The double many‐body expansion of potential energy surfaces from interacting ²S atoms

The double many‐body expansion ( DMBE ) method has been applied to the many‐valued ground‐state potential energy surfaces from three and four interacting 2 S atoms. New forms are suggested where the degeneracy manifold of the two Riemann sheets of the potential surface can be made identical to that predicted from the London equation that is correct to first‐order. Considerable flexibility can be introduced through adjustable coefficients to fit ab initio data, whereas the potential energy terms can be chosen to maintain the correct analytic properties at the conical intersection. Hence, the new forms may be used on the extrapolation of the potential energy from the lowest‐ to the upper‐Riemann sheets. Long‐range effects are semiempirically described from the general formalism of the DMBE method.