A semi-empirical CNDOVSB computer program has been developed to calculate the second-order nonlinear optical polarisabilities of molecules. The program has been parameterised by comparison of calculated and experimental values of molecular properties over a large wavelength range. The use of the program is described, both in the evaluation of the potential of specific compounds and also to study trends in series of related molecules. In particular, the effect of conjugation length on the nonlinear optical properties of polyphenyls and polyenes is described.