In this case, the SFG response is approximately given by ( ) Hamiltonian, and is the eigenvalue vector of the exciton Hamiltonian. Here is the vibrational decay lifetime, fixed at 1.3 picoseconds in our calculations. The average is over all configurations in a molecular dynamics (MD) simulation. The elements of the exciton Hamiltonian, which has vibrational frequencies on the diagonal and vibrational couplings on the off-diagonal, and the transition polarizability and dipole are computed using the Skinner group's electrostatic map relating local electric fields to these quantities, as we have used in past studies. We devise a method to isolate the symmetric or asymmetric stretch components. If intermolecular couplings are neglected, the eigenvectors of the exciton Hamiltonian have the form (18) where x and y can be both positive, both negative or of opposite signs. If x and y have the same sign, we consider the eigenvector to belong to the symmetric stretch, and the symmetric stretch response is given by (19) We calculate the asymmetric stretch response similarly by considering only those eigenvectors where x and y are of opposite sign. Notably, the asymmetric and symmetric stretch responses in the calculations are not exactly opposite because of the difference in symmetric and asymmetric frequency eigenvalues that arises from coupling.