The productivity of a number of bis(phenoxyamine)Zr(IV) -based catalysts (bis(phenoxyamine) = N,N-biS (3-R-1-5-R-2-2-O-C6H2-CH2)-N-N'(R-3)(2)-(NCH2CH2N)) in ethene and propene polymerization was evaluated for different R-1/R-2/R-3 combinations. In previous studies on this class we demonstrated that the cations that form upon precatalyst activation (e.g., by methylalumoxane) adopt a "dormant" mer-mer geometry, and an endothermic isomerization to the active fac-fac geometry is the necessary first step of the catalytic cycle. Herewith we report a clear correlation between catalyst activity and the DFT-calculated energy difference Delta E-i between the active and dormant state. The correlation only holds when the calculations are run on ion pairs, which is less obvious than it may appear because the anion in these systems is not at the catalyst front. This finding provides a comparatively simple and fast method to predict the activity of new catalysts of the same class.