It is not possible to calculate the ratio between the transition frequencies in different atoms with anywhere near sufficient accuracy for metrological applicat

It is not possible to calculate the ratio between the transition frequencies in different atoms with anywhere near sufficient accuracy for metrological applications. Therefore, only Cesium clocks can serve as primary standards for the SI second. Other clocks can be calibrated to Cesium clocks in order to serve as secondary standards, but it is not possible to measure a frequency in Hz with an accuracy that is better than that of the best Cesium clocks. Probably the definition of the SI second will be changed at some point, but currently there are still too many possible candidate species and transitions without a clear favorite.