which has specific centers of molecules xJ . This ÛN is still a boost operator on pj (of the j th charged particle that belongs to the J th molecule), given that we assume the individual molecules are neutral, much smaller than the wavelength of the mode, and can be well described by their dipoles. Under these approximations, ÛN pj Û † N = pj + q j Â( xJ ). We can also evaluate ÛN âk Û † N as, ÛN âk