The dependence of exchange and coulomb energies on wave functions of the interacting systems

First-order energies have been calculated for the He2 system using a number of He wave functions. The convergence to the Hartree-Fock limit has been studied and a two-term expansion in Slater orbitals gives about 98 per cent of this limit. The effect of correlation has been examined by adding s′2 and p 2 configurations to the wave function and a change of approximately 10 per cent in the effective first-order energy has been noted.