Where the i index runs over the number of electrons. Each component of the position operator is a one-electron operator, they can be represented in second quantization as follows: x ^ ^ = ∑ ∑ i j ⟨ i | x ^ ^ | j ⟩ a i † † a j where i, j run over orbitals. The expectation values of the position components are the first moments of the electrons' position.