MODERN VALENCE BOND DESCRIPTIONS OF MOLECULAR EXCITED STATES : AN APPLICATION OF CASVB

The valence bond description of excited states is considered in the context of the CASVB approach, with emphasis on the simultaneous optimization of orbitals and structure coefficients for highly general types of wave function, as well as the close connection to the complete active space self-consistent field (CASSCF) method. In addition to fully variational optimization of modern valence bond (VB) wave functions, one may also generate compact representations of CASSCF wave functions, essentially irrespective of their nature. Consequences of generalizing the CASVB methodology to other than the lowest-lying state of a given symmetry are explored for both the overlap- and energy-based optimization criteria. The methodology is illustrated by means of applications to the 21Ag state of trans-1,3-butadiene, the c˜1A1 state of methylene, and the first excited singlet state of benzene. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 637–650, 1998