How to determine occupied and closed orbitals for a Molpro CASSCF calculation?

Question

I am currently working with Molpro, to which I am completely new, trying to run some calculations on $\ce{PdO}$. From literature I have gathered that the ground state is given by $^3\Sigma^-$ state, which translates to a $^3A_2$ state in the $C_{2\mathrm{v}}$ point group, or a wave function card {wf,54,4,2,0}. Moreover, the literature gives a possible active space of ($4a_1$, $2b_1$, $2b_2$, $1a_2$). I now want to run a CASSCF calculation using this active space, but I am not sure how to determine the occupied (OCC) and closed shell (CLOSED) orbitals.

How do I determine these values in general?

Edit

So I have been working on this a little more, and maybe I have found an approach that could work. I can run a simple HF calculation and get a set of orbitals whereby Molpro gives me the corresponding IRREPS. In the case for $\ce{PdO}$ I get a set of occupied orbitals ($13a_1$, $7b_1$, $6b_2$, $2a_2$), whereby two are singly occupied. To get my desired active space of ($4a_1$, $2b_1$, $2b_2$, $1a_2$), I would simply have to close orbitals according to ($9a_1$, $5b_1$, $4b_2$, $1a_2$).

Is this a sensible approach?

Answer 1

The occupied orbital pattern of 13 7 6 2 that you got from your Hartree-Fock calculation is not unique. For example, I've just run an RHF calculation on PdO with the ANO-RCC basis set and got a different occupancy pattern compared to you:

 Final alpha occupancy:  14   6   6   2
 Final beta  occupancy:  14   5   5   2

I would recommend to find the experimental ground state configuration, which is something that for most diatomic molecules is not too hard to find. For example, here's an unpublished table from a project I worked on with an undergraduate student:

You cannot rely on theory (in general) to arrive at the ground state configuration, even for a simpler (homonuclear) diatomic molecule like $\ce{Fe2}$.

There is another complication, which is that, while the best basis sets for diatomic molecules like PdO are arguably the correlation-consistent basis sets, there are no correlation-consistent basis sets for Pd, so many of the electrons will be treated by a pseudopotential (and therefore not treated in the normal way).

If you want to pick the occupied orbitals based on your (non-unique!) HF calculation, then you can proceed with:

{casscf
 closed,9,5,4,1
 occ,4,2,2,1
 wf,54,4,2,0};      

But I would highly recommend surveying the literature first to see if the experimental ground state configuration is known, and otherwise doing a systematic study to decide which theory calculation is the one on which you wish to rely (already your HF calculation disagrees with mine!).

I may be able to help you more if it turns out we need to determine the occupation numbers based on theory alone, but we'd have to also make sure we're using the same basis set and bond length.