title: Full Configuration Interaction Excitations of Ethene and Butadiene: Resolution of an Ancient Question.
abstract: We employ the recently developed full configuration interaction quantum Monte Carlo (FCIQMC) method to compute the π → π* vertical excitation energies of ethene and all-trans butadiene. These excitations have been the subject of extensive theoretical studies, and their location with respect to the corresponding absorption band maximum is the source of a long lingering debate. Here, we reliably estimate the vertical excitations of ethene and butadiene by performing FCIQMC calculations for spaces as large as 10(18) and 10(29) Slater determinants, respectively. For ethene, we obtain a vertical excitation energy in the range 7.89-7.96 eV, depending on the particular equilibrium ground-state geometry employed, and definitely higher than the absorption maximum located at 7.66 eV. For the computationally more challenging case of butadiene, our calculations provide a robust estimate of about 6.3 eV for this excitation, that is, 0.4 eV higher than the corresponding absorption band maximum. Our FCIQMC excitation energies represent a reliable benchmarking reference for future calculations.
authors.affiliation: MESA+ Institute for Nanotechnology, University of Twente , P.O. Box 217, 7500 AE Enschede, The Netherlands.
authors.forename: Csaba
authors.initials: C
authors.lastname: Daday
authors.affiliation: University of Cambridge , Chemistry Department, Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: Simon
authors.initials: S
authors.lastname: Smart
authors.affiliation: University of Cambridge , Chemistry Department, Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: George H
authors.initials: GH
authors.lastname: Booth
authors.affiliation: University of Cambridge , Chemistry Department, Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: Ali
authors.initials: A
authors.lastname: Alavi
authors.affiliation: MESA+ Institute for Nanotechnology, University of Twente , P.O. Box 217, 7500 AE Enschede, The Netherlands.
authors.forename: Claudia
authors.initials: C
authors.lastname: Filippi