A geometric parameter that changes during the conversion of one (or more) reactant @M03986@ into one (or more) product molecular entities and whose value can be taken for a measure of the progress of an @E02035@ (for example, a @BT07003@ or bond @A00346@ or a combination of bond lengths and/or bond angles; it is sometimes approximated by a non-geometric parameter, such as the @B00707@ of some specified bond). In the formalism of '@T06470@', the reaction coordinate is that coordinate in a set of curvilinear coordinates obtained from the conventional ones for the reactants which, for each @R05178@, leads smoothly from the configuration of the reactants through that of the @T06468@ to the configuration of the products. The reaction coordinate is typically chosen to follow the path along the @G02669@ (path of shallowest ascent/deepest descent) of potential energy from reactants to products. The term has also been used interchangeably with the term @T06455@, applicable to the coordinate in the immediate vicinity of the potential energy maximum. Being more specific, the name @T06455@ is to be preferred in that context.