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Recent years have seen an explosion of interest in organic radicals due totheir promise for highly efficient organic light-emitting diodes (OLEDs) andmolecular qubits. However, predicting their electronic structure hashistorically been challenging, especially for excited states, due tospin-contamination. Computationally inexpensive methods commonly use anunrestricted basis, leading to highly spin-contaminated excited states, whereashigh-level methods such as Multiconfigurational SCF and Full-CI can giveaccurate, spin-pure results but are very computationally expensive. In thisarticle we task ourselves with finding the simplest possible method to computespin-pure excited states of radicals accurately. To do this, we note previoussuggestions that adding a small number of double excitations into a radical CIScalculation could resolve the spin-contamination problem and combine this ideawith the semi-empirical approach of Pariser-Parr-Pople (PPP) theory. To oursurprise, this method, which we call ExROPPP (Extended Restricted Open-shellPPP theory), produces excited state energies and oscillator strengths withsimilar accuracy to experimental results as high-level GMC-QDPT for a series ofhydrocarbon radicals and at minimal computational cost. Furthermore, byderiving Hamiltonian and dipole moment matrix elements we obtain a series ofrules for the excited states of all alternant hydrocarbon radicals which shouldaid molecular design. These results pave the way for the highly accurate andefficient computation and prediction of the excited states of organic radicals.

Fast, Accurate and Spin-Pure Calculation of the Low-Lying Electronically Excited States of Organic Hydrocarbon Radicals