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.