On the Boys Bernardi–method to correct interaction energies calculated using Møller-Plesset perturbation theory

The Boys and Bernardi method has been used to correct the interaction energy of a molecular association involving two molecules of alanine interacting through their respective carboxylic groups; while the SCF calculation predicts the association to be very stable as expected considering the two hydrogen bonds involved, the second-order Møller–Plesset perturbation theory provides a strongly repulsive interaction energy; it is shown that a number of virtual orbitals with a relatively large contribution from a ghost basis set may be responsible for this.