# MO methods

## 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$P.$-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).