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How Bond Length Alternation and Thermal Disorder Affect the Optical Excitation Energies of π-Conjugated Chains: A Combined Density Functional Theory and Molecular Dynamics Study.

We dissect the sources of error leading to inaccuracies in the description of the geometry and optical excitation energies of π-conjugated polymers. While the ground-state bond length alternation is shown to be badly reproduced by standard functionals, the recently adapted functionals PBEh* and ωPBE* as well as the double hybrid functional XYGJ-OS manage to replicate results obtained at the CCSD(T) level. By analysis of the bond length alternation in the excited state, a sensitive dependence of the exciton localization on the long-range behavior of the functional and the amount of Hartree-Fock exchange present is shown. Introducing thermal disorder through molecular dynamics simulations allows the consideration of a range of thermally accessible configurations of each oligomer, including trans to cis rotations, which break the conjugation of the backbone. Thermal disorder has a considerable effect when combined with functionals that overestimate the delocalization of the excitation, such as B3LYP. For functionals with a larger amount of exact exchange such as our PBEh* and ωPBE*, however, the effect is small, as excitations are often localized enough to fit between twists in the chain.