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On The Vertical And Adiabatic Excitation Energies Of The 21a(G), State Of Trans-1,3-Butadiene, Jason Lappe '00, Robert J. Cave

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The excitation energy to the 2¹A_g 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 2¹A_g state relative to the HOMO → LUMO excitation (1¹B_u) 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 …