Term: natural population analysis (NPA)
https://doi.org/10.1351/goldbook.NT07080

Definition:
The analysis of the electron density distribution in a molecular system based on the orthonormal natural atomic orbitals. Natural populations, n i A are the occupancies of the natural atomic orbitals. These rigorously satisfy the Pauli exclusion principle: 0 < ni(A) < 2. The population of an atom n A is the sum of natural populations n A = ∑ A n i A. A distinguished feature of the NPA method is that it largely resolves the basis set dependence problem encountered in the Mulliken population analysis method.

Related Terms:
1) electron density (http://doi.org/10.1351/goldbook.E01986).
2) pauli exclusion principle (http://doi.org/10.1351/goldbook.PT07089).
3) basis set (http://doi.org/10.1351/goldbook.BT06999).

Source: PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1954 (https://doi.org/10.1351/pac199971101919)

Citation: 'natural population analysis (NPA)' 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.NT07080

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