Fig. 2 shows the optimum structure found for the cyclic tetramer. With four molecules in the cluster, the distortion from T-shaped pair arrangements to form a ring is less than that in the trimer; however, the energetic advantage of a linear approach of HCN to an acetylene cannot be realized in this cluster. Even so, the stability of this cyclic form is strong enough that the addition of another acetylene, according to the MMC model potential, does little to distort the ring. The extra
acetylene simply finds a favorable orientation in a location above the plane of the 4-membered ring (Fig. 2) in the MMC global minimum structure of HCN–(HCCH)4.