The theoretical basis of nuclear quadrupole resonance

As any text-book on electrostatics shows, the following equations define the three principal values of an electric quadrupole moment tensor. 

The three components of an electric field vector are defined by the partial derivatives of the electrostatic potential with respect to the positional coordinates. The three principal values of the electric field gradient tensor are thus  

The trace of this tensor is, like that of the quadrupole moment, equal to zero but, unlike the nuclear quadrupole moment tensor, it is not necessarily cylindrically symmetric and two independent quantities are needed to define it. With the convention  

This definition implies that the asymmetry parameter lies in the range 0 r 1.  

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