A simple method to calculate potential curves of two-electron molecules at intermediate nuclear distances

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Abstract
A simple configuration interaction method to calculate accurate electronic energies of two-electron molecules at intermediate internuclear distances is presented. The correlated basis functions are constructed from H2+ molecular orbitals (exact solution of the two-centre Coulombic problem), and include an explicit dependence on the interelectronic distance in order to describe the major part of the electronic correlation. The first Green transformation is used to give an important simplification of the expression for the matrix elements of the Hamiltonian. The method is shown to give accurate energies for the H2 molecule in the ground 1 Sigma g state at intermediate distances, and is applied to determine new potential curves of highly excited states. The simplicity of the wavefunction will allow it to be used relatively easily in subsequent calculations of dynamical properties.