Gaussian AO bases

We now turn to considering calculations with the AOs represented by sums of Gaussians. This approach was pioneered by Boys[33], and is used almost universally today. We will settle on a particular basis and investigate its use for a number of different VB-like calculations. 

We define a ten-function AO basis for the H2 molecule that has two different s -type orbitals and one / -type set on each H atom. It will be recalled that Weinbaum allowed the scale factor of the s orbital to adjust at each intemuclear distance. Using two different sized 5-type orbitals on each center accomplishes a similar effect by allowing the variation theorem to choose the amount of each in the mixture. Our orbitals are shown in Table 2.2. The 5-type orbitals are a split version of the Huzinaga 6-Gaussian H function and the p-type orbitals are adjusted to optimize the energy at the minimirm. It will be observed that the p and scale factors are different. We will present an interpretation of this below. 

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