Ambiguities in surface nonlinear spectroscopy calculations

► We explain ambiguities in simulations of nonlinear optical surface spectroscopy. ► There is an ambiguity in the definition of the electric-dipole-order surface signal. ► There is also an ambiguity in the definition of the quadrupole-order bulk signal. ► These two ambiguities are equal and opposite, so the total signal is unambiguous. ► Our simulations of the water-air interface support this picture.