 He made major contributions to the theory of approximate molecular orbital (MO) calculations, starting with one identical to the one developed by Rudolph Pariser and Robert G. Parr on pi electron systems, and now called the Pariser-Parr-Pople method.Subsequently, he developed the methods of Complete Neglect of Differential Overlap (CNDO) (in 1965) and Intermediate Neglect of Differential Overlap (INDO) for approximate MO calculations on three-dimensional molecules, and other developments in computational chemistry.In 1970 he and David Beveridge coauthored the book "Approximate Molecular Orbital Theory" describing these methods.