The monopositive scandium and titanium carbides, ScC + and TiC + , have been studied by multireference methods coupled with quantitative basis sets. We have constructed potential energy curves for the ground and twelve low-lying excited states for both species, focusing on dissociation energies and bonding mechanisms. We have found that the ground states of ScC + and TiC + are of 3 Π and 2 Σ + symmetries, with binding energies (D 0 ) of 68.8 (71.5) and 85.2 (86.7) kcal/mol at the MRCI (MRCI+Davidson correction) level of theory. These values are compared favorably with the corresponding experimental findings, D 0 (ScC + ) ) 77.0 ( 1.4 and D 0 (TiC + ) ) 93.4 ( 5.5 kcal/mol.

We test the recently developed state-specific multireference Brillouin-Wigner coupled cluster ͑MRBWCCSD͒ method against the single reference CCSD method by examining theoretically the competing X 3 ⌺ Ϫ and 5 ⌺ Ϫ states of the ͑experimentally unknown͒ isovalent calcium and zinc carbide diatomics ͑CaC, ZnC͒. At the CCSD level, CaC is ''incorrectly'' predicted to have a ground 5 ⌺ Ϫ state; however, the MRBWCCSD treatment restores the correct state ordering, and improves significantly the energetics for both molecules. Further comparison with various single-and multireference treatments shows that the latter are absolutely necessary for obtaining meaningful results for the ground states in both molecules.

The gas-phase and in-solvent absorption and emission spectra of pyrene, 1,6-dithiapyrene, and tetrathiafulvalene are studied theoretically in the visible spectral region with the complete active space self-consistent field method, the complete active space second order perturbation theory method, and the resolution-of-identity second order perturbative corrected coupled cluster doubles (RICC2) method, with basis sets up to augmented polarized triple-zeta quality. The time-dependent density functional theory (TDDFT) formalism is also used employing a series of functionals. The nature of the excited states is discussed. With respect to literature theoretical values of the absorption and emission wavelengths of these three molecules, substantial improvements are achieved and comparison with experiment is favorable. Moreover, theoretical absorption and emission spectra of 1,6-dithiapyrene are presented for the first time. It is also exhibited that in most cases, a TDDFT treatment with hybrid functionals combined with a modest basis set (6-31G( *)) appears to be capable of providing reliable estimates for absorption and emission in all three molecules with relatively low computational cost. Furthermore, the RICC2 method (standalone or in conjunction with TDDFT) provides a satisfactory ab initio alternative, providing a good compromise between accuracy and computational effort.

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