This research is devoted to the vibrational spectroscopy inverse problem solution that gives a possibility to design a
molecule and make conclusions about its geometry. The valence angle finding based on the usage of inverse spectral
vibrational spectroscopy problem is a well-known task. 3N-matrix method was chosen to solve the proposed task. The
usage of this method permits to make no assumptions about the molecule force field, besides it can be applied to
molecules of matter in liquid state. Anharmonicity constants assessment is an important part of the valence angle finding.
The reduction to zero vibrations is necessary because used matrix analytical expression were found in the harmonic
approach. In order to find the single-valued inverse spectral problem of vibrational spectroscopy solution a shape
parameter characterizing “mixing” of ω1 and ω2 vibrations forms must be found. The minimum of such a function Υ called a divergence parameter was found. This function characterizes method’s accuracy. The valence angle assessment was reduced to the divergence parameter minimization. The β value concerning divergence parameter minimum was interpreted as the desired valence angle. The proposed method was applied for water molecule in liquid state: β = (88,8 ±1,7)° . The found angle fits the water molecule nearest surrounding tetrahedral model including hydrogen bond curvature in the first approximation.