Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In chemistry, a reaction coordinate is an abstract one-dimensional coordinate which represents progress along a reaction pathway. It is usually a geometric parameter that changes during the conversion of one or more molecular entities. These coordinates can sometimes represent a real coordinate system (such as bond length, bond angle...), although, for more complex reactions especially, this can be difficult (and non geometric parameters are used, e.g., bond order). Reaction coordinates are often plotted against free energy to demonstrate in some schematic form the potential energy profile (an intersection of a potential energy surface) associated to the reaction.