Suppose that the potential V is the sum of N non-overlapping potentials, V = ∑ ∑ i = 1 N v i . The physical meaning of this is that it describes the interaction of the electron with a cluster of N atoms having nuclei located at positions R i . Define an operator Q i so that T can be written as a sum T = ∑ ∑ i = 1 N Q i . Inserting the expressions for V and T into the definition of T leads to