Abstract
Three body forces acting on lithium, oxygen, and hydrogen atoms have been calculated directly by ab initio molecular orbital methods for more than a thousand relative intra‐ and interionic Li+‐OH configurations in order to evaluate the accurate interactions. The forces have been decomposed in two ways. The first method expresses the forces on O and H atoms of OH ion in terms of the translational, the rotational, and the vibrational components. The vibrational forces have been optimized to the differential form of the Morse function and some spectroscopic parameters for OH stretching mode have been evaluated. It is clarified that they are largely dependent on the coordinating position of Li+ ion. A Li+ ion on the oxygen side causes a blue shift of the spectrum while the ion in the hydrogen side takes a red shift. These are directly related to the inhomogeneous broadening in Raman scattering and infrared absorption spectra. The second method leads the forces to a set of pairwise additive interatomic interaction forces. It has been also shown that the interatomic forces obtained here can be applied to higher order many body systems with good precision. The evaluation of the many body force is remarkably improved by considering the three body forces for the pair of the ions.