Photochemical and excited-state processes in conjugated organic molecules are central to many important physical and biological phenomena, including electrocyclic reactions, photoisomerization and photosynthesis.  Conjugated polyenes are important prototypes to study and in this work we focus on the two lowest-lying singlet excited states of ethylene and trans-butadiene.  We present systematic ab initio benchmarks of the vertical excitation energies using an incremental equation-of-motion coupled-cluster scheme, including an estimate of up to quadruple excitations, as well as explicitly multireference calculations based on the density-matrix renormalization group approach.  We aim to achieve an accuracy of approximately 1 kcal/mol (0.04 eV) and to thereby resolve some of the outstanding questions regarding the comparison of theory with experiment that remain unanswered after more than 40 years of effort in this area.

Predicting the low-lying excited states of ethylene and butadiene to chemical accuracy

244th National Meeting (2012)

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