# How to determine energy convergence threshold and force convergence threshold in Quantum Espresso [closed]

In the computational investigation of $$MoS_2$$ monolayer for its catalytic activity in the hydrogen evolution reaction, how are parameters such as forc_conv_thr and etot_conv_thr determined within the Quantum Espresso calculation framework?

• could you provide a few more details? We'd be happy to help but it would be useful to understand a bit more about the setup of your calculations. Apr 3 at 18:27
• The system is 001 surface of MoS2 [(2*2*1) SuperCell ] with 15 angstrom vacuum inserted.It has total 12 number of atoms .I am using PAW. pseudopoential and XC functional PBE.The property i am looking for is total energy . my question intends to know the criteria behind setting this parameters to a specific value.(say for example when i used a QE input generator the values it gave are 10^-4 for both energy and force conv thr and in &ELECTRONS conv_thr is given 10^-9.)so why it selected this values(on what criteria) ? Apr 3 at 19:00

The parameters etot_conv_thr and forc_conv_thr determine the convergence thresholds for the ionic minimization. When the change in total energy is smaller than etot_conv_thr and the changes of

all components of all forces are smaller than forc_conv_thr[,]

the calculation is considered converged with respect to the ionic minimization. I have taken the quote from the QE input description found here. It is always advisable to check the function of any input field in the description to understand its role and behaviour.

The correct choice of these parameters depends upon your scientific case and cannot be answered in general. A good first step is to check the literature and see which thresholds other papers have chosen. You can check this question about standard convergence criteria.

Some words of caution:

i) QE uses atomic Rydberg units, so you might need to convert units

ii) Keep in mind that there is also the parameter conv_thr under the &ELECTRONS field. These are different thresholds that should not be mixed up

iii) Energy (and consequently the threshold) is an extensive parameter. The larger your system, the larger the energy, which implies that the same parameter etot_conv_thr imposes different level of strictness depending on the number of atoms. In the literature you'll often find units like meV/atom due to this

• Thank you @manju9 Apr 9 at 11:25