Isotope Shifts of Vibrational Frequencies

To calculate isotopic shifts using ADF do the following:

• Calculate frequencies and save TAPE21 with a different name, say result.t21
• Modify the input file as follows:
  • add "RESTART result.t21" anywhere in the input file
  • create new fragment file with different mass
  • specify the fragment file in FRAGMENTS section
• Run ADF with the new input

Please note that if you change the fragment file for an atom that has symmetry-equivalent ones then the new fragment file will be applied to all of the atoms.
Example: first calculate the NH3 frequencies in the C(3v) symmetry and then change H to D. This will mean that one calculates the frequencies of a ND3 molecule and not of NH2D as one might want to do. If one wants to calculate the frequencies of NH2D one first has to do a calculation with lower symmetry, say C(s), to be able to change isotope of only one of the hydrogens.