# NDDO

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In [computational chemistry](/source/Computational_chemistry), **NDDO** (**neglect of diatomic differential overlap**) is a formalism that was first introduced by [John Pople](/source/John_Pople); it is the basis for most [semiempirical methods](/source/Computational_Chemistry#Semi-empirical_methods). While [INDO](/source/INDO) added all one-centre two electron integrals to the [CNDO/2](/source/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.[1] Otherwise, the [zero-differential overlap](/source/Zero-differential_overlap) approximation is used. The common software program is [MOPAC](/source/MOPAC) (Molecular Orbital PACkage).

In the NDDO method, the [overlap matrix](/source/Orbital_overlap) *S* is replaced by the unit matrix. This allows the [Hartree–Fock](/source/Hartree%E2%80%93Fock_method) [secular equation](/source/Characteristic_polynomial) |H-ES| = 0 to be replaced 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 the [core electrons](/source/Core_electron) is to reduce the [nuclear charge](/source/Effective_nuclear_charge). Semiempirical calculations are usually carried out in a minimal [basis set](/source/Basis_set_(chemistry)).

## See also

- [MNDO](/source/MNDO)
- [AM1](/source/Austin_Model_1)
- [PM3](/source/PM3_(chemistry))
- [SAM1](/source/SAM1)
- [RM1](/source/RM1_(chemistry))
- [MOPAC](/source/MOPAC)

## References

1. J. Pople and D. Beveridge, *Approximate Molecular Orbital Theory*, McGraw-Hill, 1970

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