Performance of multi-configurational calculations for a 1,4-bis(phenylethynyl)benzene derivative conjugated molecule

The theoretical challenge of finding a single method that quantitatively reproduces both the experimental low-lying excitation energies and the torsional barrier of a prototypical conjugated molecule, which could act as a molecular wire, has been addressed here. The results indicate that this goal can be reasonably achieved when multi-reference perturbation theory up to second order (MRMP2) based on a complete active space self-consistent field (CASSCF) wave function using large active spaces is used. The results obtained were also used to compare with less expensive Kohn-Sham (KS) density functional theory (DFT) calculations when applied to these properties. The results obtained with BLYP and B3LYP exchange-correlation functionals indicate that quantitative agreement with all the experimental data cannot be obtained with this methodology, with a clear dependence on the exchange-correlation form selected. We thus encourage a careful testing of pure and hybrid density functionals whenever KS DFT is used for the rational design of conjugated materials for charge conduits.