Computer simulation study of liquid CH2F2 with a new effective pair potential model

A new effective pair potential model is proposed for computer simulations o f liquid methylene fluoride and used in Monte Carlo simulations on the isot hermal-isobaric ensemble at two different temperatures. The new model is ab le to reproduce the thermodynamic (internal energy, density, heat capacity, vapor-liquid equilibrium) and structural (neutron diffraction data) proper ties of liquid methylene fluoride with good accuracy. The structure of liqu id methylene fluoride is analyzed in detail on the basis of the present sim ulation at 153 K. It is found that, unlike in liquid water, the preferentia l location of the nearest neighbors is in the direction of the face centers of the tetrahedron of the central molecule. However, the four nearest neig hbors do not surround the central molecule in a highly tetrahedral arrangem ent: the obtained distribution of the tetrahedral angular order parameter i s rather similar to that in liquid argon. Preferential head-to-tail type or ientation is found for nearest neighbors, accompanied by a slight preferenc e for antiparallel dipole-dipole arrangement. The orientational correlation of the molecules is found to be rather long ranged, extending over the fir st coordination shell. The observed preferential nearest neighbor arrangeme nt is resulted from the competition of steric and electrostatic interaction s. No evidence for C-H ... F type hydrogen bonding is found in liquid methy lene fluoride. (C) 1999 American Institute of Physics. [S0021-9606(99)51506 -7].