zero differential overlap (ZDO) approximation

https://dx.doi.org/10.1351/goldbook.ZT07132
An approach to the systematic neglect of the small-in-value electron repulsion integrals which is used in a number of approximate self-consistent field molecular orbital schemes. It means that all the products of atomic orbitals χ μ χ ν are set to zero and the overlap integralS μ ​ ν = δ μ ​ ν (where δ μ ​ ν is the Kronecker delta). The ZDO approximation greatly simplifies the computation of wavefunctions by eliminating many of two-electron integrals. At the ZDO approximation all three- and four-centered integrals vanish.
Source:
PAC, 1999, 71, 1919 (Glossary of terms used in theoretical organic chemistry) on page 1970