A classical thermodynamic model for the equilibrium pressures of different
guests of structure I and II gas hydrates is proposed that removes the need
for reference energy parameters used in the van der Waals and Platteeuw (v
dWP) type models. The assumption of a constant crystal lattice for differen
t guests within a structure, which is not in agreement with quantum chemist
ry calculations, is removed. This model uses published Kihara cell potentia
l parameters determined from viscosity and second virial coefficient data,
unlike previous models that fit these parameters to hydrate data. Quantum m
echanical calculations were used to reduce the number of fitted parameters
in the new model. This model greatly improves upon the accuracy of previous
models and is applicable over a wide temperature range. Percent absolute a
verage deviations in the predicted equilibrium pressures is 3.27% with two
or three guest-hydrate specific parameters compared to more than 11% with t
hree adjustable parameters in the vdWP type models and 8.55% using a recent
thermodynamic model that uses five guest-hydrate parameters.