Fig. 1. Minimum energy structures (I–IV) of HCN–HCCHÞ2 obtained from geometry optimization at the cc-pVTZ/MP2 level.
Structure I, the global minimum energy structure, is planar and cyclic, whereas II, III, and IV have a C2 symmetry axis and
are simple juxtapositions of T-shaped and linear forms of the corresponding dimer subunits. The orientation angles for I and the separation distances between monomer mass centers for I–IV determined at the cc-pVTZ/MP2 level are shown for each structure. Corresponding values from the MMC model potential are shown in parentheses.
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.