Dimer centred basis set in the calculations of the first-order interaction energy with CI wavefunction

Assuming the monomer wavefunction in the form of a CI expansion in terms of orbitals of the whole dimer, the expression for the Heitler-London interaction energy is analyzed and implemented in the calculations of the He dimer. It is proved that the ‘zeroth-order exchange’ term Δ vanishes if the CI expansion is a full one. If the CI wavefunctions are complete up to n-fold excitations the contributions to Δ can be related only to excitations higher than n-fold ones. If the gaussian orbitals are used the problem of a gaussian asymptotic behaviour of the CI ansatz appears to be effectively circumvented.