Convergence Problems

My calculation didn't work!

This is a common complaint when performing molecular modeling simulations. You start the calculation, and come back many hours later to find it didn't finish correctly or didn't converge. This usually means either (1) the calculation crashed (hopefully with error messages that you can google) or (2) the calculation is taking a very long time, despite restarting the calculation multiple times.

Below we discuss several issues related to converging simulations. Recall that there are two convergence loops that DFT calculations will run through: wavefunction optimization (i.e. scf convergence) and geometry optimization. The program first determines the wavefunction for a given geometry, which gives the forces and energies of that geometry. Then the program will update the geometry to a new structure based on the calculated forces. It will re-calculate the wavefunction, then geometry, and repeat until the system is finally converged and the geometry is optimized.