mnDO

MNDO (Modified Neglect of Diatomic Overlap) is a semiempirical quantum chemical method in the Neglect of Diatomic Differential Overlap (NDDO) family. It simplifies electronic structure calculations by replacing many two-electron integrals with parameterized forms and by neglecting differential overlap between most diatomic orbitals. The method uses a minimal basis set of Slater-type orbitals and employs empirically derived parameters to reproduce experimental data such as heats of formation, bond lengths, and vibrational frequencies.

In MNDO, the total energy is composed of core-core repulsion, the one-electron energy, and two-electron repulsion terms that are evaluated under the NDDO approximations. Core electrons are represented by effective potentials, which reduces computational cost and enables the study of larger organic molecules and radicals compared with ab initio methods. Parameterization covers a broad set of main-group elements, allowing geometry optimizations and qualitative energetics for a variety of chemical systems.

Historically, MNDO was introduced in the 1980s as an early member of the NDDO family, alongside CNDO/2 and INDO. It served as a stepping stone to later semiempirical models such as AM1, PM3, and RM1, which refined the parameterization and functional forms to improve accuracy across larger and more diverse chemistries.

Limitations include reduced reliability for transition metals, noncovalent interactions, and systems requiring accurate dispersion or hydrogen-bond descriptions. Despite these constraints, MNDO remains of historical importance in the development of semiempirical methods and is encountered in legacy software and pedagogical demonstrations, as well as used for rapid screening in some contexts.