The structure ofpara‐chlorobenzyl alcohol (pCBA) andpara‐bromobenzyl alcohol (pBBA) crystals are characterized by the O&sngbnd;H&cellip;O hydrogen bonded chains along c axis. The direction of the hydrogen bond in the low temperature phase (LTP) is opposite to that in the room temperature phase (RTP). These transitions are related to the hydrogen bonds. We performed the molecular dynamics simulation ofpCBA andpBBA crystals in the LTP and RTP using forces determined by gradient of the energy within semiempirical molecular orbital calculation (PM3). From the distribution of the hydrogen atom of −OH group, we found that the hydrogen atoms in the LTP vibrate at the equilibrium position within one site or asymmetric potential. In the RTP, the hydrogen atoms ofpBBA are jumping between the two sites in the symmetric double minimum potential. © 2004 American Institute of Physics