The geometrical structures and the electronic states of the electron donor-acceptor (EDA) complexes for both ground and excited states have been studied by means of the ab-initio MO method. As models of the EDA complex, sigma-type EDA complexes composed of ammonia and halogen molecules NH3-X(2) (X = F and Cl) were chosen. For comparison, the NH3-X(2) complexes (X = Br and I) were also studied with only a small basis set. Geometry optimization of the complexes gave the van der Waals (vdW) structure whose the ammonia molecule is weakly interacting to the halogen molecule for the ground state and the distorted structure whose ammonia molecule is largely deformed for the excited charge-transfer (CT) state. This large distortion at the CT state is due to the fact that a dipole induced by bending deformation of the ammonia cation coordinates the halogen anion. Potential energy curves (PECs) for the formation of the EDA complexes were calculated by singly excited CI and MP2 methods for both the ground and CT states. The first excitation band of the intramolecular transition in X(2) of the CT complex showed a considerably large blue shift relative to that of the monomer molecule. This result was in qualitative agreement with the experimental feature. For the CT state, the transition energy is largely red-shifted due to the complex formation if the geometrical distortion is permitted. The structural model for the CT state was proposed on the basis of the theoretical results.