15.1.5.1.1 AB INITIO

The term “Ab Initio” is Latin for “from the beginning.” This name is given to com­putations, which are derived directly from theoretical principles, with no inclusion of experimental data. Most of the time this is referring to an approximate quan­tum mechanical calculation. The approximations made are usually mathematical approximations, such as using a simpler functional form for a function or getting an approximate solution to a differential equation.

The most common type of Ab initio calculation is called a Hartree Fock calcu­lation (abbreviated HF), in which the primary approximation is called the central field approximation. This means that the Coulombic electron-electron repulsion is not specifically taken into account. However, it’s net effect is included in the cal­culation. This is a variational calculation, meaning that the approximate energies calculated are all equal to or greater than the exact energy. The energies calculated are usually in units called Hartrees (1 H = 27.2114 eV). Because of the central field approximation, the energies from HF calculations are always greater than the exact energy and tend to a limiting value called the Hartree Fock limit.