Subject: MOPAC vibrations negative for a ground state?

From: [email-masked] (Doug Smith)
Date: Mon, 8 Jul 1996 15:21:36 -0400

Dear All:
        A question was recently posted from one of my co-workers, but there
have been no satisfactory answers... indeed, most of the answers completely
missed the point of the question.  Let's try again, please.

        We have been examining (poly)nitrocubanes with and without other
functional groups using MOPAC 93 and the PM3 Hamiltonian/parameters.  Most
structures optimize just fine, but a few give a very peculiar problem.  

        The job in question, 1-amino-2-nitrocubane, was optimized using PM3
GEO-OK GNORM=0.01 PRECISE LET DDMIN=0.0 EF.  The structure optimized using
eigenvector following.  SCF field was achieved with a GNORM of 0.00565.

        The jobs are run using the keywords:  PM3 NODIIS GEO-OK GNORM=0.01
LET DDMIN=0.0 FORCE THERMO(273,1273) after the optimization has been
completed.  The gradient norm was 0.00545.  The output from this force
calculation gives a first root and first frequency of -296.6, which should
indicate a transition structure.  However, the program states that the
"system is a ground state".  The output follows:

<---------------------------------------------------------------------------
--------->
Root No.     1          2       3       4         ............(etc.)
            1 A"       2 A"    3 A"    1 A'       ............
           -296.6      78.2    191.1   203.4      ............

        Description of Vibrations
Vibration 1     1A"             Atom Pair       Energy Contribution     Radial
Freq.         -296.56           N12 -- H14              27.5%            5.6%
T-Dipole          .3676         N12 -- H13              27.5%           11.3%
Travel            .1621         C 8 -- N12               7.6%             .0%
Red. Mass         .4311         C 1 -- C 7               2.2%             .0%

        System is a ground state
<---------------------------------------------------------------------------
-------->

        Next, in order to try to understand this system and result, we reran
just the force calculation in MOPAC 6.0 using the MOPAC 93 optimized
geometry.  The keywords for this run were: PM3 NODIIS GEO-OK GNORM=0.01 LET
FORCE.  We did not perform the thermodynamics calculation.  The keyword
DDMIN is not available in MOPAC 6.0.  The gradient norm for this job was
0.16015. The output from this run gave root 1 as 17.68167 and the first
printed vibrational frequency (number 2 - the first is not printed because
it is less than 50) as 89.15.  Obviously, this is not a transition structure.

        WHAT GIVES?!?!?!?!

        IS THIS A TRANSITION STRUCTURE OR A GROUND STATE (MINIMUM)??!?!?!?!

Please reply to the list and we will monitor the answers and discussion.
Thanks in advance.

Doug
--
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---

From: "Victor M. Rosas Garcia" <[email-masked]>
Date: Mon, 8 Jul 1996 21:36:27 -0400

Hello everybody,

I found the problem posted by Doug Smith very interesting.  An excerpt follows.

On Jul 8,  3:21pm, Doug Smith wrote:
> Subject: CCL:M:MOPAC vibrations negative for a ground state?

So I ran the job myself, using MOPAC93 R2 on an Indigo2Extreme, IRIX 5.3, CPU
R4400.  First, let's make sure I didn't screw up the compound, for some reason
I'm never sure about my nomenclature.  The molecule I used is:

            2HN     NO2
               \____/
              /_|__/|
              | |_|_| <-----This is supposed to be a cube!  :)
              |/__|/

All the keywords were cut and pasted from the e-mail message to avoid typos or
missing keywords.  My results are as follows:

******************************************************************************
Gradient after geometry optimization:  0.0093  The point group was Cs.

Gradient for the FORCE calculation:    0.02308

Roots:
   Root No.    1       2       3       4       5       6       7       8

              1 A"    1 A'    2 A"    3 A"    2 A'    4 A"    3 A'    4 A'

              18.4   155.4   178.8   207.8   292.4   304.8   403.3   434.5
[snip, snip]

          DESCRIPTION OF VIBRATIONS

 VIBRATION   2  1A'       ATOM PAIR      ENERGY CONTRIBUTION     RADIAL
 FREQ.      155.41       C 1 --  N 9            8.1% (228.7%)     0.0%
 T-DIPOLE   0.5083       C 2 --  C 4            4.9%              0.1%
 TRAVEL     0.0892       N 9 --  H18            4.3%             17.7%
 RED. MASS  2.5416       N 9 --  H17            4.3%             17.7%

[more snip, snip]
********************************************************************************

So I didn't get any imaginary freqs.!
A few more details:  I drew the molecule using PCMODEL v4.0 for SGI and used it
to generate the Z-matrix for MOPAC after minimization with MMX.

My only guess as to what could be the difference:
* Check the revision of MOPAC93 that you are using.  I'm using RELEASE 2.  That
could explain the difference between your MOPAC run and mine.
* See if the imaginary freq you are getting corresponds to the rotation of the
amino group (although in this case I don't know why the program would identify
the molecule as a ground state).

Hope this helps.

Victor

-- 
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Victor M. Rosas Garcia                   * "How can we contrive to be 
[email-masked]                *  at once astonished at the  
Virginia Tech doesn't necessarily share  *  world and yet at home in it?"
the opinions you just read.	         *  G. K. Chesterton
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