When molecular collisions are the principal line broadening mechanism the absorption at line centers and in the line wings can be calculated in impact approximation that provides the Lorentz contour for well separated lines. Apart from such lines there are regions in the IR spectra with very dense line structure (Q-branches or band heads for example) where band shape can not be expressed as the sum of Lorentzian lines. These deviations have generally been attributed to line mixing effects. We present here a theory which can be used in shape calculation of overlapped lines if the intermolecular potential is known. The quantum mechanical methods are developed for a limited number of systems and are usually too difficult to be used because of very complicated computations. Instead of this we have used the semiclassical method which can be treated in the simple model representation. The classical impact theory of shape was the basis of our method and then we modernized this theory to apply to a molecular system with discrete spectra. We shall consider here the case of the band shapes for linear molecules.