The orbital of the parent molecule is described by the basic functions of the atomic center defined by the basi s set. The basic functions of the fragment is used to expand the orbital of the parent molecule. Since the number of basic function remains the same, there will be no loss of information in this way. The resulting expansion coefficient can be used to obtain the composition values of the fragment orbital corresponding to the parent molecule orbital. Xiao and Lu proposed the generalized CDA (GCDA) method. GCDA deals with alpha and beta electron separately which solves the problem that CDA can only be used in closed -shell systems. In this work, we use the expansion coefficient to draw the orbital interaction diagram to investigate the main components of parent molecule orbitals.