Abstract
Following a brief outline of various developments and exploitations of the unitary group approach (UGA), and its extension referred to as Clifford algebra UGA (CAUGA), in molecular electronic structure calculations, we present a summary of a recently introduced implementation of CAUGA for the valence bond (VB) method based on the Pariser‐Parr‐Pople (PPP)‐type Hamiltonian. The existing applications of this PPP‐VB approach have been limited to groundstates of various π‐electron systems or, at any rate, to the lowest states of a given multiplicity. In this paper the method is applied to the low‐lying excited states of several archetypal models, namely cyclobutadiene and benzene, representing antiaromatic and aromatic systems, hexatriene, representing linear polyenic systems and, finally, naphthalene, representing polyacenes.
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