Ferroelectric molecular crystals are a promising class of materials with lower cost and environment-friendly preparation process. In this work, by combining first-principles calculations and molecular dynamics simulations, we report a systematic study of of ferroelectricity in three molecular ferroelectrics: di-isopropyl-ammonium chloride (DIPAC), di-isopropyl-ammonium bromide (DIPAB) and di-isopropyl-ammonium iodide (DIPAI). Our results agree with previous electronic structure calculations and provide new insights into the mechanism of the phase transition.