I have created a graphene supercell using Quantum Espresso and performed a vc-relaxation on it. Now, I need to verify whether the created structure is stable and demonstrate its stability with numerical values. Can I use formation or cohesive energy for this purpose?

I have a multilayer structure formed by C,O,H,N and Cu, for a total of 174 atoms. I am thus using U+J Hubbard methodology as well as Grimme corrections within VASP code. I am doing some convergence studies on the cut-off energy, however by enhancing the ENCUT I don't seem to observe an asymptotic...

In my phonon calculation utilizing DFPT in VASP for a monolayer material with space group 187, I've noticed that while most of the phonon bands are positive, two of them appear to be straight. What could be the explanation for this observation? [1]: https://i.sstatic.net/Q5eCS.png