 Although the rotational and vibrational motions of the nuclei in a molecule cannot be fully separated, the Eckart conditions minimize the coupling between these two.The Eckart conditions can only be formulated for a semi-rigid molecule, which is a molecule with a potential energy surface "V"(R1, R2..R"N") that has a well-defined minimum for R"A"0 (formula_1).These equilibrium coordinates of the nuclei—with masses "M""A"—are expressed with respect to a fixed orthonormal principal axes frame and hence satisfy the relations
Here λi0 is a principal inertia moment of the equilibrium molecule.