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We report results of a simulation of the phase transitions in CsLiSO4. These are based on our previously developed method for calculating parameter-free potential-energy surface for ionic molecular crystals. Our lattice-dynamical and molecular-dynamics studies show that the roomtemperature (Pnam) phase is unstable and transforms to the observed low-temperature (P1121/n) phase over approximately 200–280 K. The unstable modes of the Pnam phase have maximum instability at the zone center, which indicates a possible phase transformation without a cell multiplication. The rotational ordering of tetrahedral SOM4 2- was found to be the driving mechanism of these phase transitions. The quality of the agreement between theoretical and experimental structural parameters and transition temperatures confirms that our potentials for Li+ containing sulfates are of comparable accuracy to those for other alkali sulfates.