Theoretical study of intermolecular potential energy and second virial coefficient in the mixtures of CH4 and H2CO gases

To get a mole of a gas, it is necessary to calculate the intermolecular interaction. These intermolecular interactions can be depicted by drawing the potential energy of a pair molecule in relation to the distance. The intermolecular potential energy surface in the mixtures of CH4-H2CO gases from ab initio calculations has been explored. In ab initio calculations the basis set superposition error (BSSE) is important. This error can be eliminated to some extent by using the counterpoise correction method (CPC). In this work ab initio calculation performed at the secondorder Moller-Plesset theory, MP2, with the 6-311+G(2df,2pd) basis set, for six relative orientations of two CH4- H2CO molecules as a function of CH4-H2CO separation distance. Then, the adjustable parameters of Buckingham potential energy function are fitted to the ab initio MP2/6-311+G(2df,2pd) interaction energies for six different orientations. Assuming a given set of parameters, we obtained theoretically second virial coefficients for CH4- H2CO system in different temperatures.