In order to validate the model that explains the "up-down" (or "down-up") line shape of the chiral SFG response of water as presented in Figure , we must prove that 1) the symmetric and asymmetric stretches are at different frequencies, and 2) the symmetric and asymmetric stretches of water chiral SFG responses are equal but have opposite signs. Point 1 is known to be true, as intramolecular coupling and differences in the local environments of the two O-H groups in a water molecule cause the symmetric and asymmetric stretches to be non-degenerate.