Calculating the ionization energy of atoms is a simple process that requires basic knowledge of the electron configuration arrived at through Koopman's theory. Ionization energy is the energy an electron takes to detach itself from a neutral atom. Some elements have more than one ionization energy.
One of the two main methods of calculating the ionization energy is the Koopman’s theory, which involves the HOMO. In this method, the ionization energy of a molecule or an atom is equal to the orbital energy from which it was ejected. The equation is given by Ii = -ei.
Where ei is the orbital energy and i is the orbital where the electron was located before the ejection. This formula is ideal for hydrogenic atoms. In fact, it gives an approximation for other atoms because it does not account for the movement of the electron after its ejection. However, it assumes that the electrons remain in the same orbitals after the ionization process.
The subtraction method is the second method of calculating the ionization energy of atoms. However, it involves some experimentation whereby the value of the energy of the ion must be determined. The energy of the neutral atom is then subtracted from the experimental value to get the ionization energy.