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Many transition metal dichalcogenides (TMDs) host interesting properties arising from charge and spin ordering, superconducting pairing, and topological electronic effects, among which charge density wave (CDW) formation is common, but the detailed mechanisms remain murky in many cases. Models including Fermi surface nesting, static or dynamic phonon coupling, excitonic interaction, and many-body correlation effects are potential candidates. Specifically, TiSe₂ in the bulk form shows a simple commensurate (2×2×2) CDW transition, while a single layer of TiSe₂ shows a similar (2×2×2)  transition but at a higher transition temperature. The underlying driving mechanism for the CDW formation is still under debate. To gain a better understanding, we have performed first-principles calculations of the electronic and phononic structures for various distorted structures. The goal is to develop a method to predict the CDW transition temperature and the nature of the CDW phase. This talk will report our results.