The analysis of the
electron density distribution in a molecular system based on the orthonormal natural atomic orbitals.
Natural populations,

are the occupancies of the natural atomic orbitals. These rigorously satisfy the
Pauli exclusion principle:

.
The population of an atom

is the sum of natural populations

.
A distinguished feature of the
NPA method is that it largely resolves the
basis set dependence problem encountered in the Mulliken population analysis method.
Source:
Cite as:
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
https://doi.org/10.1351/goldbook.