A Unified Treatment of Single and Double Electronic Excitations and Corresponding Delocalization Lengths in pi-conjugated Materials

Understanding the lowest excited states of materials is of great interest for investigating biological processes (where light-harvesting complexes act to convert light into chemical energy) and emerging technologies (where organic dyes are used as the active materials). In large linear π-conjugated materials, the lowest-energy singlet state has significant double excitation character, corresponding qualitatively to the promotion of two electrons from an occupied state to an unoccupied state. However, the accurate description of both the single (one-electron) and double (two-electron) excitations for complex materials has been a significant challenge for electronic structure methods. We present the accurate calculation of single and double excitation energies for the prototypical pi-conjugated material polyacetylene using our newly developed functional based on the particle-particle random phase approximation (pp-RPA).This approach allows for several experimental observations to be rationalized and new predictions to be made in regards to the impact of state-dependent correlation lengths on higher-energy excited states.