# Doubt about energy convergence with different k points

During the convergence test of k points, I observed my energy start increasing instead of decreasing. I am doubtful is this correct or I am doing something wrong. can I take the appropriate K point in this type of converged graph in which energy is inceasing. I am doing this convergence test for bulk material (spin polarised calculation) and K points I took like 2X2X2 then 3X3X3 and so on and observed the corresponding energy.

Here are my input file settings:

SYSTEM = GRPH Electronic Relaxation 1 ISTART = 0 ICHARG = 2 ISPIN = 2 LWAVE = .F. LELF = .F. MAGMOM = 128*1.0 PREC = Accurate ENCUT = 500 LREAL = A NELM = 100 NELMIN = 4 EDIFF = 1E-5 EDIFFG = -1E-2 NSW = 500 IBRION = 2 ISIF = 3 LORBIT = 10 #DOS related values ISMEAR = 0; SIGMA = 0.05

• Welcome to our community!
– Camps
Commented Sep 27, 2021 at 22:45
• Kindly tell your system, input setting as well for clearity Commented Sep 28, 2021 at 6:05
• +1 energy should decrease as you change the energy cutoff because it is a variational parameter (you increase the number of plane waves in your basis), but there is no reason why the energy should decrease as a function of the number of k-points, so this looks ok to me. Commented Sep 28, 2021 at 6:22
• It is a bulk system with 68 atoms in the unit cell. Lattice parameter I took from the literature. Here is my input file setting. SYSTEM = GRPH Electronic Relaxation 1 ISTART = 0 ICHARG = 2 ISPIN = 2 LWAVE = .F. LELF = .F. MAGMOM = 128*1.0 PREC = Accurate ENCUT = 500 LREAL = A NELM = 100 NELMIN = 4 EDIFF = 1E-5 EDIFFG = -1E-2 NSW = 500 IBRION = 2 ISIF = 3 LORBIT = 10 #DOS related values ISMEAR = 0; SIGMA = 0.05 Commented Sep 28, 2021 at 8:27
• +1. Welcome to our new community and thank you for contributing your question here! We hope to see much more of you in the future !!! Commented Sep 30, 2021 at 21:06

Let's just remind ourselves what k point sampling is. It is used to approximate the integral of a function over the Brillouin zone by replacing the integral with a sum over a finite number of points: $$\int_{BZ} F({\bf k})d{\bf k} \approx \sum_j w_j F({\bf k_j})$$