Potential energy surfaces of H
2, H
2+ and H
2- molecules calculated by MP2, B3LYP with 6-311G** basis set are compared with those calculated by Hartree-Fock (HF). H
2+ has no exchange correlation, and no electron correlation. H
2 has exchange correlation and electron correlation. H
2- has larger exchange correlation and electron correlation than H
2. In the case of H
2, potential surfaces of HF and B3LYP are almost the same not only in the equilibrium region but also in the long inter-atomic distance region. However, the potential surfaces of H
2+ and H
2- generated by B3LYP are different from those of HFs, especially in the long inter-atomic distance region. The potential surfaces have maxima at about 3Å and are qualitatively wrong at long distances.Potential energy surfaces for H
2 and H
2- obtained by MP2 are qualitatively the same as HF.
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