# Permissible amount of forces on ions in a relaxed structure

An equilibrium structure would be at a local minimum but that doesn't happen with all relaxation calculations. Relaxed structures tend to have some resultant forces on ions. What amount of these forces can be accepted? Is 0.05 eV/$$\unicode{xC5}$$ acceptable?

• It's a political choice but using very low values cause the consumption of lots of resources with no measurable gain in results. I mostly stick with 0.01 eV/A. Commented Aug 16, 2020 at 19:14
• Yeah for vibrational calculations mostly much lower values could be used, but you need to take into account higher time and resources consumption in this case. Commented Aug 16, 2020 at 22:34
• I think it's difficult to answer this question as-is because it depends on what property is being computed. If only the total energy is important, the maximum forces that are acceptable are larger than what would be required for vibrational calculations for materials with soft vibrational modes. There's also the accuracy/time tradeoff mentioned above. Without being more specific, it's hard to give an answer other than "it depends". Commented Aug 18, 2020 at 20:25
• The numerical value also isn't enough to specify what is being measured: maximum force? maximum force component? rms force? If the maximum force / maximum force component is small, you might still be an appreciable distance from the minimum if you have a lot of atoms. Commented Aug 18, 2020 at 20:43
• This completely depends on what you're looking to model and the type of material you're modeling. Rule-of-thumb is generally no greater than 0.05 eV/Å, but even that can often be too large, particularly (but not exclusively) in the case of highly flexible materials or molecular crystals. I like 0.03 eV/Å, but that's for a very specific class of materials I study (MOFs). For other materials, that may not be sufficient. There is unfortunately no proper answer to this question. You simply have to test it and see. Commented Aug 19, 2020 at 1:21