In the field of detonation science or shock physics, predicting the thermodynamic properties and chemical compositions of detonation products of CHON systems (outcome of the decomposition of explosives) over a wide range of temperature and pressure with a very small computational cost is of great importance. Thermochemical codes such as the CARTE code use several methods published in the literature to compute the properties of these CHON systems. In this work, we evaluate the accuracy of the recently improved KLRR perturbation method and we propose a new mixing rule to reduce the fluid mixture to an effective pure fluid. Then, the results obtained with these two contributions are presented.