Measurements of molecular transition frequencies with accuracies of 10−16

Measurement of vibrational transition frequencies of X⁶Li and X²³Na molecules in an optical lattice and XH⁺ molecular ions in a string crystal are proposed (X:⁴⁰Ca, ⁸⁸Sr, or ¹⁷⁴Yb). Measurements of these transition frequencies with an uncertainty of less than 10^-16 are useful for the test of the variation in the proton-to-electron mass ratio.