quantum chemistry - Is it reasonable to use natural orbitals of an unrestricted HF calculation as a basis to start a CASSCF calculation for a radical

Let's say we want to run a CASSCF calculation of a radical, for example the cation of a neutral closed-shell molecule (therefore, we calculate an open-shell radical cation). MOLCAS, as well as probably every other program package out there, needs starting orbitals when performing a CASSCF calculation. For a neutral species, these are normally assumed to be the regular old Hartree-Fock orbitals. Usually, when dealing with a cation, it seems that the Hartree-Fock-SCF calculation is performed for the neutral species or the di-cationic species to yield the necessary HF starting-orbitals. They are in these cases the result of closed-shell calculations. MOLCAS also permits the use of a UHF calculation as the basis for a subsequent CASSCF calculation. Then, the natural orbitals of the unrestricted calculation are used.

Is the usage of the natural orbitals of an unrestricted HF calculation reasonable as a basis for a subsequent CASSCF calculation? If not, why? When is it a good idea / when is it a bad idea? What are the differences in using natural UHF orbitals in contrast to the SCF orbitals of the neutral or di-cationic species? What are the pitfalls?

Edit: I am aware (or at least it is my understanding) that conceptually it is no problem to perform the CASSCF using the natural UHF orbitals, as it could for example also be performed using only the alpha orbitals of the UHF calculation. My question is therefore aimed at finding out in which cases it is generally a good or bad idea to use the natural UHF orbitals.

Answer

The best starting orbitals for a CASSCF calculation are optimised orbitals of another CASSCF (or RASSCF) calculation. That sounds a bit ridiculous, but this is probably the best way to figure out the active space.

At first you are probably choosing something quite small, like CAS(2,2) to CAS(4,4). For these calculations it barely matters what kind of starting orbitals you are choosing. The change will be negligible and probably depending heavily on the system you are using.

The advantage of calculating the cations by dismissing the radicals is often a good choice since you can run a very fast RHF calculation and the orbitals are symmetric, which is what you need for CAS.
Natural orbitals of an UHF calculation are also a quite sensible choice, because they already allow for fractional occupation and should also be symmetric. The only downside I can think of is, that this kind of start orbital generation takes a lot longer.
Similarly you can choose to run a ROHF calculation, if you are uncomfortable with too much cationic charge.
As you have said you can choose also an UHF calculation by throwing away one set. I would advise caution here. Sometimes - probably especially when CAS is necessary - spin contamination in UHF is significant. Therefore one set might not give you anything close and you could end up choosing the wrong CAS.

I am not aware of any major pitfalls except for the just mentioned choosing of a completely wrong active space.

In any case it is a good idea to give the CASSCF calculation a few cycles and see how the active space is developing. Sometimes you need to adjust the active space long before the calculation converges and then you will have already a much better guess than with your initial starting orbitals.

I personally use the following routine

R(O)HF [x+] -> CAS(x, ⌈½x⌉ +1) 
            -> RAS(x+2n, ⌈½x⌉ +n +1) 
            -> CAS(x+2n, ⌈½x⌉ +n +1) 
            -> RAS(...) 
            -> CAS(...) -> ...

to narrow down the active space and include all orbitals I want and need. In most cases I am not converging the intermediate calculations, except for the minimal CAS.

As you can see the procedure that follows after your start orbitals is much™ more time consuming, so it really does not matter.