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Herein we employ density functional theory (DFT) and linear response time-dependent density functional theory (LR-TDDFT) together with our own n-layered integrated molecular orbital and molecular mechanics (ONIOM)-based quantum mechanical/molecular mechanics (QM/MM) methods to study the room-temperature phosphorescent (RTP) micro-mechanism of several benzonitrile-carbazole (CzBz-X) molecules (i.e. CzBz-H, CzBz-F, CzBz-Cl, CzBz-Br) in liquid and solid state. Based on the calculated the ground- and excited-state geometric and electronic structures, the absorption and emission spectra are simulated and agreed well with previous experimental observation. The intersystem crossing (ISC) rate constants of S1 -> T1 obtained by the formula derived from the Fermi golden rule are small in liquid state, while the ISC rate constants are comparable to the relative radiative rate constants of S1 ->S0 in solid state. Molecular vibrations are restricted in solid state, which lead to the decrease of reorganization energies and Huang-Rhys factors, and the increase of ISC rate constants. Both the heavy-atom effect and aggregation effect play important roles in improving the RTP performance in CzBz-X compounds. Through quantum chemistry calculations, the present work not only elucidates the RTP mechanism and the significance of heavy-atom and aggregation effects in CzBz-X, but also provides new insights for designing novel RTP materials.