This paper, divided into two parts, is devoted to the transport properties at local thermodynamic equilibrium: the first part shows the influences of partition functions through the plasma composition and the second part the influence of interaction potentials. In the first part, for complex chemical mixtures the determination of the partition functions of different species is considered: monatomic, diatomic and polyatomic. In the plasmas the monatomic species are important; we study thoroughly the partition functions of monatomic neutrals and ions. We introduce two cut-off criteria. We test the influence of the two criteria on the partition functions and consequently onto the plasma composition and transport properties. We applied the study to Ar-Cu mixtures. In the second part, an historic study shows that the collision integrals used in calculating the transport properties become more accurate leading to more reliable values of the transport coefficients: application to N-2 plasma. Now we have to calculate transport properties of complex mixtures and in these cases, for numerous interactions, a lack of data means that model potentials have to be used to determine collision integrals. In this paper, we have used two potential models: the first, for neutral-neutral and ion-neutral interactions, is an improvement of the Lennard-Jones function and the second is developed, from Stockmayer potential, for polar gases. We compare, for the collision integrals, the results obtained by these two models with those determined with more accurate potentials: applications to CO2 plasma and H-2-N-2 mixtures.