I have searched through the GAUSSIAN 16 manual and have not been able to solve these two related issues:

(1) I want to be able to save the CI vector and load it as an initial guess for a different calculation. Or at least to be able to manually set the initial CI vector.

(2) During a geometry optimization, the initial guess for the CI vector changes on each geometric iteration. This does not make much sense to me, as the final CI vector is almost always very similar to the one for the previous geometry! I have noticed that this can lead to huge orbital rotations (like starting with a CI[1] = 1 vector and swapping the HOMO for a core orbital) and then this leads to convergence issues. How do I force it to use the final CI vector from the previous iteration as the starting guess?

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    $\begingroup$ I have not used CASSCF before, but it seems like what you want for the first part of your question is the StateGuess keyword. gaussian.com/cas $\endgroup$
    – Tyberius
    Apr 12, 2019 at 21:57


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