With an operating Atacama Large Millimeter Array telescope and the recently launchedJames Webb Space Telescope, astronomy enters its golden age. Physical conditions of the interstellar medium (ISM) are derived from molecular spectra. The interpretation of these spectra requires to know the population of energy levels of the chemical species. However, most of the astrophysical media do not fulfill local thermodynamic equilibrium conditions. are determined by taking into account both radiative, characterized by Einstein coefficients, and collisonal, characterized by rate coefficients, transitions. Nowadays, the rate coefficients, which are system-specific, can only be computed for ”small” molecules colliding with light partners. However, in media like cometary or planetary atmospheres, the dominant colliders are ”big” molecules, such as H2O, CO, etc. Our understanding of the chemical composition of the universe is limited by computer power. Therefore, numerical strategies need to be elaborated to compute accurate rate coefficients for ”big” collisional systems. In this work, limitations and strategies that can be put into place to treat colliding systems involving H2O will be presented through the example of CS-H2O and HCN-H2O. This study starts by quantum calculations in order to reach convergence for as many transitions as possible. The approaches used are the ”exact” Close-Coupling method, that will be then supported with Coupled-State or infinite order sudden approximations. In a second part, collisional data are computed with statistical methods. The accuracy of the results obtained with the different methods will be evaluated by a comparison with the results obtained with quantum calculations