molecular modeling, the computer simulation, by various means, of chemical structures or processes. Special computer-graphics programs can display three-dimensional images of molecular structures and chemical processes, showing the distances and angles of chemical bonds and the modifications that result when atoms or groups of atoms are introduced or substituted. In computer-assisted organic synthesis, the molecular structure of a compound is analyzed to discover how it might be synthesized; special databases can be searched for all occurrences of a particular molecular substructure and for ways to achieve a particular molecular transformation. Calculation-based modeling uses theoretical chemistry and complex mathematical equations to optimize chemical reactions by predicting such things as the energy required or released by reaction between two molecules. Molecular modeling has altered the way research is conducted. When results can be predicted before experiments are performed, experiments that have the highest probability of success can be chosen, potentially reducing the cost of research.