From owner-chemistry@ccl.net Sun Apr 29 01:39:00 2007 From: "Jim Harrison jim456harrison a yahoo.com" To: CCL Subject: CCL: Vibrational frequencies with RHF and R-B3LYP - ethylene Message-Id: <-34153-070427033749-27062-o8CW68VLV4InJgBD5nYXWg.@.server.ccl.net> X-Original-From: Jim Harrison Content-Transfer-Encoding: 8bit Content-Type: multipart/alternative; boundary="0-891618644-1177655861=:4469" Date: Thu, 26 Apr 2007 23:37:41 -0700 (PDT) MIME-Version: 1.0 Sent to CCL by: Jim Harrison [jim456harrison^_^yahoo.com] --0-891618644-1177655861=:4469 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit if you are searching for the optimized structure of ethylene (i.e the minimum), none of the results (with 1 neg frequency or 3 negative freq) is correct. the structures u have obtained correspond to a saddle point and not the minimum.also i do not understand what u mean with a particular ethylene geometry(do u mean its anion or its cation or a specific geometry or something else). ethylene has a D2H symmetry and obviously u will obtain the optimized structure (without any imaginary frequency) with the D2H point group. now if u are looking for the transition state of ethylene the structure with the neg freq is a true transition state. the structure with the 3 neg frequency is higher order saddle point but is not the transition structure that connects two minima. hope that this will help jim "Sriraj Srinivasan ss537a/drexel.edu" wrote: Sent to CCL by: "Sriraj Srinivasan" [ss537]-[drexel.edu] Hi, I have been trying to obtain the vibrational frequency of a particular ethylene geometry, i encountered the following scenario By performing my Hessian calculations with only RHF i obtained one negative frequency, and when i did the same with R-B3LYP i got 3 negative frequencies. I was wondering if anyone could tell me why this has happened, and which of the results do i consider to be correct. I performed all my geometry optimizations with R-B3LYP. Thanks, Srirajhttp://www.ccl.net/cgi-bin/ccl/send_ccl_messagehttp://www.ccl.net/chemistry/sub_unsub.shtmlhttp://www.ccl.net/spammers.txt--------------------------------- Ahhh...imagining that irresistible "new car" smell? Check outnew cars at Yahoo! Autos. --0-891618644-1177655861=:4469 Content-Type: text/html; charset=iso-8859-1 Content-Transfer-Encoding: 8bit if you are searching for the optimized structure of ethylene (i.e the minimum), none of the results (with 1 neg frequency or 3 negative freq) is correct. the structures u have obtained correspond to a saddle point and not the minimum.also i do not understand what u mean with a particular ethylene geometry(do u mean its anion or its cation or a specific geometry or something else). ethylene has a D2H symmetry and obviously u will obtain the optimized structure (without any imaginary frequency)  with the D2H point group.
now if u are looking for the transition state of ethylene the structure with the neg freq is a true transition state. the structure with the 3 neg frequency is higher order saddle point but is not the transition structure that connects two minima.

hope that this will help
jim

"Sriraj Srinivasan ss537a/drexel.edu" <owner-chemistry..ccl.net> wrote:

Sent to CCL by: "Sriraj Srinivasan" [ss537]-[drexel.edu]
Hi,

I have been trying to obtain the vibrational frequency of a particular ethylene geometry, i encountered the following scenario

By performing my Hessian calculations with only RHF i obtained one negative frequency, and when i did the same with R-B3LYP i got 3 negative frequencies.

I was wondering if anyone could tell me why this has happened, and which of the results do i consider to be correct. I performed all my geometry optimizations with R-B3LYP.

Thanks,

Sriraj






Ahhh...imagining that irresistible "new car" smell?
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