Fig. 5.

a) Charge on the central donor molecule (for definition of charge see eqns. 4, with the text below) for all 36 clusters, grouped by donor molecule indices. Acceptor molecule indices for each curve are not shown, because they group tightly enough that they are not important. Note that the charge on the molecule when the two half clusters have separated at large distance is approximately 0.013 to 0.014 charges for an increment of one in X, and about -0.010 to - 0.012 for an increment of 1 in W. The charge on the central molecule does not go to zero at infinite separation (except fortuitously, for the 21 YZ clusters), because of charge transfer within the half cluster. The charge transferred for short bonds is much greater, which is not a surprise. Even at large distance, the difference between having the central molecule being a donor twice and never an acceptor (20YZ), and being an acceptor twice and donor only the one time required by definition (12YZ), is >0.03 electronic charges, which is appreciable.

b) Charge on the half clusters : This time, the charge must go to zero at large distances. The curves appear to cross, because of conflicting effects of donor and acceptors. However, the lowest curve (largest negative charge) again goes with the 20YZ (double donor, no acceptor) cluster, the least negative is the 12YZ cluster. The difference in charge at 2. 9Å is >0.01 e, large enough to be important.