The projection of the first hyperpolarizability onto the dipole vector (βvec(-2ω;ω,ω)), which can be sampled by an electric field-induced second harmonic generation (EFISHG) experiment, is also one of the molecular response properties often concerned in experiments. Due to their C2v point-group symmetry, the studied C18 derivatives exhibit dipole moments only in the direction of principal axis, i.e., the y-axis of H-C18-H and the x-axis of H-C18-NO2, NH2-C18-H, and NH2-C18-NO2, and have unique nonzero Cartesian components of the first hyperpolarizability in the corresponding direction (βy(-2ω;ω,ω) of H-C18-H and βx(-2ω;ω,ω) of H-C18-NO2, NH2-C18-H, and NH2-C18-NO2), as shown in Table . Therefore, it can be deduced that for all the C18 derivatives, the |βvec(-2ω;ω,ω)| and the absolute value of their non-zero Cartesian components of the first hyperpolarizability are exactly equal. The sign of the βvec(-2ω;ω,ω) depends on the components of the dipole moment and the first hyperpolarizability that are not-zero, that is, βvec(-2ω;ω,ω) is positive if the two parameters are oriented in the same direction, i.e., their polarity is consistent; otherwise, βvec(-2ω;ω,ω) is negative. Due to these facts, only the H-C18-NO2 has positive βvec(-2ω;ω,ω) value, while the βvec(-2ω;ω,ω) is negative for all other molecules.