The development of simple analytical method(s) for solving the first-order wave functions and the second-order energy for two-electron atoms has remained a major challenge of theoretical atomic physics. The failure to overcome this challenge has motivated progress in the variational-perturbation method. However, the main drawback of the method is that it fails to directly account for the mechanisms which cause nuclear charge screening usually introduced as one of the variation parameters. In the book, we have developed a simple analytical solution to the Schrodinger equation for two-electron...