From owner-chemistry # - at - # ccl.net Mon Sep 24 17:51:00 2018
From: "Robert Molt r.molt.chemical.physics:-:gmail.com"
To: CCL
Subject: CCL:G: is CAS-SCF first-order density matrix idempotent
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X-Original-From: Robert Molt
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Date: Mon, 24 Sep 2018 17:49:47 -0400
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Sent to CCL by: Robert Molt [r.molt.chemical.physics===gmail.com]
a.) Yes, using Molcas, GAMESS, or Molpro
b.) There is no restriction to integer values for natural orbitals in CAS.
See Rodney Bartlett's book on coupled cluster theory (Many-Body Methods
in Chemistry and Physics), or Peter Szalay's review article on MR-CI in
Chemical Reviews, 2012, or Mark Gordon's tutorials on MC-SCF
http://www.msg.ameslab.gov/tutorials/mcscf2.pdf
c.) I have no idea what Gaussian does. Sorry.
On 9/24/18 4:08 PM, Thomas Manz thomasamanz~~gmail.com wrote:
> Dear colleagues,
>
> We recently ran some CAS-SCF calculations in Gaussian 16. When
> visualizing the orbitals in GaussView 5 or outputting the natural
> orbitals to a WFX file (e.g., pop=NO output=wfx), the natural
> orbitals are listed as fully occupied or fully empty. (In other words,
> the CAS-SCF first-order density matrix is claimed to be idempotent.)
> This is different than the behavior that occurs with Gaussian's MP2,
> coupled-cluster, SAC-CI, or CISD calculations, which print fractional
> orbital occupancies, and generally speaking have non-idempotent
> first-order density matrices. The systems we are studying are some
> small diatomic molecules (e.g., Be2, B2, etc.)
>
> I am confused about whether the claimed whole number occupancies of
> the CAS-SCF natural orbitals is a real effect or whether it is a bug
> in the Gaussian software? According to literature references, an
> N-representable first-order density matrix can in general have
> fractional natural orbital occupancies (i.e., it can be
> non-idempotent). But also, there is not a one-to-one map between the
> first-order density matrix and physical observables: two calculations
> with identical electron density distributions can have different
> first-order density matrices. Consequently, I have not yet been able
> to rule out whether this is just a representation issue.
>
> Can anyone offer specific insights into the following questions:
>
> (1) Has anyone observed a non-idempotent first-order density matrix
> (i.e., fractional natural orbital occupancies) when performing CAS-SCF
> calculations using any software program?
>
> (2) Can anyone offer a concrete answer as to whether the CAS-SCF
> first-order density matrix should or should not in general be
> idempotent? (Please omit discussions of trivial one or two-electron
> systems, since they are too simple.) Is there theory or literature
> references to back this up?
>
> (3) If the CAS-SCF first-order density matrix is not supposed to be
> idempotent, is there a reasonably easy and fast way to extract the
> CAS-SCF natural orbital fractional occupancies from Gaussian 16
> calculations? (Caution: This needs to be based on actual experience,
> rather than conjectures.)
>
> By the way, this issue is not new to Gaussian 16; we observed the same
> behavior for Gaussian 09. At this point, I do not understand whether
> this behavior is "real" or a "bug". Can anyone provide clarification?
>
> Sincerely,
>
> Tom Manz
> associate professor
> Chemical & Materials Engineering
> New Mexico State University
--
Dr. Robert Molt Jr.
r.molt.chemical.physics-,-gmail.com