Hi, I am having difficulty optimizing a MECP geometry for a molecule with high spin contamination. For example, if I want to consider the lowest open shell singlet (S0, mecp_state1 [0,0]) and lowest triplet state (T1, mecp_state2 [1,1]) for a diradical molecule, what should I do to remove the triplet spin contamination in the first state and the singlet spin contamination in the second and so a correct optimization?
Another question is whether is there any way to keep the root from changing during an excited state optimization?
This is a pernicious problem in excited-state optimizations and there’s no general solution. There are state-tracking algorithms that have been proposed in the literature but I don’t think any of them is available in a general way in Q-Chem, so there’s no black-box solution. You just need to carefully monitor what is going on and restart accordingly.