Thermodynamic Properties for Applications in Chemical Industry via Classical Force Fields - Multiscale Molecular Methods in Applied Chemistry - page 232

distance of 0.08 ˚ from the nitrogen nucleus. The LJ parameters, partial charge
magnitudes, and the position of the displaced nitrogen charge were optimized to
vapor-liquid equilibrium data.
Eckl et al. [ 97 ] introduced a semi-empirical force field for ammonia also based
on one LJ 12-6 site and four partial charges that are located at the nitrogen and
hydrogen positions. The geometry was calculated at the self-consistent field HF
level of theory with a 6-31G basis set. The resulting geometry 0136 ˚ ð r NH ¼ 1 : ;

0124 ˚ 99 Þ

∢ HNH ¼ 105 : is very close to the experimental data ð r NH ¼ 1 : ;

67 Þ

∢ HNH ¼ [ 248 ]. Eckl et al. [ 97 ] adjusted the partial charge magnitudes to

the results from a single point QM calculation at the MP2 level of theory with the
polarizable basis set 6-311G(d,p) using the COSMO [ 90 ] method to account for the
liquid polarizability. Only the two LJ parameters were adjusted to experimental
data on saturated liquid density, vapor pressure, and enthalpy of vaporization.

106 :

6.2 Vapor-Liquid Equilibria of Ammonia

Both the GEMC and the grand equilibrium method have been applied to evaluate
vapor-liquid equilibrium data for ammonia. Krist´f et al. [ 246 ] calculated the vapor
pressure and saturated densities using the force field by Impey and Klein [ 108 ] and
found systematic deviations from experimental data; cf. Fig. 3 . Therefore, they
proposed a new ammonia force field that was optimized to vapor-liquid equilibria
[ 246 ], achieving a better accuracy. Simulated saturated densities and enthalpies
based on this force field agree with the experimental data within 1 and 3%,
respectively. However, it shows a mean deviation of 13% from experimental

Fig. 3 Saturated densities of ammonia on the basis of different force fields by Impey and Klein ( open
diamonds )[ 108 ], Krist´fetal.( open squares )[ 246 ], Eckl et al. ( open circles )[ 97 ], as well as Zhang
and Siepmann ( open inverted triangles )[ 247 ]. The simulation results are compared with a reference
equation of state ( solid line )[ 249 ]. The calculated critical points ( full symbols ) are also shown