Dirac–Fock finite element method (FEM) calculations for some diatomic molecules

A two-dimensional, fully numerical approach to the four-component first-order Dirac–Fock differential equation using the finite element method (FEM) is employed for diatomic systems. Using this method we achieve for the molecules H2, LiH, BH, HF, N2, CO and HCl a relative accuracy better than 10−8 for total energies. Up to 6561 grid points are used. Unexpectedly large truncation errors of 1–2 mhartree for HF and HCl are exhibited for even rather big basis-set calculations.