Using Valence Bond Theory to Compute and Conceptualize Excited States

Chapter 7 describes semiempirical and nonempirical VB calculations of radical- and ionic-excited states for molecules starting from H 2 and on to polyenes. It shows that the VB results are at par with CASPT2 and CIS data. VB calculations (e.g., BOVB) require small numbers of VB structures (see Chapter 12), while CASPT2 and MRCISD require up to hundreds of million configurations. Generally, the lowest excited states of large molecules are covalent. Higher ones are monoionic followed by states of increased ionicity (see Scheme 6.1). The chapter shows the states for H 2 , and for the analogous states of a π-bond, for which the first excited state is the resonating ionic state. Excited states of H 3 and allyl are described and generalized for larger radicals. This is followed by derivations of the excited-states for benzene, cyclobutadiene, naphthalene, and polyacenes. The exercises and their solutions are insightful.