The ability of certain canonical orbitals of isolated molecular fragments to transfer largely unchanged to the molecular environment is examined. The separation of fragment canonical orbitals from the total molecular electronic problem is compared with the more familiar separations of atomic core orbitals and fragment localized orbitals. The specific example of the carbonyl functional group in formaldehyde is examined in detail. These studies lead to a new concept of valence electron only calculations in which the molecular valence electrons are assumed to move in an effective field provided by frozen molecular fragment canonical cores. In addition, for the case of assumed fragment canonical orbital transfer, perturbation theory analysis is found to be an efficient method of assessing the quality of the approximate wavefunction, thus eliminating much of the uncertainty as new systems are studied. The methods developed in the course of these studies offer certain practical advantages for the construction of approximate wavefunctions for large molecules. The details of application of these concepts to existing molecular orbtial methods are also presented.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry