Density descriptor

It transpires that the second moment of the function I(x) =, v(x) — l]2 depends mono-tonically on 17 for the amorphous materials of relevance in explosives detection. Hence, it represents a quantitative parameter that is directly related to density for a monatomic species. Its use here as a density descriptor for amorphous materials corresponds to defining a monatomic species having atomic volume equal to the mean volume of the polyatomic molecules comprising the material. 

Density Organic explosives tend to have higher density than equivalent harmless plastic materials Diffraction profile yields density descriptor based on analysis of molecular interference function... 

In addition to the steric and electrostatic descriptors, it was proposed to use other 3D molecular fields characterized by the sampling over the rectangular grid - in particular, the hydrophobic field/molecular lipophilic potential (MLP), ° hydrogen bonding and quantum-chemical parameters, e.g., orbital densities.Descriptor selection techniques are often recommended to enhance the stability, predictivity and interpretability of the CoMFA models. ... 

Another model was proposed based on atomic composition of the molecule only, ignoring the influence of the charge density descriptors on the logF calculation [Klopman et al, 1985]. The best fitted proposed model is ... 

The first model for the prediction of log P proposed by Klopman and Iroff [Klopman and Iroff 1981] was based on the assumption that partition coefficients of molecules depend on the charge densities on each atom of the molecule. The following equation was proposed including both atom and group counting descriptors and charge density descriptors ... 

Thus, the group of descriptors Ti , can be related to the quantity of unfilled space per projected atom and have been called directional WHIM density descriptors (or d-WDEN indices, or WHIM emptiness) the greater the r m values the more the projected unfilled space. The X m descriptors are used in place of the kurtosis descriptors (previously proposed). 

Howard, S.T and Krygowski, TM. (1997) Benzenoid hydrocarbon aromaticity in terms of charge density descriptors. Can.]. Chem., 75, 1174—1181. 

The most common method of field-based descriptor generation draws a 3D grid around a molecule and then determines the value of the property of interest (steric, electrostatic, or hydrophobic) at each lattice intersection of the grid. This method originally came from work by Carbo et al. [72] to calculate electron density descriptors, and has been extended by others (e.g., Refs. 73 74 75) to include electrostatic potentials and steric fields. 

The intimate relation between the nuclear distribution and the electronic density distribution is a natural bridge that connects the more conventional, essentially classical, ball-and-stick models, and the more accurate, quantum-chemical electronic density descriptors of molecular shape. It is somewhat surprising that relatively little effort has been devoted to the natural relation between the purely nuclear interactions and the electronic density. In this contribution this connection will be discussed from a specific viewpoint, leading to a 3D representation of molecular shape and to an interpretation of chemical bonding. 

Finally, accurate and robust models for corrosion inhibition efficiencies have been found for multiple sets of corrosion inhibition data. This finding is rather remarkable as corrosion inhibition depends on a number of molecular properties such as hydrophobicify (LogP) and Lewis Base strength as well as the solubiUfy of the inhibitor. This corroborates the concept that similarity based on electron density descriptors may in fact capture more than one aspect of the phenomenon being studied. 

Saleh G (2014) Chemical paradigms seen through charge density descriptor lenses, Ph.D. thesis, Universita degli Studi, Milano, Italy... 

On the other hand, techniques like Principle Component Analysis (PCA) or Partial Least Squares Regression (PLS) (see Section 9.4.6) are used for transforming the descriptor set into smaller sets with higher information density. The disadvantage of such methods is that the transformed descriptors may not be directly related to single physical effects or structural features, and the derived models are thus less interpretable. 

FIGURE 13.5 Isosurface plots, (a) Region of negative electrostatic potential around the water molecule. (A) Region where the Laplacian of the electron density is negative. Both of these plots have been proposed as descriptors of the lone-pair electrons. This example is typical in that the shapes of these regions are similar, but the Laplacian region tends to be closer to the nucleus. 

Pet food purchases are based on the satisfaction of the owner, and pet food proliferation is enormous with accompanying advertising descriptors including natural, Hte, low calorie, high calorie, low protein, and high protein. New therapeutic series, sizes, densities, colors, and attractive packaging have also added to the proliferation. 

Two models of practical interest using quantum chemical parameters were developed by Clark et al. [26, 27]. Both studies were based on 1085 molecules and 36 descriptors calculated with the AMI method following structure optimization and electron density calculation. An initial set of descriptors was selected with a multiple linear regression model and further optimized by trial-and-error variation. The second study calculated a standard error of 0.56 for 1085 compounds and it also estimated the reliability of neural network prediction by analysis of the standard deviation error for an ensemble of 11 networks trained on different randomly selected subsets of the initial training set [27]. 

For (AGh 0), these two regimes are approximately balanced, and a maximum in the exchange current density is predicted. This is the optimal value of the descriptor, and it immediately suggests that a reasonable goal for a computational, combinatorial electrocatalyst search is to find alloys with AGh values close to zero. 

MolSurf parameters [33] are descriptors derived from quantum mechanical calculations. These descriptors are computed at a surface of constant electron density, with which a very fine description of the properties of a molecule at the Van der Waals surface can be obtained. They describe various electrostatic properties such as hydrogen-bonding strengths and polarizability, as well as Lewis base and acid strengths. MolSurf parameters are computed using the following protocol. 

Table 3.41. NBO/NRT descriptors for B4H10 (see Fig. 3.103(a)), showing (a) alternative styx codes (with specific bond types) and associated non-Lewis density error (b) NHO hu and percentage polarization for NBOs of the...

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