A choice of the semirigid bender model of the water molecule

In order to minimize momentum coupling between small- and large-amplitude internal motions special intrinsic semirigid models should be used. The intrinsic model for the water molecule is constructed as a Taylor series in the bending angle. Using sets of ab initio force constants up to quartic, expansions can be found up to and including cubic terms. A polynomial fit to the ab initio value of the bond length of the linear saddle configuration is used to find the quartic term in the expansion.