In computational chemistry, NDDO is a formalism that was first introduced by John Pople; it is the basis for most semiempirical methods. While INDO added all one-centre two electron integrals to the CNDO/2 formalism, NDDO adds all two centre integrals for repulsion between a charge distribution on one centre and a charge distribution on another centre. Otherwise, the zero-differential overlap approximation is used. The common software program is MOPAC.