P. Schwerdtfeger and H. Silberbach were the first to succeed in solving integrals of the operator (1) between Cartesian Gaussian basis functions in full generality. Their derivation distinguishes different subcases which are solved separately. As a side node we would like to mention that Smit has presented an alternative formulation, which makes use of the McMurchie-Davidson recurrence relations. While this recursive approach is expected to be more efficient than Schwerdtfeger's, we are not aware of any published implementation.