The most obvious simplification to the Schrodinger equation involves separation of variables, replacement of the many-electron wavefunction by a product ofone-electron wavefunctions. The simplest acceptable replacement, termed a Hartree-Fock or single- determinant wavefunction, involves a single determinant of products of one-electron functions, termed snin crbitals. Each spin orbital is written as a product of a space part, y, which is a function of the coordinates of a single electron and is referred to as amolecular orbital, and one of possible spin parts, o or B. Only two electrons may two occupy a given molecular orbital and they must be of opposite spin
The Hartree-Fock approximation leads to a set of coupled differential equations(the Hartree-Fock equations, each involving a single electron. While they may be solved numerically, it is advantageous to introduce one additional approximation.