Reliability of HF/IQA, B3LYP/IQA, and MP2/IQA data in interpreting the nature and strength of interactions

Mainly focusing on the B3LYP level, the reliability of level of theory (LoT)-dependent IQA-defined energy terms (self- and additive atomic energies, interaction energy and its components) in interpreting interactions was investigated at three LoTs using default settings in AIMAll software explicitly implementing the actual B3LYP exchange–correlation functional. Reliability was quantified using relative errors (REs) defined as, for example, RE = B3LYP/IQA(computed) − B3LYP/IQA(expected), using reference CCSD/BBC1/IQA data to obtain the LoT/IQA(expected) terms. On average, B3LYP produced the most accurate IQA energies among the LoTs investigated, affording REs an order of magnitude smaller than those at the HF level. The B3LYP/IQA description of the O4⋯H6 and O3⋯O4 interactions in glycol conformers compared well with the CCSD/BBC1/IQA-generated picture. Exceptionally reliable data were obtained at the B3LYP level for changes in the IQA energies computed for structural changes in glycol. The FAMSEC-based interpretation produced exact qualitative description and perfectly quantitatively comparable values to CCSD/BBC1/IQA data. The ΔEIQA = ΔE criterion (representing changes between the final and any suitably selected initial structure of a molecular system) is reported to ‘validate’ or predict the usefulness of changes in the LoT/IQA computed energy terms with respect to interaction interpretability. This was supported by (i) the smallest errors in the IQA energy changes being obtained at MP2/Müller, despite EIQA largely overestimating E (by −170 kcal mol−1 due to large REs in self-atomic energies) and (ii) reasonable HF-generated FAMSEC descriptors, regardless of the largest REs in the HF/IQA data. Finally, adding Grimme's D3 empirical dispersion correction had no significant effect on the REs.