Short-Range Interactions and Orbital Expansions

In the standard models of quantum chemistry of Chapter 5, the A -electron wave function is expanded in products of the one-electron basis functions introduced in Chapter 6. In the present chapter, we shall examine the orbital expansion of the many-electron systems by considering a simple two-electron system the ground-state helium wave function. In this manner, we shall obtain a clearer picture of the possibilities and limitations offered by the orbital expansion for many-electron systems. The information obtained by studying the helium system will later prove useful when we go on to construct molecular basis sets from AOs in Chapter 8. 

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