Zhaoxi Sun , Thank you for your response. I would try to make my question more clear. When absorption band strengths obtained with different basis sets for neutral molecules were compared, the measured intensities are found higher compared to gas phase experimentally calculated values. The situation gets better if we go for higher functional and basis set, but it is still considerably higher (by about 30%). The MP2 caculation gives much a better result, but computationally time demanding. A recent research calculated the strength for certain neutral PAH molecules with MP2/6-311+G(3df,3pd) and estimated a scaling factor to scale values obtained with B3LYP/6-311+G **. The purpose of this is to reduce overestimation of theoretically measured values and bring it close to gas phase values. Now, the calculation can be done with a smaller basis set that would take less time and after using the scaling factor, it would give the same accuracy as MP2/6-311+G(3df,3pd). Since, these whole estimation of scaling factor is measured from neutral molecules, my question is does it make sense if I use the same scaling factor for ions in order to make it experimental values . Or the use of different basis sets, no matter how high it is, not going to make any change in the absorption band strength for cations?

If I understand your response correctly, you say that use of different basis sets don't change the output results, say absorption intensity of different vibrational modes? and it would be same for all basis sets. Am I correct in my understanding?

Zhaoxi Sun , I think I undertsand your point. About the test, I have the same opinion. I even tried to make calibration calulcation with B3LYP/6-311+G ** and MP2/6-311+G(3df,3pd). But for MP2 calulcations, it takes so much time and after waiting for long, I get error. I am now thinking of starting the calculation with small molecules. Thank you for your suggestion.