where ω is the frequency of the optical phonons, c is a constant, e is the electron charge, V is the volume, and r is the electron coordinate. The case of weak coupling of the electrons to the field of the optical phonons, when Hint can be treated in perturbation theory, was considered for the first time in [1]. The case of strong coupling, when Hint cannot be treated as a perturbation, was first considered in [2]. This paper gives a semiclassical theory of the motion of an electron in an ionic crystal, the electron being treated quantum mechanically but the field classically. A consistent solution of the quantum problem (I) in the case of strong interaction of the particle with the field was given in [3] on the basis of the adiabatic approximation with allowance for the translational degeneracy of the Hamiltonian (1). In the zeroth approximation, the wave equation has the form [3] (H-E)ψ=0 ψ= ψ( ( )( ) pq i e / λ )θ  (...Qf... ), p=-i ∂ /∂q, where q is the part of r corresponding to uniform rectilinear motion, λ is the fluctuating part of r, and Qf are the field coordinates. For φ(λ), the following equation was obtained in [3]: