Subject: Instability

From: <[email-masked]>
Date: Wed, 5 Apr 1995 11:43:08 -0500

Dear Netters,

    I studied Ab Initio calculations of NaOONO. There are some unusual
  results observed:

  (a). Unusual high O=N stretching in MP2/6-311+G* calculation.
            EXP.   O=N: 1437 cm-1 in frozen argon.
       HF/6-311+G* O=N: 1718 cm-1
      MP2/6-311+G* O=N: 2408 cm-1 -- unusual high!

  (b). Unusual IR intensity in O=N stretching.
       MP2/6-311+G* O=N: 55012 km/mol (IR intensity
       is 500 km/mol for instense band)

  (c). O=N stretching is very sensitive to different basis set.
       MP2/6-311+G* O=N: 2408 cm-1 [55012] km/mol -IR intensity
       MP2/6-311+G  O=N: 1797 cm-1 [7432]
       MP2/6-311 G* O=N: 2416 cm-1 [55314]
       MP2/6-311 G  O=N: 1829 cm-1 [7676]
   I think these unusual results are caused by the instability of
   HF wavefunctions. So, I calculate instability analysis in G92/DFT.
   I got this messages:
     " Unable to determine singlet-triplet character of expansion vector.
   Closed Shell can not continue.
   Error termination in Lnk1e. "

   The G92/DFT was able to determine what instability this wavefunction has,
   RHF -> UHF, but not the orbitals involved.  We found that there is a
   triplet state that is lower in energy than the singlet at HF levels of
   theory, but singlet is lower for MP2 and DFT levels of theory.  Triplet
   states were searched for, but all of those obtained from the INDO guess
   were high in energy.  The lowest triplet was not found by the INDO guess.
   There are four references about instability:
   (1). J. Cizek and J. Paldus, J. Chem. Phys. 47 (1967) 3976.
   (2). J. Paldus and J. Cizek, Chem. Phys. Letters 3 (1967) 1.
   (3). J. Paldus and J. Cizek, J. Chem. Phys. 52 (1970) 2919.
   (4). J. Paldus and J. Cizek, J. Chem. Phys. 54 (1971) 2293.

   ANY INFO and advise will appreciate!

                                  Gavin Tsai