Testing electronic structure methods for describing intermolecular H ⋯ H interactions in supramolecular chemistry

In this article a wide variety of computational approaches (molecular mechanics force fields, semiempirical formalisms, and hybrid methods, namely ONIOM calculations) have been used to calculate the energy and geometry of the supramolecular system 2-(2′-hydroxyphenyl)-4-methyloxazole (HPMO) encapsulated in β-cyclodextrin (β-CD). The main objective of the present study has been to examine the performance of these computational methods when describing the short range H ⋯ H intermolecular interactions between guest (HPMO) and host (β-CD) molecules. The analyzed molecular mechanics methods do not provide unphysical short H ⋯ H contacts, but it is obvious that their applicability to the study of supramolecular systems is rather limited. For the semiempirical methods, MNDO is found to generate more reliable geometries than AM1, PM3 and the two recently developed schemes PDDG/MNDO and PDDG/PM3. MNDO results only give one slightly short H ⋯ H distance, whereas the NDDO formalisms with modifications of the Core Repulsion Function (CRF) via Gaussians exhibit a large number of short to very short and unphysical H ⋯ H intermolecular distances. In contrast, the PM5 method, which is the successor to PM3, gives very promising results. Our ONIOM calculations indicate that the unphysical optimized geometries from PM3 are retained when this semiempirical method is used as the low level layer in a QM:QM formulation. On the other hand, ab initio methods involving good enough basis sets, at least for the high level layer in a hybrid ONIOM calculations, behave well, but they may be too expensive in practice for most supramolecular chemistry applications. Finally, the performance of the evaluated computational methods has also been tested by evaluating the energetic difference between the two most stable conformations of the host(β-CD)-guest(HPMO) system.