This work aims at providing theoretical values of the refractive index of some gas hydrates using up-to-date physical data. Indices of refraction have been computed with a modified Lorenz-Lorentz model for a pressure between 2 and 15 MPa, a temperature between 0 and 12degreesC and wavelengths in the extended visible domain. Given the prohibitive time required to perform the calculations with the standard procedure (up to 20 min for one P-T-lambda set), we considered separately the crystalline structure and the molecules that occupy its sites. Thus, we obtain a series of tensors, characterising the sole structure, that we can easily use to get the refractive indices within less than 1 s. In this paper, all data are given to calculate the absolute or relative refractive index of any gas hydrate, provided the host gas polarisability is known. The numerical results are in agreement with experimental data.