Benchmark calculations: highly accurate ab-initio computations for molecules in the gas phase.

Implementation 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 [1,6,7]. A few illustrative examples will be presented. The comparison of the computed data with experimental results allows us to show how ab initio computations are able to accurately predict experimental data. [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). [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); C. Puzzarini, G. Cazzoli, A. Gambi, J. Gauss, "Rotational spectra of 1-chloro-2-fluoroethylene. II. Equilibrium structure of the cis and trans isomers", J. Chem. Phys. 125, 054307 (2006). [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 and 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 (Cu, Ag, Au) and 12 (Zn, Cd, Hg) elements", Theor. Chem. Acc. 114, 283 (2005).