nddo

NDDO stands for Neglect of Diatomic Differential Overlap, an approximation used in semi-empirical quantum chemistry to simplify the calculation of electronic structure. By neglecting certain differential overlap integrals between atomic orbitals on different atoms, NDDO reduces the computational complexity of the Fock matrix, enabling faster evaluations of molecular energies and geometries.

NDDO is a key component of a family of semi-empirical methods that includes MNDO, AM1, and PM3. These methods parameterize the remaining integrals to reproduce experimental data such as heats of formation, bond lengths, and spectroscopic properties. The result is a computationally efficient approach suitable for studying large organic systems, screening reaction pathways, and performing rapid geometry optimizations.

The approach relies on empirical parameterization, which means its accuracy and transferability depend on the quality and relevance of the training data. NDDO-based methods generally perform well for many organic molecules but can be unreliable for systems outside the parameterization set, such as transition metal complexes, unusual bonding situations, or weak noncovalent interactions where the underlying assumptions of the approximation are stressed.

Strengths of NDDO methods include low computational cost and the ability to provide reasonable molecular geometries and qualitative energetics for a wide range of organic compounds. Limitations include dependence on parameterization, potential inaccuracies for non-standard chemistries, and reduced reliability for systems with significant multi-reference character or dispersion effects.

See also: semi-empirical quantum chemistry, NDO, MNDO family, AM1, PM3.