An analytical model for adding a spacedependent potential to the φ4 field equation of motion is presented, by constructing a collective coordinate system for the solitary solutions of this model. The interaction of φ4 solitons with a delta function potential barrier and also delta function potential well is investigated. Most of the characters of interaction are derived analytically while they are calculated by other models numerically. We will find that the behaviour of a solitary solution is like a point particle which is moved under the influence of a complicated effective potential. The effective potential is a function of the field initial conditions and also of parameters of the added potential.

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