Polarizability is the relative tendency of an electron cloud (a charge distribution) of a molecule to be distorted by an external electric field. The more stable an ionized (charged) site is the more polarizable its vicinity is. This is why atomic polarizability is an important factor in the determination of pKa and why it is considered in our pKa Plugin. Atomic polarizability is altered by partial charges of atoms. The Polarizability Plugin is able to calculate the atomic and molecular polarizability values. We use two methods to calculate polarizability: one of the calculations is based on the atomic parameters in Miller et al., while the other is based on the parameters in Jensen et al.