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

 Sent to CCL by: "Johannes Hachmann" [jh388]*[cornell.edu]
 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
 ---------------------------------------------------------------
 Johannes Hachmann
 Chan Research Group
 140D Baker Laboratory
 Department of Chemistry and Chemical Biology
 Cornell University
 Ithaca, NY 14853-1301
 USA
 ---------------------------------------------------------------
 > -----Original Message-----
 > From: owner-chemistry+jh388==cornell.edu_-_ccl.net
 > [mailto:owner-chemistry+jh388==cornell.edu_-_ccl.net] On Behalf
 > Of Yutao Yue Yutao.Yue#,#gmail.com
 > 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- -gmail.com] (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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