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The Van der Waals force between atoms is not strong enough in state X to explain the presence of a potential well that when low (the depth is in the order of kilotorr) can contain between 12 and 20 vibrational levels (see Table 3). The relative increase in the binding energy of state X compared to state A can also be explained by taking into account the configuration interaction. State A is also very lightly bound with binding energy half that of state X.