The sum frequency generation ͑SFG͒ spectrum of CH 2 and CH 3 modes in the self-assembled monolayer of octadecanethiol is modeled in order to understand the reasons of the very low SFG intensity of CH 2 modes and to investigate quantitatively the two types of L-shaped conformations qualitatively suggested in the literature. CH 2 modes are assumed local in order to calculate easily the spectrum of any conformation, and ab initio molecular hyperpolarizabilities are used. It is found that the absence of CH 2 bands does not imply an all-trans conformation and vice versa. Several conformations are compatible with SFG, but only one of them can be arranged on Au͑111͒ and completely agrees with x-ray diffraction. All conformational details of the L-shape molecules are obtained.

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