Quantum-Mechanical Theory of Linear and Nonlinear Susceptibility

Abstract

The wave function ψ (q, t), or the vector of state, corresponds to the most complete description of a system in quantum mechanics. It is quite often, however, especially for large systems, that a statistical description is necessary: we can only speak of a probability of the system occupying a particular pure state which is described by the wave function. The state of the system is then characterized by the density matrix (density operator).