in which the mass flux is given by:
in which indicates integration of the enclosed quantity over the momentum space for a molecule of species (the only contribution in the case of a dilute monatomic gas), and indicates an integration over all the internal coordinates of a molecule of species (needed because of the extension of the polymer molecule in space). The is the set of appropriately chosen "connector vectors" (often coincidental with the springs).
When we make the choice of B in Eq. 2.10, we then get the equation of motion:
in which the momentum flux is given by :