<div dir="ltr">Hi all, this may be a very entry-level question, but please help me understand something that currently confuses me. Let's say we want to calculate static energies and forces of a small unit cell (or very few repetitions) of a metallic crystal, much like the "How to Calculate Energy and Forces" howto. I find that there is a large difference between multigrid method and k-point sampling (when parameters of both have been correctly converged). For all systems I tried this on, most individual energy terms are very close, but the "Core Hamiltonian energy" is about 5-10% higher using MGRID than using KPOINTS. From other sources I know the correct cohesive and isolated atom energies, and I find that only k-point sampling returns the correct value. Example from 1x1x2 cells Al, basis DZVP-MOLOPT-SR-GTH, potential GTH-PBE-q3, PBE exchange-correlation: <div style="background-color: rgb(250, 250, 250); border-color: rgb(187, 187, 187); border-style: solid; border-width: 1px; overflow-wrap: break-word;" class="prettyprint"><code class="prettyprint"><div class="subprettyprint"> &MGRID NGRIDS 4 CUTOFF 300 REL_CUTOFF 50 &END MGRID </div></code></div><div style="background-color: rgb(250, 250, 250); border-color: rgb(187, 187, 187); border-style: solid; border-width: 1px; overflow-wrap: break-word;" class="prettyprint"><code class="prettyprint"><div class="subprettyprint"> Overlap energy of the core charge distribution: 0.00000000001890 Self energy of the core charge distribution: -45.13516668382051 Core Hamiltonian energy: 10.22296505539078 Hartree energy: 25.42600995275711 Exchange-correlation energy: -6.48331280168588 Electronic entropic energy: -0.00263408295383 Fermi energy: 0.23983128515630 Total energy: -15.97213856029344 </div></code></div>vs. <div style="background-color: rgb(250, 250, 250); border-color: rgb(187, 187, 187); border-style: solid; border-width: 1px; overflow-wrap: break-word;" class="prettyprint"><code class="prettyprint"><div class="subprettyprint"> &KPOINTS SCHEME MONKHORST-PACK 17 17 34 ! converged to 5 decimals around 13 13 26 FULL_GRID T WAVEFUNCTIONS COMPLEX &END KPOINTS</div></code></div><div style="background-color: rgb(250, 250, 250); border-color: rgb(187, 187, 187); border-style: solid; border-width: 1px; overflow-wrap: break-word;" class="prettyprint"><code class="prettyprint"><div class="subprettyprint"> Overlap energy of the core charge distribution: 0.00000000001890 Self energy of the core charge distribution: -45.13516668382051 Core Hamiltonian energy: 9.67133497320798 Hartree energy: 25.41644035557356 Exchange-correlation energy: -6.46349454507014 Electronic entropic energy: -0.00027958913223 Fermi energy: 0.25517849633624 Total energy: -16.51116548922202</div></code></div> The total energy of that particular system should be around -16.51 Ha. Why is that? I would very much like to use the multigrid solution, as it is a lot faster for such systems - but obviously I can't if the results are off. I have read one post here that recommended using k-points for such systems, but not why, and where one could place a limit of usefulness between the two methods (if that is possible). Thanks in advance, Sebastian Hütter</div>