Full-variational treatment of GTF basis sets for molecular orbitals: application to interactions in the helium dimer under an electrostatic field

The full-variational optimization method at the SCF and MP2 levels is applied to a calculation of the intermolecular interaction energy of the helium dimer under an electrostatic field. The results under an external field show that the basis set superposition errors are suppressed by this method and that the intermolecular interaction energies calculated with s functions only are comparable to those of conventional calculations with polarization functions.