Lcao-Xα calculations of rotational energy barriers—prototypes of chemical reactions

The LCAO-X α method is used to compute the X α rotational barrier for ethane both in the rigid-rotor approximation and for optimized staggered and eclipsed geometries. The problem of how to pick points and weights in three dimensions for fitting the nonanalytic exchange and correlation functionals of local density-functional theory is examined; a variational weighting procedure is developed which greatly reduces the size of errors due to the 3-D grid as compared to simple volume-element weighting. The accuracy of the X α results for the optimized geometries and the barrier height of ethane is found to be comparable to that of large-basis Hartree–Fock calculations.