Benchmarking quantum chemistry with rotational spectroscopy or benchmarking rotational spectroscopy with quantum chemistry?

Quantum chemistry has nowadays reached such an advanced level that highly accurate results can be achieved for energies and properties of small to medium-sized molecules. For these high-level calculations the requirements are efficient treatment of electron correlation via coupled-cluster theory, basis-set extrapolation techniques, incorporation of core correlation, relativistic as well as vibrational eects together with the use of suitable additivity schemes. Nevertheless, despite all the progress made so far, it is still essential to benchmark the results from quantum-chemical calculations. Data from rotational spectroscopy are ideally suited for this purpose, as this technique provides, in particular for small molecules in the gas phase, highly accurate results. On the other hand, however, measurements and analyses of rotational spectra are not often straightforward. State of-the-art quantumchemical computations are therefore needed to guide the investigation and in particular to assist in the determination of the spectroscopic parameters of interest. Quantum chemistry in this way allows to verify ("benchmark") results from rotational spectroscopy. A statistical analysis of the accuracy of theoretically predicted rotational constants will be presented as an example for the benchmark of quantum chemistry via rotational spectroscopy [1]. On the other hand, the determination of the hyperne parameters of dihalogencarbenes (CF2 and CCl2) will show the need of "benchmarking" results from experiments [2]. Based on all the considerations given above, the answer to the "title question" turns out to be not clear-cut. What we suggest instead is to exploit a fruitful interplay of theory (quantum chemistry) and experiment (rotational spectroscopy). The power of such an interplay will be demonstrated by a few examples. In particular, the determination of an absolute 17O NMR scale via the analysis of the rotational spectrum of H217O (Ref. 3) and the evaluation of equilibrium structures of substituted diacetylenes via the corresponding rotational constants [4] will be presented. 1. C. Puzzarini, M. Heckert, and J. Gauss, J. Chem. Phys., 128, 194108 (2008). 2. C. Puzzarini, S. Coriani, A. Rizzo, and J. Gauss, Chem. Phys. Lett., 409, 118 (2005). 3. C. Puzzarini, G. Cazzoli, M. E. Harding, J. Vazquez, and J. Gauss, J. Chem. Phys., 131, 234304 (2009). 4. S. Thorwirth, M. E. Harding, D. Muders, and J. Gauss, J. Mol. Spectrosc., 251, 220 (2008); P. Botschwina and C. Puzzarini, J. Mol. Spectrosc., 208, 292 (2001); G. Cazzoli, L. Cludi, M. Contento, and C. Puzzarini, J. Mol. Spectrosc., 251, 229 (2008).