State-specific theory and method for the computation of diatomic molecules: Application to He22+1Σg+

We present a state-specific approach to the calculation of correlated wave-functions and potential-energy surfaces (PES's) of ground and excited states of diatomic molecules. Its emphasis is on the optimal choice of zeroth-order and of correlation-function spaces, which are computed separately. The zeroth-order Fermi-sea multiconfigurational wave function is obtained numerically, using McCullough's partial-wave multiconfiguration self-consistent field [Comput. Phys. Rep. 4, 265 (1986)] program PWMCSCF and numerical one-electron two-center orbitals as input. The correlation functions are obtained from partial and total configuration interaction (CI), using two-center virtual molecular orbitals, optimized by minimizing the energy. The method is demonstrated on the prototype He22+1Σg+. A number of wave functions of acsending accuracy have been calculated. The most accurate one is composed of 116 configurations (multiconfiguration Hartree-Fock plus higher-order correlations), arising from 43 orbitals. It yields results which are lower over the entire PES than those obtained from the conventional linear combination of atomic orbitals including full CI with 158 basis functions and 2282 configurations. Compared with a published large CI calculation with rij-dependent basis sets, only at the equilibrium position, where the influence of the Coulomb cusp increases, does the present approach yield a slightly (4x10-4 a.u.) higher energy. For the rest of the PES, our calculation yields the lowest energies yet