The variable-size simplex optimization method is used to reparametrize the I + A and / 3 parameters of an INDO approximation to the perturbed Hartree-Fock calculation of 13C chemical shifts in hydrocarbons. The absolute shifts for 39 nuclei in a set of molecules containing up to four carbons are reproduced within a standard error of 9.9 ppm for an unconstrained optimization and to a standard error of 10.0 ppm for an optimization constrained to yield gross atomic charges in agreement with double-zeta ab initio calculations.
INTRODUCTIONSecond-order properties such as nuclear spinspin couplings and chemical shifts are sensitive probes of the electronic structure of molecules in that variations in these quantities often indicate subtle electronic changes. To relate such observed changes to the proper physical phenomenon it is, of course, necessary to have a theory capable of reproducing these second-order effects as well as the equally important lower-order properties of
Authoritative values for the parameters of harmonic thermal motion have been used as criteria for various least-squares refinements of the structure model for beryllium metal. A change in the absolute scale of Brown
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