Hylleraas-Configuration Interaction calculations on the 11S ground state of helium atom

Hylleraas-Configuration Interaction (Hy-CI) calculations on the ground 1(1)S state of helium atom are presented using s-, p-, d-, and f-Slater orbitals of both real and complex form. Techniques of construction of adapted configurations, optimization of the orbital exponents, and structure of the wave function expansion are explored. A new method to evaluate the two-electron kinetic energy integrals occurring in the Hy-CI method has been tested in this work and compared with other methods. The non-relativistic Hy-CI energy values are approximate to 10 picohartree accurate, about 2.2 x 10(-6) cm(-1). The Hy-CI calculations are compared with Configuration Interaction (CI) and Hylleraas (Hy) calculations employing the same orbital basis set, same computer code, and same computer machines. The computational required times are reported.