Ab initio calculations have been performed to determine the molecular structure and proton affinity of a set of molecules. The basis sets were developed for pseudopotentials using the GCM procedure, This technique is potentially useful for large molecules for which similar procedures (such as the G2 method and variations) were not feasible. This method achieves performance similar to the G2 method at a lower computational cost, The mean absolute deviation and the mean deviation of the results from experimental are 3.5 and 1.7 kJ mol(-1), respectively, compared with 4.6 and 2.2 kJ mol(-1) for the G2 method.