Modulated perturbation theory for molecular interactions. III. Variational calculations of the second‐order energy for the ground state of H2+

Hylleraas variational calculations of the second‐order energy for the ground state of H2+ have been performed in the modified form of Murrell–Shaw–Musher–Amos (MS–MA) perturbation theory including exchange described in Paper I. Simple first‐order trial functions consisting in a single optimized Slater function for each multipole are found to be effective in representing nonexpanded multipole components of the interaction energy. The modulated form of the theory includes a large part of the spherical component of the polarization function already in first order, leaving dipole polarization as the dominant second‐order contribution.