Methods which use parameters derived from experimental data to simplify computations. The simplification may occur at various levels: simplification of the Hamiltonian (e.g.
, as in the Extended Hückel method), approximate evaluation of certain molecular integrals (see, for example, zero differential overlap (ZDO) approximation), simplification of the wavefunction (for example, use of a Pi (π) electron approximation as in Pariser–Parr–Pople method), etc
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1962 (https://doi.org/10.1351/pac199971101919)