Molecular geometry and the valence-shell electron pair repulsion model

5 Molecular geometry and the valence-shell electron pair repulsion model 

For example, for the H2 molecule such a balance of forces occurs for an intemuclear distance of 74 pm (the bond length of the molecule)  

For large molecules, however, the computer requirements become increasingly prohibitive, especially when conformationally flexible compounds are tackled. Alternative approaches to quantum-mechanical methods are known, based on potential functions and parameters derived from detailed analysis of vibrational spectra. These so-called force field methods are now joined in what is called molecular mechanics, an empirical method that considers the molecule as a collection of spheres (possibly deformable) bound by harmonic forces (eventually corrected with cubic and quartic potentials). The energy 

As far as qualitative quantum-mechanical models are concerned, the first attempts applied the principles of valence-bond theory. For example, the bent geometry of the water molecule was initially interpreted in terms of the overlap of the Is atomic orbitals of the H atoms with two 2p atomic orbitals (with mutually perpendicular axes) of the O atom, each one contributing one electron  

This description is supported by the fact that the (degenerate) states of lowest energy for an isolated O atom involve one 2p orbital fully occupied and two half-filled 2p orbitals. 

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