Isotopic sum rules and compliance scheme

It is known that compliance scheme does not provide simple linear isotopic sum rules in λ ^-1 similar to the ones in λ in the force constant scheme. The advantage of not having such sum rules in the compliance scheme is discussed in relation to the uniqueness problem of the molecular force field. Incidentally, for the molecules H ₂O, D ₂O, and HDO, two quadratic relations in terms of the inverse frequency parameters λ ^-1 are obtained. These supplement the three obvious product rule relations.