natural population analysis (NPA)

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: $0 < n i A < 2$ . 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.

Source:

PAC, 1999, 71, 1919 (Glossary of terms used in theoretical organic chemistry) on page 1954