Similarity transformed equation of motion coupled-cluster study of excitedstates of selected azabenzenes

The recently developed similarity transformed equation-of-motion coupled-cl uster method (STEOM-CC) is applied to calculate the vertical excitation spe ctra and various 0-0 transitions of selected azabenzenes. The primary purpo se of these studies is to gain further experience with the STEOM-CCSD appro ach, and to gauge its accuracy. It is shown that the accuracy of the method exceeds the accuracy of EOM-CCSD, and that results for valence excited sta tes are often close to EOM-CCSD((T) over tilde). A number of low lying 0-0 transitions are determined in pyridine and the diazines. The geometries and vibrational frequencies of excited states are determined at the CI singles level, while final electronic energies are calculated through single point STEOM-CCSD. The manifold of excited states, ionized and electron attached states are analyzed as the geometries are distorted from the ground state e quilibrium structures. (C) 1999 Elsevier Science B.V. All rights reserved.