## The most important calculation steps

For a semi-empirical SCF-MO calculation, the following steps are usually run through in the computer program:

- Reading the input file (* .M05) and building the 3D structure
- Assigning the semi-empirical parameters contained in the program
- Calculation of the overlap integrals, the one- and two-electron integrals and construction of the Fock matrix from a starting$\mathrm{P.}$-Matrix (start coefficients) for the considered electrons
- SCF iteration process up to self-consistency of energies and electron structure
- Calculation of the total energy = electron energy + core-core repulsion
- Determination of electron densities (atomic charges) and dipole moment
- optional calculation of gradients of the energy (forces, first derivative of the energy with respect to the structural degrees of freedom) to optimize the structure of the molecule
- optional calculation of the second derivative of the energy (force constant matrix) for a vibration analysis
- Output of the results (* .M06, etc.)

These steps can be followed using the example of a MOPAC calculation (ethanol).