Hydrazoic acid (HN3) is used as a case study for investigating the accuracy and precision by which a molecular structure—specifically, a semi-experimental equilibrium structure (reSE)—may be determined using current state-of-the-art methodology. The influence of the theoretical corrections for effects of vibration–rotation coupling and electron-mass distribution that are employed in the analysis is explored in detail. The small size of HN3 allowed us to deploy considerable computational resources to probe the basis-set dependence of these corrections using a series of coupled-cluster single, double, perturbative triple [CCSD(T)] calculations with cc-pCVXZ (X = D, T, Q, 5) basis sets. We extrapolated the resulting corrections to the complete basis set (CBS) limit to obtain CCSD(T)/CBS corrections, which were used in a subsequent reSE structure determination. The reSE parameters obtained using the CCSD(T)/cc-pCV5Z corrections are nearly identical to those obtained using the CCSD(T)/CBS corrections, with uncertainties in the bond distances and angles of less than 0.0006 Å and 0.08°, respectively. The previously obtained reSE structure using CCSD(T)/ANO2 agrees with that using CCSD(T)/cc-pCV5Z to within 0.000 08 Å and 0.016° for bond distances and angles, respectively, and with only 25% larger uncertainties, validating the idea that reSE structure determinations can be carried out with significantly smaller basis sets than those needed for similarly accurate, strictly ab initio determinations. Although the purely computational re structural parameters [CCSD(T)/cc-pCV6Z] fall outside of the statistical uncertainties (2σ) of the corresponding reSE structural parameters, the discrepancy is rectified by applying corrections to address the theoretical limitations of the CCSD(T)/cc-pCV6Z geometry with respect to basis set, electron correlation, relativity, and the Born–Oppenheimer approximation, thereby supporting the contention that the semi-experimental approach is both an accurate and vastly more efficient method for structure determinations than is brute-force computation.
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The CCSD(T)/cc-pCV5Z optimized parameters of hydrazoic acid reported in Table VI and Fig. 3 of Ref. 5 are incorrect; specifically, they are inconsistent with the CCSD(T)/cc-pCV5Z optimization output file in the supplementary material of that same work. The values that are present in the output file of that supplementary material were replicated in this work, and all references and comparisons herein involving the previous CCSD(T)/cc-pCV5Z refer to these correct CCSD(T)/cc-pCV5Z values.
The use of the word “worst” here refers to the degree to which information provided by the isotopologue in question is consistent with the information provided by the other isotopologues in the dataset and does not necessarily reflect the quality of the spectroscopic data or computed corrections for that isotopologue. As seen in pyridazine,4 the inclusion of the final isotopologue resulted in a dramatic increase in the δreSE but was required for accurate determination of several parameters.