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

Abstract

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