Quantum Espresso energy cutoff for different pseudopotential

Question

I've been told that I should use the potentials from the Standard Solid-State Pseudopotential (SSSP) library in my simulations. I am interested in $\ce{Pt}$, $\ce{O}$, and $\ce{H}$. Oxygen is Projector-Augmented Wave (PAW) while platinum and hydrogen are Ultra Soft Pseudopotentials (USPP) (I am surprised that quantum espresso is able to run multiple PP descriptions at the same time).

The advised energy cutoffs for platinum, hydrogen, and oxygen are $35$, $60$, and $\pu{50 Ry}$ respectively. My system is made of $85\%$ platinum and a few oxygen/hydrogen atoms, do I still need to use ecutwfc = 60? It seems like a waste of time in this case?

Answer 1

You can do a convergence test and see if ecutwfc=60 is really needed. Depending on your application, you also might find the treatment of hydrogen may result in energies shifting but no relative difference changes for example (All OH groups treated equally bad). This is a form of error cancellation, so be careful about it, but if you never change the state of the hydrogen and it doesn't affect your answer, I do not see why you need to go all the way to ecutwfc=60.

I would predict you are doing some sort of Pt surface with H2, OH, H2O adsorbates etc. In this case, the H may play a significant role and you will likely need ecutwfc=60 or even higher (showing the importance of convergence tests!). You could also consider trying to find a different pseudopotential with a lower cutoff, since you can mix types under some conditions as you have discovered.