Benchmark calculations for molecules in the gas phase: state-of-the-art coupled-cluster computations

none1noImplementation of very accurate ab initio methods on one hand and improvements in computer facilities on the other hand allow the determination of structural, molecular, thermochemical and spectroscopic properties of small- to medium-size molecules to a very high accuracy [1]. The predictive capabilities have such an accuracy that theoretical calculations can guide [2], support [3] and even challenge [4] experimental determinations. Therefore, nowadays ab initio calculations are suitable for benchmarking purposes, even for large systems [5]. To perform benchmark calculations, post-HF methods, such as the coupled cluster ones, should be employed in conjunction with extrapolative and additive techniques in order to account for basis set and wave function truncation errors as well as to include important corrections, such as those related to core correlation and relativistic effects [1,3,6,7]. A few illustrative examples will be presented. The comparison of the computed data with experimental results allows us to show how quantum chemical computations are able to either accurately predict experimental data or cast doubts on them. Perspectives for extension of extrapolation techniques to the structures and properties of open-shell systems with an effectiveness and reliability comparable to that well documented for closed-shell systems will be also discussed [8]. [1] C. Puzzarini, "How accurately can structural, spectroscopic and thermochemical properties be predicted by ab initio computations?", Lecture Series on Computer and Computational Science 6, 416 (2006); C. Puzzarini, M. Heckert, Jürgen Gauss, "The Accuracy of rotational constants predicted by high-level quantum-chemical calculations. I. Molecules containing first-row atoms", J. Chem. Phys. 128, 194108 (2008); F. Pawłowski, P. Jørgensen, J. Olsen, F. Hegelund, T. Helgaker, J. Gauss, K. L. Bak, J. F. Stanton, "Molecular equilibrium structures from experimental rotational constants and calculated vibration–rotation interaction constants", J. Chem. Phys. 116, 6482 (2002); M.E. Harding, J. Vázquez, B. Ruscic, A.K. Wilson, J. Gauss, J.F. Stanton, "HEAT: High Accuracy Extrapolated Ab initio Thermochemistry. III. Additional Improvements and Overview", J Chem Phys. 128, 114111 (2008). [2] G. Cazzoli, C. Puzzarini, A. Gambi, J. Gauss, "Rotational spectra of 1-chloro-2-fluoroethylene. I. Main isotopologues and deuterated species of the trans isomer", J. Chem. Phys. 125, 054313 (2006); G. Winnewisser, F. Lewen, S. Thorwirth, M. Behnke, J. Hahn, J. Gauss, E. Herbst, "Gas-Phase Detection of HSOH: Synthesis by Flash Vacuum Pyrolysis of di-tert-butyl sulfoxide and Rotational-Torsional Spectrum", Chem. Eur. J. 9, 5501 (2003). [3] C. Puzzarini, G. Cazzoli, A. Baldacci, A. Baldan, C. Michauk, J. Gauss, "Rotational spectra of rare isotopic species of bromofluoromethane: determination of its equilibrium structure from ab initio calculations and microwave spectroscopy", J. Chem. Phys. 127, 164302 (2007). [4] C. Puzzarini, S. Coriani, A. Rizzo, J. Gauss, "Critical analysis of the spin-rotation constants of CF2 and CCl2: a theoretical study", Chem. Phys. Lett. 409, 118 (2005). [5] P. Jurecka, J. Sponer, J. Cerny, P. Hobza, "Benchmark database of accurate (MP2 and CCSD(T) complete basis-set limit) interaction energies of small model complexes, DNA base pairs, and amino acid pairs", Phys. Chem. Chem. Phys. 8, 1985 (2006). [6] M. Heckert, M. Kàllay, J. Gauss, "Molecular equilibrium geometries based on coupled-cluster calculations including quadruple excitations", Mol. Phys. 103, 2109 (2005); M. Heckert, M. Kàllay, D.P. Tew, W. Klopper, J. Gauss, "Basis-set extrapolation techniques for the accurate calculation of molecular equilibrium geometries using coupled-cluster theory", J. Chem. Phys. 125, 044108 (2006). [7] K.A. Peterson, C. Puzzarini, "Systematically convergent basis sets for transition metals. II. Pseudopotential-based correlation consistent basis sets for group 11 ...noneC. PuzzariniC. Puzzarin