A factor involving activity coefficients that appears as a multiplier in the rate equation. For a @M03989-1@ reaction the factor is \[\frac{y_{\text{A}}\ y_{\text{B}}}{y_{\ddagger}}\] where \(y_{\text{A}}\), \(y_{\text{B}}\) and \(y_{\ddagger }\) are the activity coefficients for the reactants A and B and the @A00092@, respectively. For a @M03989-2@ reaction the factor is \[\frac{y_{\text{A}}}{y_{\ddagger}}\] and for a trimolecular reaction it is \[\frac{y_{\text{A}}\ y_{\text{B}}\ y_{\text{C}}}{y_{\ddagger}}\]