The calculated stabilities of the HCN–HCCHÞ2 cluster structures are given in Table 2. Also given are the 3-body contributions. These values have been obtained by taking the stability of a trimer structure (with counterpoise correction for the ab initio values) and subtracting the pair interactions (with counterpoise correction) for each pair of monomers held in the positions that they have in the trimer. The three-body contributions are small but noticeable. They are stabilizing for two of the structures, but destabilizing for the two that are least stable. It is interesting that in the absence of three-body effects, structure I would be only 24 cm1 more stable than structure II. Clearly, the structural distortion from T-shaped and linear pair arrangements needed to form a cyclic trimer gives up considerable pair interaction strength, such that the
cyclic form is only slightly more stable than thequadrupole interactions which are similar, whether HCN or HCCH is the partner.
the cc-pVTZ/MP2 and aug-cc-pVTZ/MP2 values;
i.e., the error is of the size of what may be the
errors in cc-pVTZ/MP2 values from lingering basis
set deficiency. However, the MMC stability differences
are in the other direction (undervaluing).
The effect of counterpoise correction [23], isolated
in values collected in Table 1, remains important
even with the aug-cc-pVTZ basis. The results of
the DFT calculations are about midway between
cc-pVDZ/MP2 and cc-pVTZ/MP2 results.