CCL:G: (correction) Energy change upon swapping two orbitals in Gaussian 98

Thank you very much Johannes, I did swap the two highest OCCUPIED beta orbitals. I was also doubting that the original solution was not stable. I compared the solutions before and after swapping, the MO compositions are almost identical for frontier orbitals, and only some minor differences for some deeper orbitals. Plus, the S2 values, the NBO charges and spins of some characteristic atoms (metal, etc) are almost exactly the same, up to quite a few digits.

On the other hand, although with highly similar MO compositions, the MO energy themselves are fairly different through all MOs including the deepest, e.g., the difference for a single MO is on the order of 0.01 a.u..

By the way, I used the default SCF convergence level (which is equivalent to conver=4). But I guess that's far from introducing errors of this magnitude right?

I wonder is it possible that Gaussian does something artificial (and arbitrary? because in one case energy went up and the other case it went down) to the MO energies, once the initial guess swapping is introduced?


On Thu, Mar 12, 2009 at 12:50 PM, Johannes Hachmann jh388~! <> wrote:

Sent to CCL by: "Johannes Hachmann" [jh388]*[]
Dear Yutao,

sounds to me like you might have landed on an electronic instability the
first time. SCF for (transition-)metal compounds can be very hard to
converge. Try STABLE=OPT starting from your original calc without the
orbital swap and see whether that lowers the energy (in that case, your swap
has introduced some mixing/symmetry breaking and led to a more stable
state). You can check the S2 value of your two results to see, whether your
state has changed. You might also want to check your SCF convergence
threshold and DFT grid (although it should not make a difference of 20mH).

Generally, swapping the highest two HOMOs should not change the energy at
all, but maybe you ment the HOMO-LUMO pair? A vanishing HOMO-LUMO gap
indicates a system with nondynamic correlation, and you might not want to
use a single reference method like DFT in the first place.

Hope that helps. Best


Johannes Hachmann

Chan Research Group
140D Baker Laboratory
Department of Chemistry and Chemical Biology
Cornell University
Ithaca, NY 14853-1301

> -----Original Message-----
> From:
> [] On Behalf
> Of Yutao Yue Yutao.Yue#,
> Sent: Thursday, March 12, 2009 10:55 AM
> To: Hachmann, Johannes
> Subject: CCL:G: (correction) Energy change upon swapping two
> orbitals in Gaussian 98
> Sent to CCL by: "Yutao  Yue" [Yutao.Yue-] (sorry
> the energy changes are 0.02 a.u., NOT 0.2 a.u.)
> Dear All,
> I have a successful G98 single point calculation
> (UBLYP/6-31G*) on a metal-containing system of around 70
> atoms. I use the result MOs as initial guess, but swapped the
> two highest occupied beta orbitals (which are supposed to be
> highly competing with each other) upon reading in the initial
> guess, to do another G98 single point calculation on the same
> system. I then arrived at a solution that, the frontier
> orbitals are almost identical to the original solution
> including the ordering of the beta orbitals, while the single
> point energy is 0.02 a.u. lower!
> Similar calculations was done with UBPW91/6-31G*, and again
> the orbitals go back to original ordering, but the single
> point energy is 0.02 a.u. higher now!
> Anybody have any idea about those energy changes?
> Thanks a lot and wish everybody a nice day!
> -Yutao Yue
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