Chemical accuracy definition

An important factor to be considered is the computational cost. Continuum methods are noticeably less expensive than simulation methods based on discrete models. On the other hand, simple properties, as the solvation energy AGso of small and medium-size solutes are computed equally well with both continuum and discrete methods, reaching chemical accuracy (Orozco et al., 1992 Tomasi, 1994 Cramer and Truhlar, 1995a). There is large numerical evidence ensuring that the same conclusion holds for other continuum and discrete methods as well. The evaluation of A(jso at TS has given almost identical results in several cases, but here numerical evidence is not sufficient to draw definitive conclusions. 

G3X theory gives significantly better agreement with experiment for the G3/99 test set of 376 energies (Table 27.3). Overall, the mean absolute deviation from experiment decreases from 1.07 kcal/mol (G3) to 0.95 kcal/mol (G3X). Thus the mean absolute deviation in G3X not only beats our target accuracy of 2 kcal/mol but also meets the more rigorous definition of chemical accuracy (1 kcal/mol). Even more impressively, the mean absolute deviation for the 222 enthalpies of formation decreases from 1.05 kcal/mol (G3) to 0.88 kcal/mol (G3X). This is clearly due to the improvement for non-hydrogen systems where the mean absolute deviation decreases from 2.11 to 1.49 kcal/mol. 

Aromaticity is one of the most useful concepts in chemistry. We teach it in undergraduate courses, and we need it in our everyday work as chemists. To abandon this concept because it doesn t have a rigorous physical basis would mean that we would lose a very important tool to interpret and predict the properties of molecules on a qualitative basis. A theoretical physicist or chemist might argue that nowadays properties of molecules can be predicted with almost chemical accuracy. So why do we need qualitative concepts like aromaticity Ifs because chemistry is about more than the prediction of numbers. Chemistry is not a science in a strictly reductionist definition. There is a large part of an engineering-type of reasoning involved in chemistry. The analytic part of chemistry may be covered by quantum mechanics, but the constructive part is not accessible to theoretical methods (e.g., try to set up a wavefunction to solve the problem of chemical synthesis). 

For our reference sample, the mean absolute error is 1.32 kcal mol on the values. An improved version of the scheme known as G2M(RCC) brings down the mean absolute error to 0.82 kcal mol While this definitely qualifies as chemical accuracy, it does not pass muster as benchmark quality . 

Definition and Uses of Standards. In the context of this paper, the term "standard" denotes a well-characterized material for which a physical parameter or concentration of chemical constituent has been determined with a known precision and accuracy. These standards can be used to check or determine (a) instrumental parameters such as wavelength accuracy, detection-system spectral responsivity, and stability (b) the instrument response to specific fluorescent species and (c) the accuracy of measurements made by specific Instruments or measurement procedures (assess whether the analytical measurement process is in statistical control and whether it exhibits bias). Once the luminescence instrumentation has been calibrated, it can be used to measure the luminescence characteristics of chemical systems, including corrected excitation and emission spectra, quantum yields, decay times, emission anisotropies, energy transfer, and, with appropriate standards, the concentrations of chemical constituents in complex S2unples. 

The three historical approaches to certification mentioned above were recently expanded to identily seven modes that are used at NIST for value assignment for chemical composition (May et al. 2000). These seven modes and the resulting values are summarized in Table 3.13. The basic principles of value assignment remain unchanged however, these modes now provide a well-defined link between the process used for value assignment and the definition of the assigned value (i.e. certified, reference, or information value). The terms described above provide a clear indication of the level of confidence that NIST has in the accuracy of the assigned value. The definition of a certified value implies that NIST must be involved in the measurement process for the value to be classified as a NIST certified value (see modes 1-3 in Table 3.13). Thus, modes 4 and 7, which do not involve NIST measure-... 

The pharmacophore concept, like that of molecular/chemical structure, assumes different meanings, and consequently different definitions, according to the accuracy and resolution level of its information content. 

Aromaticity, defined as a structural feature, was used as a predictive tool for compounds that had not been prepared previously, whereas reactivity, bond length, or magnetic criteria had to await the isolation of a compound and its experimental investigation. Only recently has the development of quantum-chemical methods reached the point where one can predict with sufficient accuracy the magnetic properties, the bond lengths, and the reactivity patterns of aromatics. The multidimensional character and the definition and measurement of aromaticity generated confusion and conflicts.43 A recent review discussed the multidimensional character of aromaticity and theoretical and experimental approaches to aromatic structures and their predictions, and references are indicated extensively.66... 

The concept of the similarity of molecules has important ramifications for physical, chemical, and biological systems. Grunwald (7) has recently pointed out the constraints of molecular similarity on linear free energy relations and observed that Their accuracy depends upon the quality of the molecular similarity. The use of quantitative structure-activity relationships (2-6) is based on the assumption that similar molecules have similar properties. Herein we present a general and rigorous definition of molecular structural similarity. Previous research in this field has usually been concerned with sequence comparisons of macromolecules, primarily proteins and nucleic acids (7-9). In addition, there have appeared a number of ad hoc definitions of molecular similarity (10-15), many of which are subsumed in the present work. Difficulties associated with attempting to obtain precise numerical indices for qualitative molecular structural concepts have already been extensively discussed in the literature and will not be reviewed here. 

When talking about quality of chemical measurements trueness, precision, accuracy and error are some of the more important keywords. Therefore a clear definition is necessary (see also chapter 11, slides 36 and 44). 

There was, then, a certain degree of accuracy in the alchemical description of the processes we now call chemical changes, as being the removal of the outer properties of the things which react, and the manifestation of their essential substance. But there is a vast difference between this description and the chemical presentment of these processes as reactions between definite and measurable quantities of elements, or compounds, or both, resulting in the re-distribution, of the elements, or the separation of the compounds into their elements, and the formation of new compounds by the re-combination of these elements. 

The sensitivity of explosives has been defined by Koenen et al (Ref 10) as the minimum amount of energy that must be imparted to the explosive, within limited time and space, to initiate explosive decomposition. This definition is, accdg to Ma ek (Ref 13, p 60) meaningful and can serve as a basis of quantitative fundamental treatments provided the imparted energy is thermal and provided its initial distribution in time and space is known. The accuracy of treatments of thermal explosion described in Section IIA of Mafcek s paper is then limited mainly by the accuracy of chemical kinetic data... 

Miscellaneous physical chemical measurements, some quite empirical, are of great importance to the petroleum industry because they are used for control in manufacture and are included in customer s specifications. Macro methods are of course available, but occasionally the sample is too small and this is frequently the case when the problem is particularly important. Micro modifications of these macro methods have often proved extremely helpful. Microchemistry is not a fad but it is not a panacea either. It should be employed where it is necessary, as where the sample is very small or where it offers a definite and substantial advantage in accuracy, precision, or economy of time or materials. If in a particular case it offers none of these advantages there is no good reason to employ it. 

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