Term: semi-empirical quantum mechanical methods https://doi.org/10.1351/goldbook.ST07099 Definition: 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. Related Terms: 1) extended hückel method (http://doi.org/10.1351/goldbook.ET07032). 2) zero differential overlap (zdo) approximation (http://doi.org/10.1351/goldbook.ZT07132). 3) pariser–parr–pople (http://doi.org/10.1351/goldbook.PT07088). Source: PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1962 (https://doi.org/10.1351/pac199971101919) Citation: 'semi-empirical quantum mechanical methods' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.ST07099 License: The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. Disclaimer: The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using.