The steric factor is a quantity used in collision theory, usually denoted by ρ. It is also known as the probability factor and is defined as the ratio between the experimental value of the rate constant and the one predicted by collision theory. Physically, the steric factor can be interpreted as the ratio of the cross-section for reactive collisions to the total collision cross-section. The steric factor is the probability of the reactant molecules colliding with the right orientation and positioning to achieve a product with the desirable geometry and stereospecificity/Kinetics/06%3A_Modeling_Reaction_Kinetics/6.01%3A_Collision_Theory/6.1.05%3A_Introduction). Steric factors or effects refer to the different repulsive interactions between neighboring atoms. The steric factor is very difficult to assess on paper and is determined experimentally/Kinetics/06%3A_Modeling_Reaction_Kinetics/6.01%3A_Collision_Theory/6.1.05%3A_Introduction). Usually, the more complex the reactant molecules, the lower the steric factors. However, some reactions exhibit steric factors greater than unity, such as harpoon reactions, which involve atoms that exchange electrons, producing ions. The deviation from unity can have different causes, such as the molecules not being spherical, so different geometries are possible, not all the kinetic energy is delivered into the right spot, or the presence of a solvent (when applied to solutions). There is usually no simple way to accurately estimate steric factors without performing trajectory or scattering calculations.