AbstractThe variable‐size simplex optimization method is used to reparametrize theI+Aand β parameters of an INDO approximation to the perturbed Hartree–Fock calculation of13C 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‐zetaab initiocalcul