Definition of long-range orientational order

The order parameter can be directly related to certain experimentally determinable quantities - say, for example, the diamagnetic anisotropy of the liquid crystal.Let us choose a space-fixed cartesian coordinate system xyz with z parallel to n. If and are the principal diamagnetic susceptibilities of the molecule referred to its own principal axes, the average z component of the susceptibility per unit volume in the nematic phase is evidently 

NMR and dielectric measurements (see 2.3.5) indicate that there is a fair degree of rotational freedom of the molecules about their long axes. Nevertheless the assumption that the molecules are cylindrically symmetric is not valid in general and can sometimes lead to errors in the determination of For a molecule of arbitrary shape, (2.3.1) may be written in the generalized form  

In the uniaxial nematic case, = 22-If the molecule is flexible more parameters are required to define the degree of alignment of its different parts.However, in a large part of the discussion that follows on the theory of the nematic state we shall ignore all these details and assume the molecules to be cylindrically symmetric rods. 

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