From chemistry-request@ccl.net Wed Jan 8 09:44:24 1992 Date: Wed, 8 Jan 1992 07:27 CST From: Andy Holder Subject: TRANSITION STATE LOCATION To: CHEMISTRY@ccl.net Status: R On the subject of transition states, ONE negative frequency indicates a transition state. (Since the frequencies are computed directly from the force constants, it is not neccessary to check them as well.) If you have more than one, a true transition state has not been reached. If there are two, you are on a "ridgeline". You must analyze the motion of the smaller value and "goose" the geometry along this motion and re-optimize. (Yes, I know, incredibly ted- ious.) Very small negative frequencies usually correspond to things like methyl rotations. I don't know the exact structure you are working with, but that is likely. If you have more than two negative frequencies, this can be a problem. People doing reactions spend the majority of their time stomping out spurious negative frequencies. As if that wasn't enough, you must make sure that your TS corresponds to the one for the reaction you are examining. To do this, examine the motions associated with the ONE negative frequency and again "goose" the geometry along these coordinates. If you end up with product, that's good. You can also goose it backwards and hopefully end up with reactant. I have mentioned twice "analyze the motion". You can do this is several ways. After doing a FORCE calculation, the cartesian components of motion for each atom are printed under the force constant listing. Making sure that the molecule is oriented the way the force calculation has indicated (its in there) just look at what the atoms with large components actually do for a partic- ular vibration. The second method (the one I use) is to perform an LTRD calculation. This will give you the components of nega- tive frequencies in internal coordinates. This is usually much easier to interpret. Again figure out which components are large and count optimizable geometric parameters to match them up. Hope this abbreviated version helps. Andy Holder =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= DR. ANDREW HOLDER Assistant Professor of Computational/Organic Chemistry Department of Chemistry || BITNET Addr: AHOLDER@UMKCVAX1 University of Missouri - Kansas City || Internet Addr: aholder@vax1.umkc.edu Spencer Chemistry, Room 502 || Phone Number: (816) 235-2293 Kansas City, Missouri 64110 || FAX Number: (816) 235-1717 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=