A basic postulate of wave mechanics is that the wave function of a microscopic system develops in time according to the equation iℏ∂ψ/∂ t = H ψ, where H , the Hamiltonian, is an operator which in general depends upon the time. If, and only if, the Hamiltonian is time-independent, then the solutions of this equation take the form ψ( q, t ) = ∑ ncnѰn ( q )e
-1
Ent
/ℏ
, (2) where the individual terms Ѱn ( q ) are functions of the co-ordinates alone and the En are the corresponding eigenvalues of the Hamiltonian, satisfying HѰn = EnѰn . (3)