Constant pressure molecular dynamic simulations of gas hydrates with guests of different size indicate that the equilibrium cell volume increases with guest size. The effect of lattice stretching due to guest size on the reference chemical potential difference between the empty lattice and water is calculated, and a new thermodynamic model with a guest dependent reference chemical potential difference, Delta mu degrees, is used for predicting the equilibrium conditions for single component (one gas + water) gas hydrates and multicomponent gas hydrates. This model avoids potential parameter adjustment (fitting) for prediction of equilibrium conditions. A relationship for obtaining the reference chemical potential difference for multicomponent gas hydrates (mixture gas hydrates) from the hydrate phase composition is proposed.