Multipole Expansion for the Calculation of Field-Dependent Molecular Ionization Potentials

The field-dependent ionization potential of six molecules relevant for transformer insulating liquids has been examined by an analytical expression based on a multipole expansion. Molecular properties were calculated by density functional theory at zero field. We found that many molecules are explained well by a classical Frenkel model at low to moderate field strengths. Large molecules and molecules with a significant permanent dipole moment require additional terms, especially the interactions between the electron and the dipole and polarizability of the cation.