Primitive descriptors

There exist variations on aforementioned procedures. Equations with parameters raised to a power, such as Xf, may be considered as linear in a variable Zj (= X, ). For example, a squared parameter might be used in the case of an extremum occurring in a plot of a property versus some parameter such as the molecular volume. Such a relation might be expected if there exists maximum molecular size for fitting into a receptor cavity. Other mathematical functions of the primitive descriptors can likewise be used to generate terms. Linear regression models are preferred because the terms in Eq. [1] have relatively simple physical interpretations. 

The space-lattice of Fig. 5.3 helps in the description of crystals. One must, however, observe that the lattice is only a mathematical abstraction and not the actual crystal. The lattice points may or may not coincide with actual motif positions. All motifs, however, have a fixed relationship to the lattice. The lattice descriptor is the unit cell, drawn in heavy lines. The three non-coplanar axes of the unit cell have the lengths a, b, c, and the angles a, p, y. One lattice point occupies the comers of the unit cell. Since for any given unit cell only 1/8 of its comers lies inside the unit cell, each primitive unit cell shown, contains only one lattice point. 

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