The total position spread Λ is defined as the second moment cumulant of the total electron position operator, and its units are in length square (e.g. bohr²). In order to compute this quantity, one has to take into account the position operator and its tensorial square. For a system of n electrons, the position operator and its Cartesian components are defined as: r ^ ^ = ∑ ∑ i = 1 n r ^ ^ i (total position) x ^ ^ = ∑ ∑ i = 1 n x ^ ^ i y ^ ^ = ∑ ∑ i = 1 n y ^ ^ i z ^ ^ = ∑ ∑ i = 1 n z ^ ^ i