Q i j = ∑ ∑ α α q α α ( 3 r α α , i r α α , j − − r α α 2 δ δ i j ) . Here, the indices i and j can independently take on the values 1, 2, and 3 (or x , y , and z ) corresponding to the three Cartesian axes, the index α α runs over all particles (electrons and nuclei) in the molecule, q α α is the charge on particle α α , and r α α , i is the i -th component of the position of this particle. Each term in the sum is a tensor operator. In particular, the nine products r α α , i r α α , j together form a second rank tensor, formed by taking the outer product of the vector operator r α α with itself.