<p>This study investigates molecular structure and vapor pressure of molybdenum pentachloride (MoCl₅) in the gas phase using ab-initio thermodynamics. MoCl₅ exists as either a monomer (MoCl₅) or a dimer (Mo₂Cl₁₀). While the monomer is thermodynamically favored above 215&#xa0;K, the dimer remains kinetically stable due to the higher energy required to dissociate the monomer from the solid phase than the dimer. The study’s calculations reveal that the vapor pressure of the dimer aligns closely with experimental data, suggesting that MoCl₅ likely exists as a dimer in typical atomic layer deposition (ALD) conditions. This finding challenges the assumption that monomers dominate at higher temperatures, highlighting the role of kinetic factors in maintaining the dimer structure. The results emphasize that the vapor pressure of gaseous species like MoCl₅ can be predicted using density functional theory (DFT) rather than relying solely on experimental methods.</p>