Normal vibration calculation gives IR position and intensity. But I also need the transition dipole moment (magnitude as well as direction) for each mode. Can anyone provide a Gaussian input file example for that purpose?
The dipole strengths are obtained directly using Freq=(vcd,vibrot). The magnitude value determins the length of the arrow of the dipole moment vector. The polarizability tensor components and the depolarization ratios provide you information about the direction. Thus, (alpha)xx and (alpha)yy provide you information about the dipole moment induced in the corresponding directions x and y, respectively. You could see the basic equations determining the induced dipole moment and depolarization ratio in any monograph/textbook devoted to polarized IR (or Raman) spectroscopy. You should add "output=pickett" (http://www.gaussian.com/g_tech/g_ur/k_output.htm). The output-data have the well known tabular form.
For H2O, for example, the analysis by HF/6-31G (Freq=(VCD,vibrot) output=pickett) is:
i.e., the opt and freq can not be in the same input, otherwise it does not work. It should be linked (via link1).
The output gives du/dQ(x,y, and z) as 'Dipole derivatives wrt mode 1: du/dQ(x) du/dQ(y) du/dQ(z) in units of (Km/mol)^0.5, i.e. the same units as the square root of intensity. It can be converted to the more common used units Debye per angstrom pers squareroot of amu by dividing by 6.5005.
would it be possible to provide some ressources, which give information on the units of the dipole derivatives in Gaussian and on the conversion factor of 6.5005 you mentioned above? For me it is not clear, how you found this conversion factor.
It would help to improve understanding of what Gaussian actually calculates.