If the cylindrical multipoles are exterior, this equation becomes where , and are the cylindrical multipole moments of charge distribution 1. This energy formula can be reduced to a remarkably simple form where and are the interior cylindrical multipoles of the second charge density. The analogous formula holds if charge density 1 is composed of interior cylindrical multipoles where and are the interior cylindrical multipole moments of charge distribution 1, and and are the exterior cylindrical multipoles of the second charge density. As an example, these formulae could be used to determine the interaction energy of a small protein in the electrostatic field of a double-stranded DNA molecule; the latter is relatively straight and bears a constant linear charge density due to the phosphate groups of its backbone. See also Axial multipole moments Potential theory Quantum cylindrical quadrupole Multipole expansion Spherical multipole moments Electromagnetism Potential theory Moment (physics)