Hylleraas-Configuration Interaction calculations on the 1 ^1S 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 here and compared with other methods. The non-relativistic Hy-CI values are approximately 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. 
Maria Belen Ruiz
physics.atom-ph