Example 4: Due to calibration error, the 0-1 peak may slightly differ in location on the pump and probe axes (e.g. <1 cm -1 ). Therefore, it is okay to model a calibration error along one of the axes, but modeling it along both axes, or modeling two calibration errors (one for each axis), would cause indistinguishability between the calibration error(s), the 0-1 center frequency and the anharmonic shift.