There are a number of different methods to calculate infrared intensities in psi. They can be done in either Cartesian or internal coordinates. The advantage of using Cartesian coordinates is that the programming and implementation are easier, but is is very difficult to assign normal modes to the calculated fundamentals. Internal coordinates are more difficult since they can be extremely intricate to derive for all but the simplest systems, but then assignment is unambiguous.
The two most widely used programs for this are normco and intder. Normco will calculate the frequencies and intensities in Cartesian coordinates. The only options are the isotopes array.
normco:(
isotopes = ((1 1 1 1)
(2 2 2 2)
... )
Normco will always calculate for the most common isotope, even
if not specified in the isotopes array.
Intder is definitely not necessarily trivial to use. To carry out analysis in cartesian coordinates, set the freq_anal = cart_co. It can also do calculations in internal coordinates by setting freq_anal = int_co. Setting freq_anal to both will cause intder to calculate the frequencies and intensities in both coordinate systems.
Which is better? Calculation of frequencies in internal coordinates is definitely better for assignment purposes. Fundamentals may switch in energetic orderings with different methods or basis sets which would might be missed in a Cartesian analysis of the vibrational frequencies.