The motion of a foreign atom in the field of a silicon crystalline lattice is simulated. The lattice field is an external one for a foreign atom. The Hamilton principle (principle of least action) is used to identify and determine the interaction potential of a foreign atom with the field of the crystal lattice. A local n-dimensional space is constructed for which the Lagrangian is introduced. The equations of motion for the “foreign atom+crystal” system are found. The symmetries allowed by the Lagrangian for the “foreign atom+crystal” system make it possible to reduce a multidimensional a system of sequentially connected differential equations to a one-dimensional equation. The solution to this equation is a soliton (kink), which describes the translational displacement of a foreign atom in an external field in the form of a collective consisting of the foreign atom itself and the accompanying reversible displacements of the lattice atoms. A foreign atom can only manifest itself as a particle in oscillatory mode.