From rosas@irisdav.chem.vt.edu Mon Jul 8 21:59:13 1996 Received: from irisdav.chem.vt.edu for rosas@irisdav.chem.vt.edu by www.ccl.net (8.7.5/950822.1) id VAA06724; Mon, 8 Jul 1996 21:44:47 -0400 (EDT) Received: by irisdav.chem.vt.edu (951211.SGI.8.6.12.PATCH1042/921111.SGI.AUTO) id VAA07075; Mon, 8 Jul 1996 21:36:31 -0700 From: "Victor M. Rosas Garcia" Message-Id: <9607082136.ZM7073@irisdav.chem.vt.edu> Date: Mon, 8 Jul 1996 21:36:27 -0400 In-Reply-To: dsmith@dasgroup.com (Doug Smith) "CCL:M:MOPAC vibrations negative for a ground state?" (Jul 8, 3:21pm) References: <19960708192136.AAA13987@dirac> X-Mailer: Z-Mail (3.2.0 26oct94 MediaMail) To: dsmith@dasgroup.com (Doug Smith) Subject: Re: CCL:M:MOPAC vibrations negative for a ground state? Cc: CHEMISTRY@www.ccl.net Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii 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? > 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)??!?!?!?! 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 -- ----------------------------------------------------------------------- Victor M. Rosas Garcia * "How can we contrive to be rosas@irisdav.chem.vt.edu * 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 -----------------------------------------------------------------------