How can I choose the right symmetry in CAS calculation using MOLCAS?

Question

With $C_{2\mathrm{v}}$ symmetry and a closed shell configuration, how can I choose the occupation for each irreducible representation in the input file?

Answer 1

To perform a CASSCF calculation in MOLCAS, you have to supply a "standard" SCF calculation, specifically the orbitals, in a manner that the CASSCF module of MOLCAS can read them and use them as the one-determinantal basis for the following CASSCF procedure. As your system is closed-shell, a standard Hartree-Fock calculation performed beforehand using MOLCAS will do the trick. So, before you may run the RASSCF module of MOLCAS to perform a CASSCF calculation, you need to run the SCF module of MOLCAS to perform a Hartree-Fock calculation.

If this Hartree-Fock calculation is performed utilizing the $C_{\mathrm{2v}}$ symmetry of the molecule it will yield a set of orbitals, some for each symmetry group, i.e. $\mathrm{a_1, b_1, a_2, b_2}$. As the calculation is closed-shell the occupation will either be zero or 2. If your issue is the detection of symmetry in MOLCAS, please consider the following:

MOLCAS handles symmetry for all calculations via the GATEWAY module, you can specify the generators for the symmetry you deem the molecule to have there. This specification using generators might seem a little peculiar if you are used to programs like Gaussian or Turbomole, however, there are many resources to be found explaining it. See for example here:

• Lecture on using symmetry in MOLCAS
• Manual page on "Computing high symmetry molecules"

For C2V, if I'm not mistaken, the following keywords for GATEWAY might suit you:

&GATEWAY
coord=yourStructre.xyz
basis=cc-pVDZ
group=xy y

Now, assuming the symmetry was correctly recognized during the run of the GATEWAY submodule, and you have successfully performed a HF calculation utilizing the molecular symmetry, you can the proceed to chose your active space. Picking your active space is actually the tricky part of a CASSCF calculation where experience and chemical intuition come into play. I cannot give a full tutorial on choosing an active space, as this is a huge subject of its own. You can find some information here:

• MOLCAS Manual - Tools for selection of the active space

• MOLCAS FAQ: How do I choose my active space in a CASSCF/CASPT2 calculation?

• Chemistry StackExchange: How to choose proper active space in method such as CASPT2 and CASSCF?

Once you have chosen the orbitals you want to include in your active space, you need to identify their "number" in the Hartree-Fock calculation in order to be able to specify them. The "number" is of course given due to the energetical ordering of MOs ascending in energy. If your Hartree-Fock calculation yielded for example

• 10 occupied orbitals of $\mathrm{A_1}$ Symmetry
• 8 occupied orbitals of $\mathrm{A_2}$ Symmetry
• 6 occupied orbitals of $\mathrm{B_1}$ Symmetry
• 4 occupied orbitals of $\mathrm{B_2}$ Symmetry

and you have determined (via chemical intuition, experience and black magic) that the inclusion of the energetically highest two occupied orbitals of each symmetry and the energetically lowest two unoccupied orbitals of every symmetry will yield a fine active space, your input to the RASSCF module of MOLCAS for a ground state calculation could look like this:

&RASSCF
 Symmetry
 3
 Charge
 0
 Spin
 1
 Inactive
 8 6 4 2
 Nactel
 8 0 0
 Ras2
 4 4 4 4
 LumOrb

Please notice the Symmetry keyword for the RASSCF module which is set to "3" in my example above. A $C_{\mathrm{2v}}$ symmetry yields 4 symmetry groups. Different symmetries will yield a different number if symmetry groups, for example $C_2$ symmetry will give you two ($\mathrm{A}$ and $\mathrm{B}$). The number of values used as input for the keywords Inactive and Ras2 correspond to the number of symmetry groups present. You have to specify the desired symmetry of the wavefunction via the Symmetry keyword of the RASSCF module, or else the default, which is $\mathrm{A_1}$, will be calculated.

The total inputfile for a CASSCF calculation in symmetry may look like this:

&GATEWAY
coord=yourStructre.xyz
basis=cc-pVDZ
group=xy y
&SEWARD
&SCF
&RASSCF
 Symmetry
 3
 Charge
 0
 Spin
 1
 Inactive
 8 6 4 2
 Nactel
 8 0 0
 Ras2
 4 4 4 4
 LumOrb
&GRID_IT
 All; Ascii

I hope this helped you.