So this question is linked to the other question I asked about the electronic patition function and CASSCF. When calculating thermochemical data (e.g. Gibbs Free energy) the partition functions are calculated. This includes the vibrational and rotational partition functions along with the electronic and translational terms.
Now when we use CASSCF, we are using multiple determinants, which can be considered as low lying excited states of the molecule. My first thought was that considering excited states would mean the electronic partition function term is now included in the SCF energy (which is what my earlier question is about).
But what happens to the other degrees of freedom? For example, the vibrational modes are obvioulsy calculated based on the CASSCF solution, so it contains the contribution of the vibrational modes of the excited states. Does this type of calculation actually make sense?
In my lectures, what my professor told us is that we take the ground state, and then we calculate all the partition functions independently based on the ground state. So what happens when the contribution of excited determinants are included in the vibrational and rotational terms?