ABSTRACT
The traditional methods are based on the use of determinantal wave-functions which are eigenfunctions of the total spin operator. F. A. Matsen, D. J. Klein, R. D. Poshusta, A. A. Cantu, R. W. Kramling, G. A. Gallup, I. G. Kaplan, and J. I. Musher worked out an alternative treatment which is based on the spatial functions. The use of the determinantal wave functions which include both spatial and spin functions obscures the fact that the expectation value of the spin free Hamiltonian depends only on the spatial functions. This spin free quantum chemistry is very important because it gives a deeper understanding of the electronic correlation problem and it avoids some common misunderstandings on the role of spin and it provides an alternative way for the calculation of the matrix elements of the Hamiltonian. The structure projectors contain permutations which operate on the electronic variables.
