# Molecular Modeling

## SCF iteration

The general strategy of an SCF iteration is based on the following steps:

1. The molecule to be calculated (nuclear coordinates, atomic number, number of electrons or charge and spin multiplicity) and the basic set are specified.
2. All integrals are calculated, the overlap integrals $S.νμ$who have favourited Core-Hamiltonian integrals $Hνμ$ 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.
3. The first set of coefficients $cλi$ is "guess" and from this the density matrix P is calculated (initial guess).
4. The Fock matrix is ​​calculated with the integrals and the density matrix.
5. The Fock matrix is ​​diagonalized, resulting in new ones $cλi$ and εi.
6. You test for convergence. A standard termination criterion is Δ$E.dead$ < 10-6Hartrees (= 6.275 * 10-4kcal / mol). If no convergence is achieved, a further iteration step starting with point 4 is carried out.