical excitation energies and one-electron properties are computed using the same geometries as in our previous ab initio benchmark study on electronically excited states. The CI calculations for the standard methods (MNDO, AM1, PM3) and for the orthogonalization-corrected methods (OM1, OM2, OM3) include single, double, triple, and quadruple excitations (CISDTQ) using the graphical unitary group approach (GUGA) as implemented in the MNDO code. The CIS calculations for the established INDO/S method and the reparametrized INDO/S2 variant employ a modified version of the ZINDO program. As compared to the best theoretical reference data from the ab initio benchmark, all currently applied semiempirical methods tend to underestimate the vertical excitation energies, but the errors are much larger in the case of the standard methods (MNDO, AM1, PM3). Overall, the mean absolute deviations relative to the theoretical best estimates are lowest for OM3, and only slightly higher for OM1 and OM2 (in the range of 0.4-0.5 eV). INDO/S performs similar to OM2 for the vertical excitation energies of singlet states, but deteriorates considerably for triplet states. The INDO/S2 reparametrization for oxygen improves the results for low-lying singlet states of oxygen-containing compounds, but makes them worse for high-lying singlets as well as for triplets. The ab initio reference data for oscillator strengths and excited-state dipole moments are again best reproduced by the orthogonalization-corrected approaches (OM1, OM2, OM3), which thus emerge as the most favorable semiempirical methods overall for treating valence excited states of large organic chromophores.