On the Vertical and Adiabatic Excitation Energies of the 21Ag State of trans-1,3-Butadiene

The excitation energy to the 21Ag state of trans-1,3-butadiene is examined using a variety of ab initio electronic structure techniques. While analogous states have been shown to be the lowest singlet excited states for all longer polyenes, for butadiene the position of the 21Ag state relative to the HOMO → LUMO excitation (11Bu) has been difficult to establish theoretically. We employ a variety of methods (CASSCF, CASPT2, MRSDCI, QDVPT) to examine both the vertical and adiabatic excitation energies for this state. At the ground-state geometry, the vertical excitation energies obtained by CASPT2 and Davidson-corrected MRSDCI for the 21Ag state differ by approximately 0.15 eV, but all of the methods predict that the 21Ag state has a lower 0−0 excitation energy than the 11Bu state. Possible reasons for the discrepancies between the various methods for the vertical excitation energy are discussed.