A computational study of the structures of Van der Waals and hydrogen-bonded complexes of ethene and ethyne

The structures of the weakly bonded complexes XCl2 and XHCl, where X is either C2H4 or C2H2, are studied using density functional, hybrid density functional, Hartree-Fock and MP2 calculations. While most of the methods used give reasonable agreement with experimental data for bond lengths, the performance of the density functional methods for bond energy and distortion constants are not particularly satisfactory. The hybrid density functionals are less accurate for Van der Waals than for hydrogen-bonded complexes. Our results show that the softest normal modes in the complexes are not accurately treated at all by any of the density functional methods.