semi-empirical calculations

A technique for calculating atomic and molecular quantities in which the values of integrals are found using quantities derived from experiment (such as ionization energies obtained spectroscopically). Semi-empirical calculations were formerly widely used, particularly for large molecules involving extensive computation to calculate all the integrals using ab-initio calculations. As computing power has increased, the properties of large molecules can be calculated by ab-initio calculations, therefore the use of semi-empirical calculations has steadily declined, first for small molecules and more recently for large molecules.