I am doing a CI-NEB calculation using VASP. The NEB calculation looks reasonable but when I do the CI-NEB, it gives large negative value and completely changes the shape of the barrier (see the image below). What could be wrong and how to fix it?

I have to calculate the formation energy of a material, where I need the total energy of O, F, Cl, Br, S, Se, Te. I checked on the material project for their gaseous form but there are a lot of phase available. Kindly suggest the exact structure of these elements to calculate formation energy.