I am having problems getting an open shell system to converge. What strategies will help me?
Open shell systems, especially those containing transition metals or heavy elements with many unoccupied d and f orbitals, can be especially hard to converge. Running a spin unrestricted calculation can exacerbate the problem since all other possible spin states can mix into the wavefunction. It may also be true that your particular calculation has multireference character and, by using SCF or DFT, you are forcing it into a single determinate wavefunction. That being said, to converge your system, you need to guide the code to the particular state that you want. This may be a very difficult task and different starting guesses can converge to different solutions, especially if there is symmetry breaking. Here are four different suggestions to help you with your problem: • Use Auf bau construction. Start by removing most or all of the open-shell electrons and converge the resulting closed-shell wavefunction. Incrementally add the missing electrons, maintaining a closed-shell form for the
