Experienced researchers continue to use models that were taught in high school and freshmen chemistry, however they realize that there will always be exceptions to the rules of these models. A very useful model is the Lewis dot structure description of chemical bonding. It is not a complete description of the molecule since it does not contain the kinetic energies of the particles or coulombic interactions between the electrons and nuclei. The Lewis model accounts for this pairing and for the number of energy levels available to the electrons under normal temperature and pressure. However, none of the quantum mechanics equations are used in applying this technique.
Approximations are another construct that is often seen.
Even though a theory may give a rigorous mathematical description of chemical phenomena, the mathematical difficulties might be so great that it is just not feasible to solve a problem exactly. If a quantitative result is desired, the best technique is often to do only part of the work. One approximation is to completely leave out part of the calculation. Another approximation is to use an average rather than an exact mathematical description. Some other common approximation methods are variations, perturbations, simplified functions and fitting parameters to reproduce experimental results.