![]() For these calculations the lowest energy state of the atoms had to be used. Calculations at the same levels of theory were performed for isolated atoms, which are used as reference for extracting the enthalpy of formation. ![]() The standard G2, G3, G4, 6–10 and CBS-QB3 16,17 methods were used for about 2000 molecules up to 47 atoms, and W1U and W1BD 13 were used for about 650 molecules up to 16 atoms. However, it is not practical for large flexible molecules since the number of accessible conformations increases exponentially with the number of rotatable bonds. 25–29 MS-AS has demonstrated the importance of the multistructural anharmonicity in determining absolute thermodynamic quantities by reproducing the experimental standard entropy for some small molecules such as ethanol and 1-butanol 25 with an uncertainty of about 4 J/mol K. ![]() have developed a method called multistructural approximation (MS-AS) which allows for taking a Boltzmann average on conformational space and for considering the change in rotational partition function from structure to structure hence, rotation is coupled to conformational change. These methods have improved thermochemistry predictions but they were applied to special cases only and the methods are not practical for complex molecules. Other methods have used experimentally obtained anharmonic constants, 22 second-order rotovibrational perturbation theory, 23 or quadratic correction terms 24 to take the effect of anharmonicity into account for prediction of total atomization energy and enthalpy of formation. 20,21 They found that the anharmonic correction mainly affects the entropy and isochorous heat capacity thermodynamics functions, while the anharmonicity related contributions to the enthalpy of formation only amount to a few percent of the total vibrational contribution. 20 They assumed that anharmonicity only needs to be considered for some selected degrees of freedom and described the molecular vibration of silicon hydrides by a set of independent harmonic and anharmonic modes. treated nuclear motions by taking partial asymmetrical internal rotations into account for a number of small carbon and silicon compounds. Several improvements have been suggested to alleviate these shortcomings. 18 Moreover, calculations of absolute thermodynamics such as standard entropy and heat capacity are reported much less often than the enthalpy of formation despite their importance. The empirical corrections limit their predictive capability to the datasets against which they are benchmarked. 2 Methods such as Gaussian-n, 6–10 Weizman-n, 11–15 and Petersson-style complete basis set (CBS) models 16,17 have improved accuracy in ab initio thermochemistry by combining calculations at different levels of theory and basis sets with empirical corrections in most methods. 1–5 Therefore, a large amount of effort has gone into the development of quantum chemical methods to predict thermochemistry, especially enthalpy of formation, based on a theoretical description of molecular electronic structure and nuclear motion. Prediction of thermochemistry is crucial for designing chemicals with new functionality since fundamental properties such as Gibbs free energy, enthalpy, heat capacity, and standard entropy are needed to understand stability and reaction energies of compounds. Finally, in order to facilitate the analysis of thermodynamics properties by others we have implemented a new tool obthermo in the OpenBabel program suite including a table of reference atomization energy values for popular thermochemistry methods. This paper also provides predictions of Δ f H 0, S 0, and C V for well over 700 compounds for which no experimental data could be found in the literature. With the correction we find an RMSD from experiment of ≈13 J/mol K for 1273 compounds. This allows an empirical correction of the calculated entropy for molecules with multiple conformations. We observe that the deviation of composite quantum thermochemistry recipes from experimental S 0 corresponds roughly to the Boltzmann equation ( S = RlnΩ), where R is the gas constant and Ω the number of possible conformations. This problem can be tackled in principle by performing thermochemistry calculations for all stable conformations, but this is not practical for large molecules. Quantum methods systematically underestimate S 0 for flexible molecules in the gas phase if only a single (minimum energy) conformation is taken into account. ![]() A large data set may help to evaluate quantum thermochemistry tools in order to uncover possible hidden shortcomings and also to find experimental data that might need to be reinvestigated, indeed we list and annotate approximately 200 problematic thermochemistry measurements. Large scale quantum calculations for molar enthalpy of formation (Δ f H 0), standard entropy ( S 0), and heat capacity ( C V) are presented.
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