In computational chemistry, NDDO (neglect of diatomic differential overlap) is a formalism that was first introduced by John Pople and it is now the basis of most successful 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. A common software program is MOPAC (Molecular Orbital PACkage). In the Neglect of Diatomic Differential Overlap (NDDO) method the overlap matrix S is replaced by the unit matrix. This allows one to replace the Hartree–Fock secular equation |H–ES| = 0 with a simpler equation |H–E|=0. The two-electron integrals from the NDDO approximation can either be one-, two-, three- or four-centered. The one-and two-centered integrals are evaluated approximately or parameterized based on the experimental data while the three- and four-centered integrals vanish. Usually, only the valence electrons are treated quantum mechanically while the role of core electrons is to reduce the nuclear charge. Semiempirical calculations are usually carried out in a minimal basis set. See also MNDO AM1 PM3 SAM1 RM1 MOPAC References Semiempirical quantum chemistry methods