Atomistic Modeling of PEDOT:PSS Complexes I: DFT Benchmarking

Poly­(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is a conductive polymer complex integral to both established and next-generation electronic devices. Density functional theory (DFT) and molecular dynamics (MD) studies are increasingly used to probe nanoscale details of this system inaccessible to experiments, but the tools used for these investigations have not yet been thoroughly validated. In Part I of this series, we conduct a benchmarking study of ground-state properties of the PEDOT:PSS system. Predictions by seventeen density functionals (DFs) of geometries, perturbative properties, complexation energies, delocalization error, exciton stability, and torsional barriers are assessed against the double-hybrid DF DSD-PBEP86. We find that the spin contamination of the open-shell PEDOT₃⁺ wave function, a measure of the amount of Hartree–Fock (HF) exchange in a DF, is associated with several properties studied here. The influence of HF exchange on property predictions correlated with its tendency to enhance electron localization. DFs with reduced HF exchange generally yield better vibrational energies, molecular polarizabilities, and torsion barriers. In contrast, LC DFs were necessary to accurately obtain electron delocalization in fractional electron calculations. The use of dispersion corrections more strongly predicts performance in noncovalent complexation benchmarks than that in HF treatment. Systematic errors in exciton stability, obtained through the singlet–triplet energy, are discussed. The generalized gradient approximation (GGA) DF B97-D, hybrid DF HSE06, and LC DF ωB97x-D emerge as the highest-performing functionals in the study. Based on these results, we use a combination of ωB97x-D and DSD-PBEP86 calculations to train an all-atom force field for PEDOT oligomers in Part II of this work.