Use of double-bound three-body Coulomb distorted-wave-like basis sets for two-electron wave functions

The double-bound analog of the double-continuum three-body Coulomb (C3) wave function, recently introduced by the authors, allows us to propose two different basis sets of correlated functions which can be used for describing double-bound states. The components of the two basis diagonalize the Coulomb interactions but differ in the definition of the distortion factor which depends only on the interelectronic coordinate. In the first set, this factor includes a nonlinear parameter, while the second set contains parameter-free functions. Through an angular correlated configuration interaction method it is possible to generate highly correlated wave functions for S bound states of heliumlike systems. By construction, these states form an orthogonal set, satisfy all two-body cusp conditions, and retain the C3 double-continuum character. Even when a limited number of configurations is included, rather good energy values for the ground and excited states are obtained. A systematic improvement by inclusion of more configurations can be easily achieved, in particular with the computationally efficient, parameter-free basis.