 Diffused functions: In the case of negative ions, where the electron is held far away from the nucleus, the atomic orbital is distorted from its original shape. For the correct description of such kind of system, it is required to the addition of Gaussian functions with a small exponent, known as diffused functions. These functions represent the "tail" of the atomic orbitals or the regions which are far from nuclei. The diffused basis functions are necessary for the negative ions, very electronegative atoms, excited states and weak bonds (e.g. hydrogen bonds). Furthermore, the diffused basis functions are mandatory to use for the calculation of some of the properties which depend on the tail of the wavefunction (e.g. dipole moment and polarizabilities). The basis set, 6-311+G(d,p) is an example of diffused basis set, which is generally used in the evaluation of chemical properties of interest.