Our roadmap to obtaining the imaginary component of the surface second-order nonlinear susceptibility from vibrational sum frequency spectroscopy and heterodyne-detected second harmonic measurements is out in JACS. Stay tuned till galleys are turned around. For now, you can read the piece on chemRxiv. Congratulations to Emily Ma and Ben Rehl, who led this tour-de-force effort as one of their “stuck at the desk during 2020/21” projects.