where τ σ is the positive kinetic energy density (and not the Hamiltonian, which can also be locally negative ), and ρ σ is the single spin electron density. Due to the truncation of the Taylor series, only the two terms inside the square brackets, denoted together as D σ , are taken, while the other terms outside the brackets are dropped, since they now would serve only as scaling factors. Therefore, these prefactors are removed in the mathematical passage: