Exploring the Nature of the H2 Bond. 2. Using Ab Initio Molecular Orbital Calculations To Obtain the Molecular Constants

A project for students in an upper-level course in quantum or computational chemistry is described in which they are introduced to the concepts and applications of a high quality, ab initio treatment of the ground-state potential energy curve (PEC) for H 2 and D 2 . Using a commercial computational chemistry application and a scientific spreadsheet, students learn how to obtain the energy of H 2 (D 2 ) employing configuration interaction with single and double excitations (CISD) and an extrapolation technique to estimate the energy in the complete basis set (CBS) limit. They construct the PEC from the CISD/CBS energies obtained at several internuclear separations. Students obtain the equilibrium dissociation energy and internuclear separation, as well as the rovibrational molecular constants, from the regression parameters of a sixth-order polynomial fit to the PEC. Results are in nearly exact agreement with experimental data.