Such energy barriers can be due to steric or electrostatic repulsions. Dispersions may become stable through two different phenomena&colon; charge repulsion and steric or entropic repulsion. Coordination geometry is thus determined largely by electrostatic and steric interactions. The "exo"-selectivity is achieved to minimize steric repulsion. Bulky ligands are used to control the steric properties of a metal center. Electrostatic and steric stabilization do not directly address the sedimentation/floating problem. Such distortions introduce a tension in the molecule, known as steric hindrance or strain. This treatment does not include ortho-substituents, which would introduce steric effects. These ligands are available in great diversity with respect to their steric and electronic properties. A valence bond theory study also emphasizes the importance of steric effects. N-linked glycans also contribute to protein folding by steric effects. Consequently, most threonine proteases use an N-terminal threonine in order to avoid such steric clashes. The "tert"-butyl effect is an example of steric hindrance. Hydrogen bonds are an example of a steric constraint. The effects of these substitutions seem to be mediated by steric effects.