The thermally activated delayed fluorescence (TADF) phenomenon has attracted increasing attention because it can harvest 100% of the electro-pumped carriers to form singlet bound excited state for fluorescence. It is generally believed that the small energy gap between S-1 and T-1 (Delta E-ST ) is essential for TADF to facilitate the reverse intersystem crossing (rISC). However, for a few donor-acceptor (D-A) organic compounds with small Delta E-ST, the TADF phenomenon is absent, indicating that Delta E-ST might not be a good molecular descriptor. Here, using our self-developed thermal vibration correlation function (TVCF) formalism in combination with quantum chemistry calculations, we revisit the key factors that dominate the TADF property for 11 D-A systems with small Delta E-ST. Based on our theoretical results in comparison to experiments, we conclude that the activation energy Delta G is a good molecular descriptor to characterize the TADF performance because a significantly better linear relationship is observed between Delta G and the rISC rate constant (k(rISC)) compared to that between Delta E-ST and k(rISC). These findings provide deeper understanding of the TADF mechanism, shedding light on the molecular design of high-performance TADF materials.