Dear Nick,
Thank you very much for the helpful information. Currently, I am using as reference value the energy difference between the two structures obtained with VASP and a large plane-waves energy cutoff. What I noticed, is that in both the MOLOPT and regular basis sets the value approaches the VASP one with increasing basis set size, with the MOLOPT ones performing similarly to the regular ones except for the TZVP, in which the regular one performs better than the MOLOPT.
Thus, I wanted to check which value I could get from a QZV3P basis set in order to have a complete picture of the basis set size effect on the energy differences (of course I would later on resort to smaller basis set in an MD simulation for time reasons).
I will try to increase the cutoff and see if I can get a converged wavefunction, I was not aware of this effect related to the CUTOFF.
Thanks again, best regards,
Tom