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Simple Molecules and Clocks

The precise measurement of transition frequencies in cold, trapped molecules has applications in fundamental physics, and extremely high accuracies are desirable. We determine suitable candidates by considering simple molecules with a single electron, for which the external-field shift coefficients can be calculated with high precision. Our calculations show that $\Htwop$ exhibits particular transitions whose fractional uncertainties may reach $2\times10^{-17}$ at room temperature. We also generalize the method of composite frequencies, introducing tailored linear combinations of individual transition frequencies that are free of the major systematic shifts, independent of the strength of the external perturbing fields. By applying this technique, the uncertainty should be reduced to the $10^{-18}$ range for both $\Htwop$ and $\HDp$. Thus, the theoretical results demonstrate that these molecules are of metrological relevance for future studies.