We investigate and test an algorithm suitable for the parallel calculation of the potential energy of a protein, or its spatial gradient, when the protein atoms interact via pair potentials. This algorithm is similar to one previously proposed, but it is more efficient, having half the interprocessor communications costs. For a given protein, we show that there is an optimal number of processors that gives a maximum speedup of the potential energy calculation compared to a sequential machine. (Using more than the optimum number of processors actually increases the computation time). With the optimum number the computation time is proportional to the protein size N. This is a considerable improvement in performance compared to sequential machines, where the computation time is proportional to N2. We also show that the dependence of the maximum speedup on the message latency time is relatively weak. Copyright © 1992 John Wiley & Sons, Inc.