Aiming to investigate the effects of stellar and AGN radiation fields to the excitation of molecular gas, we present an analysis of model CO, H₂O, and OH⁺ emission lines produced with the application of these radiation fields using MOLPOP-CEP. We find that the ratios of CO(J_upp) = 11, 12, 13, 14, 15) to CO(J_upp) = 1, 2, 3, 4, 5, 6, 7, 8) obtained with the application of AGN radiation are larger than the ones produced by applying stellar radiation. Similarly, some H₂O and OH⁺ ratios obtained with AGN radiation are found to be higher than the corresponding ratios produced by applying stellar radiation field, making the line ratios a diagnostic tool to disentangle AGN from star formation. In all cases, we find that the ratios vary as a function of the molecular gas density and the scale of the radiation field, the larger the scale of the radiation field is the stronger the excitation of the molecular transitions. Also, the effect of the radiation fields is found to be dependent on the molecular gas density, indicating that the effect of the radiation fields is regulated by the physical properties that are intrinsic to the molecular gas. Thus, due to the dependency of the line ratios on the scale of the radiation fields and the effect of the radiation fields on the molecular gas density, we conclude that it is not sufficient to use a single line ratio to claim the presence of AGN contribution to the molecular gas excitation.

The AGN SED data was taken from the CLOUDY documentation, whereas the sythentic stellar SED was produced by using the starburst99 code based on the assumption discussed in our paper.