Accuracy of the rotational constants

From the Determination of the Accurate Equilibrium Structure of 1-Methylthymine by Gas Electron Diffraction and Coupled Cluster Computations to the Observation of Methylation and Flexibility Effects in Pyrimidine Nucleobases

“…10% being of the same order of magnitude as the accuracy of the computations. 38 This points to the reliability of the calculated values.…”

Why it is sometimes difficult to determine the accurate position of a hydrogen atom by the semiexperimental method: Structure of molecules containing the OH or the CH3 group

“…Note that in most cases, the accuracy of the rovibrational corrections is better and, furthermore, the error is mainly systematic, see Ref. and section difluoroethene below. The reliability of the SE method has been analyzed by many authors .…”

“…When a large amplitude motion is present, the methods to derive the rovibrational corrections may be inaccurate: the numerical differentiation used to calculate the cubic force constants may be deficient, and the second‐order perturbation method may be inadequate, an example is formamide, HCONH 2 . It is also known that, for large molecules, the rovibrational correction is calculated with a systematic error of a few per cent . When a heavy atom (carbon for instance) is substituted, the rovibrational correction remains almost constant (except for oblate molecules) and, therefore, also its error.…”

A benchmark study of molecular structure by experimental and theoretical methods: Equilibrium structure of thymine from microwave rotational constants and coupled-cluster computations

“…rovibrational corrections is systematic and proportional to the moments of inertia, we arrive at the result that the accuracy of the MP2/cc-pVTZ rovibrational correction is 0.47% and that of the B3LYP/cc-pVTZ correction of 0.87%. These numbers are smaller than expected, the error being generally a few per cent [35]. This indicates some compensation of errors in the calculation of P c .…”

“…Interestingly, the ground state planar moment of inertia of this species is also an outlier and the calculation of the corresponding rovibrational correction indicates that the problem probably comes from the ground state rotational constants. The semi-experimental value of the d HH distance is slightly larger than the ab initio value, it shows that the force field, although good, is not perfect and that its error is mainly systematic [35]. The B3LYP/cc-pVTZ force field gives d HH = 1.755(1) Å indicating that this force field is perhaps slightly less accurate.…”