1 Ju l 2 00 3 Exploring the Harmony between Theory and Computation - Toward a unified electronic structure theory

The physical aspect of a general perturbation theory is explored. Its role as a physical principle for understanding the interaction among the matters with different levels of hierarchy is appreciated. It is shown that the general perturbation theory can not only be used for understanding the various electronic phenomena including the nature of chemical bonds but also serve as a unified theme for constructing general electronic structure theories and calculation schemes. Perturbation theory is regarded as one of the two major approaches for approximately solving quantum many-body problems. However, its deeper physical aspect is far more than it is currently being used just as a mathematical tool for solving the complicated issues. All the fundamental laws in physics are variational in nature, including the Schrödinger equation in 1