Flexible or rigid molecular models? A study on vapour–liquid equilibrium properties of ammonia

The influence of the intramolecular degrees of freedom on the vapour-liquid equilibrium properties of ammonia is studied for vapour pressure, saturated densities and enthalpy of vaporization. Molecular force fields with and without intramolecular degrees of freedom, keeping all other parameters unchanged, show significantly different phase envelopes. For ammonia, the angle potential is particularly important, because the hydrogen sites are more aligned in the liquid than in the vapour, leading to a significantly enhanced molecular dipole moment in the condensed phase. Based on a rigid force field for ammonia from prior work of our group [Eckl et al., Mol. Phys. 106, 1039 (2008)], a new accurate force field with intramolecular degrees of freedom is developed.