Volume 113, number 2 CHEMICAL PHYSICS LETTERS 11 January 1985 ON THE THIRD VIRIAL COEFFICIENT FOR THE ALKALI METAL VAPOURS V.M.F. MORAIS 1 Instituto de Cidncias Biomddicas Abel Salazar, Universidade do Porto, 4000 Porto, Portugal and A.J.C. VARANDAS Departamento de Qufmica, Univesidade de Coimbra, 3000 Coimbra, Portugal Received 16 January 1984; in final form 29 March 1984 Theoretical calculations of the third virial coefficient for the pure components of all alkali metal vapours have been car- ried out using recently reported potential energy surfaces for the doublet and quartet states in which the interaction of three 2S ground-state atoms may evolve. The discrepancy between the theoretical and available experimental estimates for those coefficients is pointed out. 1. Introduction Interest in the use of alkali metals as working fluids in the technology of nuclear energy has led to an in- creasing need for knowledge about thermophysical properties in the liquid and vapour phases. However, experimental measurements are difficult to perform for these systems at the temperatures of interest, which explains their scarcity and unreliability. An alternative way to improve our knowledge on the thermophysical properties of those systems is via theoretical calculations. In this work we make, to our knowledge, the first attempt to get the third virial coefficients for the al- kaly metal vapours by using the potential energy sur- faces for the relevant molecular electronic states which are obtained from three 2S colliding atoms. Our ob- jective is twofold: firstly, we wish to investigate how important are the contributions from excited poten- tial energy surfaces to the total third virial coefficient, and secondly we may compare the present results for the latter with the available experimental estimates as a function of the temperature. In section 2, the calcu- 1 Work carried out as part of an MSe degree in Theoretical Chemistry at the University of Oporto. lation of the classical third virial coefficient for a di- lute gas is briefly surveyed while section 3 describes the potential energy surfaces we employ in this study. The results are given in section 4, and the conclusions are summarized in section 5. 2. The third virial coefficient At low densities the equation of state of a real gas can be described, in a convenient way, by the virial ex- pansion PVm/RT = 1 + B(T)/V m + C(T)/V2m + D(T)/V 3 + .... (1) where P is the pressure of the gas, T is the absolute temperature, V m is the molar volume, and R is the ideal gas constant. The coefficients B (T), C(T), D(T) .... in eq. (1), which are dependent on the tem- perature and on the potential energy for the interac- tion between the particles which form the gas, are called second, third, fourth,.., virial coefficients. For a real gas, the second virial coefficient accounts for deviations from the ideal gas behaviour that are caused by two-body interactions, the third virial coefficient for deviations which are due to three-body interac- 192