Point particle of point charges cannot be maintained in an equilibrium configuration solely by the electrostatic interaction of the charges. In quantum mechanics, there is a distinction between an elementary particle (also called "point particle") and a composite particle. An elementary particle, such as an electron, quark, or photon, is a particle with no internal structure. Whereas a composite particle, such as a proton or neutron, has an internal structure (see figure). However, neither elementary nor composite particles are spatially localized, because of the Heisenberg uncertainty principle. The particle wavepacket always occupies a nonzero volume. For example, see atomic orbital: The electron
Drude particle a changing local environment. Classical Drude particles are massless virtual sites carrying a partial electric charge, attached to individual atoms via a harmonic spring. The spring constant and relative partial charges on the atom and associated Drude particle determine its response to the local electrostatic field, serving as a proxy for the changing distribution of the electronic charge of the atom or molecule. However, this response is limited to a changing dipole moment. This response is not enough to model interactions in environments with large field gradients, which interact with higher order moments. The major computational cost of simulating classical