An updated hydrate thermodynamic model is developed that uses an equation of state to represent the fluid phases that are in equilibrium with gas hydrates (PSRK). Model parameters are fit to only a subset of data for single-guest or binary-guest hydrates (when there is a hydrate structure change in the mixture) to demonstrate the ability of this model to extrapolate beyond the conditions of parameter fits. Moreover, guest-host interactions are based on quantum mechanical energies and are not fit to experimental data. The level of accuracy in the resulting model for single-guest hydrates is comparable to that for modern hydrate models (5-6% absolute average deviation from experiment, % AAD). For gas hydrates with multiple guests and electrolyte solutions, the new model is primarily predictive with 9% AAD. This model is also able to predict hydrate structure changes and indicates that stability of structure II (sII) hydrates in mixtures with propane depends on temperature conditions above the normal freezing point of water. Based on this new thermodynamic model, the thickness of the permafrost hydrate stability zone can increase by 40-50% if a natural gas mixture contains 92% methane instead of pure methane. This increased stability of AI hydrates compared to that for structure I pure methane hydrates may greatly influence estimates of natural gas hydrate reserves in the seafloor and permafrost.