Abstract:
The performance of BBC1, BBC2 and Müller approximations, in terms of reliability of IQA data, was investigated at the CCSD, CCSD(T) and MP2 levels using glycol, as a case study, in interpreting the relative stability of its conformers, one with H-bond type intramolecular interaction and the other with a steric clash between two O-atoms. The CCSD/BBC1 level appeared to be perfectly suited as a reference needed to evaluate all possible levels of theory/approximation combinations (LoT/LoA). We found the reliability trend LoT/BBC1 > LoT/BBC2 > LoT/Müller (as well as its origin) and concluded that the Müller approximation should not be used when the accuracy of IQA-defined energy terms is considered. Moreover, we have established that the requirement of reproducing, by IQA calculations, electronic energy is desirable but not a necessary requirement when a comparative approach is used, such as in FAMSEC-based analysis (FAMSEC = fragment attributed molecular system energy change). A new criterion is proposed to assess the quality of IQA data for comparative analyses, ΔE(IQA) ≈ ΔE, where ΔE(IQA) and ΔE are the IQA and electronic energy differences, respectively, between the fin-state and ref-state of a molecular system. The closer ΔE(IQA) approaches ΔE, the closer the FAMSEC data approach values obtained at the exceptionally well performing CCSD/BBC1 level, regardless of the LoT/LoA combination used. Importantly, the MP2/BBC1 level performed nearly as well as the CCSD/BBC1 level in comparative studies. The origin of the MP2/BBC1 approximation's exceptional and the MP2/Müller approximations's acceptable performance in explaining the relative stability of glycol conformers has been uncovered and discussed in detail.
