The analysis of the electron density distribution in a molecular system based on the orthonormal natural atomic orbitals. Natural populations, $n_i\left(\text{A}\right)$ are the occupancies of the natural atomic orbitals. These rigorously satisfy the Pauli exclusion principle: $0<n_i\left(\text{A}\right)<2$. The population of an atom $n\left(\text{A}\right)$ is the sum of natural populations $n\left(\text{A}\right)=\sum _{\begin{array}{c}\text{A}\end{array}}{n}_i\left(\text{A}\right)$. A distinguished feature of the NPA method is that it largely resolves the basis set dependence problem encountered in the Mulliken population analysis method.