THEORY FOR THE DIRECT CONSTRUCTION OF DIABATIC STATES AND APPLICATION TO THE HE-2(+) 2-SIGMA-G+ SPECTRUM

We present a theory for the direct construction of correlated wavefunctions representing diabatic states. Attention is given to the proper separation and subsequent computation of state-specific, multiconfigurational, optimized wavefunctions whose main features do not change significantly with geometry and whose mixing causes the breakdown of diabaticity. In the case of diatomics, the theory is implemented via a method which obtains the zeroth-order description as a numerical MCHF function and the remaining "diabatic" correlation as an expansion in terms of numerical diatomic orbitals whose effective charges are optimized from the minimization of the energy. Transparent as well as accurate wavefunctions emerge. Application to the He-2+ 2-SIGMA(g)+ valence and Rydberg states has produced results in agreement with those derived from knowledge of adiabatic curves and of d/dR matrix elements, obtained from extensive LCAO-MO configuration-interaction calculations.