I am currently attempting to learn how to setup the active space for performing CASSCF calculations.

The textbook I am using as a reference is:

Roos, Björn O. "Multiconfigurational quantum chemistry." Theory and Applications of Computational Chemistry. 2005. 725-764.

My specific problem is discussed in Section 13.1.1. I am working with Gaussian 2016.

I would like to generate a checkpoint file with orbitals localized as atomic orbitals in order to use them as a starting point when selecting my active space. What I mean is the following...

I am working with BODIPY, with 11 active orbitals (the PI framework).

From a normal SCF I obtain the typical MOs, these would be three of my active orbitals:

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If I include the option Pop=SaveNBOs, I obtain localized Natural Bonding Orbitals. These would be three of my NBO active orbitals:

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I have found that I can use IOp 4/9 to control the localization method to some extent (for example, "IOp(4/9=20212)" implements the Pipek-Mezey localization). However, after searching for a while, I still don't see how to generate the localized atomic orbitals. Could any of you suggest how to achieve this? I would then be working with the 11 pz orbitals as my active space.


  • 1
    $\begingroup$ Unfortunately I don't have the mentioned book at hand an I'm not a Gaussian user, but I want to point out that atomic orbitals are local by definition. PAOs are not localized AOs. In PAOs the contribution of your occupied molecular orbitals are projected out. They are an alternative (local) representation for the virtual orbital space. Maybe this helps you a bit in searching... $\endgroup$ – g.smith Nov 16 '18 at 8:50

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