Thermochemical properties accuracy

This factor is most important when results are reported for only a subs the entire collection of calculations. For example, some studies re results only for the atomization energies portion of the set. This subs the calculations does not include some difficult molecules presen other thermochemical properties (e.g. PO, whose geometry car challenging to model accurately), and so results for a subset can be i accurate than they would be for the entire set. This effect becomes r pronounced as overall method accuracy increases. Be cautious v general accuracy and applicability conclusions are drawn from such dal... 

Calculation of thermochemical data using high-quality ab initio electronic structure calculations has been a long-standing goal. However, the availability of supercomputers and new theoretical techniques now allow the calculation of thermochemical properties with chemical accuracy, i.e., AHf to within... 

If we use B3LYP/VTZ+1 harmonics scaled by 0.985 for the Ezpv rather than the actual anharmonic values, mean absolute error at the W1 level deteriorates from 0.37 to 0.40 kcal/mol, which most users would regard as insignificant. At the W2 level, however, we see a somewhat more noticeable degradation from 0.23 to 0.30 kcal/mol - if kJ/mol accuracy is required, literally every little bit counts . If one is primarily concerned with keeping the maximum absolute error down, rather than getting sub-kJ/mol accuracy for individual molecules, the use of B3LYP/VTZ+1 harmonic values of Ezpv scaled by 0.985 is an acceptable fallback solution . The same would appear to be true for thermochemical properties to which the Ezpv contribution is smaller than for the TAE (e.g. ionization potentials, electron affinities, proton affinities, and the like). 

In principle it is known how to compute the thermochemical properties of most molecules to very high accuracy (uncertainty of 0.5 kcal/mol). This can be achieved by accounting for electron correlation, such as can be obtained by means of coupled clustered theory with single, double, and triple excita-... 

Recent advances in computational chemistry have made it possible to calculate enthalpies of formation from quantum mechanical first principles for rather large unsaturated molecules, some of which are outside the practical range of combustion thermochemistry. Quantum mechanical calculations of molecular thermochemical properties are, of necessity, approximate. Composite quantum mechanical procedures may employ approximations at each of several computational steps and may have an empirical factor to correct for the cumulative error. Approximate methods are useful only insofar as the error due to the various approximations is known within narrow limits. Error due to approximation is determined by comparison with a known value, but the question of the accuracy of the known value immediately arises because the uncertainty of the comparison is determined by the combined uncertainty of the approximate quantum mechanical result and the standard to which it is compared. 

For relatively small molecules, it is becoming rontine to calcnlate thermochemical properties in the ideal-gas state with computational qnantum mechanics. For organic species with fewer than about 10 nonhydrogen atoms, the methods are snfficiently well developed that they can rival or even surpass the accuracy that can be obtained from experiment. 

The investigation of the vinyl system consisted in the calculation of thermochemical properties of a series of peroxides and peroxy species using DFT combined with isodesmic reactions. We showed that the vinyl radical for which high-level calculations can be performed is a good model for the phenyl. The accuracy of the DFT method, which is the primary method for this work, was checked through a number of comparisons with high-level calculations. At this point, it is worthwhile mentioning the importance of the vinyl + O2 reaction system ... 

In conclusion, this work shows that ab intio and DFT methods are excellent tools for the determination of unknown thermochemical properties. The proven accuracy of quantum mechanics calculations make them cost-effective alternatives to time-consuming, difficult experiments and we expect their role to steadily increase in the future. 

Formation heat is a basic parameter in the thermochemical calculation, which can be calculated from combustion heat of a compound according to Hess s law. The measurement accuracy of combustion heat has reached a very high level. In the design of a new explosive, in order to know its explosion properties and thermochemical properties, its formation heat can be calculated first, which is necessary to design and decide the formulation of explosive. 

In several instances, group additivity calculations have highlighted experimental measurements that are almost certainly in error. The technique thus not only allows prediction of unmeasured thermochemical properties, but provides a simple standard by which to judge the probable accuracy of published data. 

In the remainder of this section we discuss the derivation of group values for heat capacities, enthalpies and entropies and provide several examples to illustrate their application. In Section V we will discuss refinements to this simple group additivity scheme. There, the aim is to see if, by a simple expansion beyond the four parameters required for group additivity, we can improve significantly upon its accuracy in predicting thermochemical properties. 

J. D. Cox and G. Pilcher, Thermochemistry of Organic and Organometallic Compounds, Academic Press, London New York, 1970. The major feature is a tabulation of experimental measurements of A H°, heats of reaction, for organic and organometallic compounds. Values of AfH° are derived for each experimental result where phase changes are involved measurements of AH (heat of vaporization) are tabulated. Experimental uncertainties are assessed. Introductory chapters discuss the basics of thermochemistry, the types of experimental measurements involved, their accuracies and limitations. Additional chapters examine theoretical aspects of thermochemistry and various schemes for relating thermochemical properties to structural parameters. 

The database is reviewed and available reference literatore values (calculated and measured ones) and own calculation results are provided. Today this database is the biggest collection of ATcT and G3B3 calculated values that were provided for about half of the included species. In addition, the accuracy of the data and the used values are shown in detail to make the calculation results traceable and or correctable, if better data are available (e.g., quantum chemical results such as spectroscopic properties like vibrations and rotational constants, additional data used to calculate the partition functions, and finally the deviations of the fits to obtain NASA polynomial data from the temperature-dependent thermochemical properties). 

As illustrated in Fig. 1, there are essentially four methods for obtaining thermochemical data for the species in our reaction mechanism. The first choice is to find the needed data in databases or in the literature in general. This includes both published experimental data and published quantum chemical calculations, which can also be a reliable source of thermochemical data. If no information on a substance is available in the literature, one should consider whether it can be treated by group additivity methods. If a well-constructed group additivity method is available for the class of molecules of interest, the results, which can be obtained with minimal effort, will be comparable in accuracy to those from the best quantum chemistry calculations. If group additivity is not applicable to the molecules of interest, then we may want to carry out quantum chemistry calculations for them, as discussed in detail in an earlier chapter. In some cases, the effort required to carry out the quantum chemical calculations may not be warranted, and we may want to make coarser, empirical estimates of thermochemical properties. 

In the original version, G1 theory, c and C2 were determined by insisting on exact energies for H( 5 ) and H2. In G2 theory, the parameters are chosen to minimize the mean absolute error for the thermochemical properties of a fairly large set of reference compounds (the G2 set ). (It should be noted that many. 6f the values in that reference set actually have error bars equal to or greater than the target accuracy.)... 

Hartree-Fock -(-second order perturbation (MP2) Mostly small organic molecules A few tens Structures and thermochemical properties Fairly high accuracy, still reasonable computational effort... 

The prediction of thermochemical properties with chemical accuracy (deviation from true value < 1 kcal mol 4.18 kJ mol ) or subchemical... 

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