I have performed TDDFT for Heme molecule with imidazole axial ligand and obtained the absorption spectrum. Later I have added few gas molecules to Heme and the absorption spectrum is compared with that of bare heme. Now i observe Red shift and blue shift for different gas added to heme. For example, in case if the gas molecules are bonded to heme then its blue shift and if not bonded then it shows red shift. What can be the reason behind this shift peak?
Here I have attached the UV peak for bare heme.