Second virial coefficients of H2 and its isotopologues from a six-dimensional potential.

We employ path-integral Monte Carlo techniques to compute the second virial coefficient as a function of temperature for molecular hydrogen (H(2)), deuterium (D(2)), and tritium (T(2)), along with the mixed isotopologues HD, HT, and DT. The calculations utilize a new six-dimensional (6D) potential, which is derived by combining our previous high-quality ground-state 4D potential for the H(2) dimer with the 6D potential of Hinde. This new 6D potential is reduced to a set of 4D potentials by fixing the intramolecular coordinates at their expectation values for each temperature and isotopic combination. The results for H(2) are in good agreement with experimental data; the effect of the temperature dependence of the average bond length is only significant above approximately 1000 K. For D(2) and HD, the available experimental data are much more limited; our results agree with the data and provide reliable values at temperatures where no experimental data exist. For the species containing tritium, our results provide the only data available.