To derive such a Hamiltonian, we start from the minimal coupling Hamiltonian (Eq. 45), following the framework discussed in Ref. 46. It is convenient to rewrite this Hamiltonian by grouping the matter particles into well-separated molecules, where the intermolecular distances are much longer than the intramolecular;; distances. In such circumstances we can write Â(x j ) ≈ Â(x J ) for all j particles within the molecule J with center of mass of the molecule xJ and the total Hamiltonian is written as