where âiσ (â † iσ ) are the annihilation (creation) operators of the single-particle states w i (r)χ σ (i) on atom i = 1, 2 with spin σ. Note that the single-particle basis is composed of the Hückel-Slater orbitals only; this restriction represents the only approximation here; all the remaining analysis and results are exact within these limitations. Those operators, in turn lead to the expression (15) of Ĥ in a standard manner. To close the formal methodological part, the two-particle spin-singlet wave function is defined as