Phonon anharmonicity driven charge density wave transition and ultrafast dynamics in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mn>1</mml:mn> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mo>−</mml:mo> <mml:mrow> <mml:mi>Ta</mml:

Charge density wave (CDW), a symmetry-breaking collective phenomenon in condensed matter systems, exhibits periodic modulations of electron density coupled with lattice distortions, where the lattice plays a critical role via electron-phonon coupling. In transition metal dichalcogenides (TMDs) $1T\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}/\mathrm{TaS}{\mathrm{e}}_{2}$, experiments reveal rich temperature- and pressure-dependent CDW phase behaviors, along with metastable CDW states induced by ultrafast optical excitation. Nevertheless, the underlying mechanisms governing thermal/pressure-driven transitions and particularly the microscopic evolution of CDW phases remain incompletely understood. Here, we perform first-principles anharmonic phonon calculations and machine-learning force-field molecular dynamics at finite temperatures/pressures to investigate the CDW transitions in $1T\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}/\mathrm{TaS}{\mathrm{e}}_{2}$. The calculated CDW transition temperature ${T}_{\mathrm{CDW}}$ and critical pressure ${P}_{\mathrm{c}}$ are in quantitative agreement with experimental values. Our results demonstrate that the melting of CDW originates from phonon anharmonicity, with ionic fluctuations dominating the transition dynamics. We observe the microscopic evolution of CDW under varying temperature/pressure, revealing an ultrafast nucleation process of CDW (\ensuremath{\sim}3 ps). Our results emphasize the essential role of phonon anharmonicity in elucidating CDW transition mechanisms underlying, and advance fundamental understanding of CDW-related phenomena in TMDs.