normal coordinate analysis 

https://doi.org/10.1351/goldbook.08645
Calculation of the normal modes of vibrations of a molecule or crystal under the assumptions that the potential energy is harmonic and the displacements from equilibrium are infinitesimal.
Source:
PAC, 2021, 93, 647. (Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)) on page 756 [Terms] [Paper]