Contribution to the Quantum Theory of the Seond‐Harmonic Generation in a Molecular Crystal

The quantum theory of the second‐harmonic generation is developed for diamagnetic molecules through a variational perturbation method. The advantage of this method is that it requires only to know the wave function Ψ of the ground state of the non‐perturbed molecule; thus, the coefficient characterizing the second‐harmonic generation can be expressed as a function of the wave function Ψ and of vectors determined by a variation calculus; so this coefficient can be computed numerically. This calculation is achieved for the hexamethylene‐tetramine crystal.