A factor involving activity coefficients that appears as a multiplier in the rate equation. For a bimolecular reaction the factor is \[\frac{y_{\rm{A}}\ y_{\rm{B}}}{y_{\ddagger}}\] where \(y_{\rm{A}}\), \(y_{\rm{B}}\) and \(y_{\ddagger }\) are the activity coefficients for the reactants A and B and the activated complex, respectively. For a unimolecular reaction the factor is \[\frac{y_{\rm{A}}}{y_{\ddagger}}\] and for a trimolecular reaction it is \[\frac{y_{\rm{A}}\ y_{\rm{B}}\ y_{\rm{C}}}{y_{\ddagger}}\]