Journal of Chemical Physics, Vol.118, No.18, 8216-8222, 2003
Coupled cluster calculations of the vertical excitation energies of tetracyanoethylene
Coupled cluster linear-response formalism has been used to compute the vertical spectrum of ethylene and tetracyanoethylene (TCNE). We show that for both molecules the pipi(*) excitation (1)A(1g)-->B-1(1u) of the experimental spectrum is not vertical nor the 0-0 transition. For TCNE this excitation is the only experimentally observed band. We have computed vertical excitations of 5.2 eV in gas phase and 5.1 eV in acetonitrile and estimated a lower bound for the 0-0 transition in the gas phase of 4.3 eV. (C) 2003 American Institute of Physics.