The general strategy of an SCF iteration is based on the following steps:
- The molecule to be calculated (nuclear coordinates, atomic number, number of electrons or charge and spin multiplicity) and the basic set are specified.
- All integrals are calculated, the overlap integrals who have favourited Core-Hamiltonian integrals and the 2-electron repulsion integrals (νμ | λσ). Since these integrals do not depend on the expansion coefficients of the molecular orbitals, they only have to be calculated once. The calculation of the integrals is very time-consuming and, above all, saving the results requires a lot of storage space. Therefore, in "direct" methods, the integrals are calculated every time they are needed instead of storing the results.
- The first set of coefficients is "guess" and from this the density matrix P is calculated (initial guess).
- The Fock matrix is calculated with the integrals and the density matrix.
- The Fock matrix is diagonalized, resulting in new ones and εi.
- You test for convergence. A standard termination criterion is Δ < 10-6Hartrees (= 6.275 * 10-4kcal / mol). If no convergence is achieved, a further iteration step starting with point 4 is carried out.