Fairly accurate linear‐combination‐of‐atomic‐orbitals molecular‐orbital self‐consistent‐field (LCAO MO SCF) wavefunctions have been calculated for CH4, both with and without the constraint that the first two a1 MO's be the 1s and 2s SCF AO's of the C atom. The calculations were carried out for tetrahedral and distorted‐tetrahedral nuclear configurations, and it was found that the tetrahedral configuration is the most stable both with and without the constraint. To examine the effects of nonpromotional (zeroth‐order) hybridization in CH4, calculations were also carried out in which the s‐type basis functions (and four associated electrons) of the C atom were completely deleted and replaced with an appropriate effective nuclear charge. Again, a tetrahedral methane was obtained. Therefore we conclude that 2s → 2p promotion and/or hybridization does not cause the tetrahedral structure of CH4. Analogous calculations on H2O and NH3 show a decrease in bond angle due to the constraint from 106.7° to 98.1° for H2O and from [inverted lazy s] 107° to [inverted lazy s] 95° for NH3. Similar calculations for H2S, in which the six lowest‐energy MO's are constrained to be the 1s, 2s, three 2p's, and 3s SCF AO's of the S atom, show a decrease of the calculated bond angle from [inverted lazy s] 93° to [inverted lazy s] 89°. As for CH4, the shapes of the angular‐distortion potential energy curves for H2O, NH3, and H2S are nearly unaffected by the constraint. For CH4, NH3, and H2O, the molecular binding energies are decreased, respectively, 4.0, 2.2, and 1.3 eV, by the constraint.

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The term“p‐type Gaussian” is here used to imply a basis orbital which is the difference of two identical s‐type, “lobe,” Gaussians displaced equally (but in opposite directions) from the origin along the axis of the p‐type orbital. A p‐type group is a fixed linear combination of these p‐type Gaussians.
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The experimental bond angles for PH3(94°) and H2S(93°), for which almost no 3s→3p promotion occurs, support this conclusion. Also, the neraly planar nature of H3O+ is in direct contradiction to the VSEPR model.
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