From owner-chemistry@ccl.net Thu Aug 29 11:50:02 2013 From: "Billy McCann thebillywayne[a]gmail.com" To: CCL Subject: CCL: More than one imaginary frequencies Message-Id: <-49121-130829085113-6636-mj8JHQVcTn/Wa/0d9T2G6A[-]server.ccl.net> X-Original-From: Billy McCann Content-Type: multipart/alternative; boundary=047d7ba97f3a2b3d3804e5158fba Date: Thu, 29 Aug 2013 08:50:50 -0400 MIME-Version: 1.0 Sent to CCL by: Billy McCann [thebillywayne---gmail.com] --047d7ba97f3a2b3d3804e5158fba Content-Type: text/plain; charset=UTF-8 Ichraf, That you found a transition structure with one and only one negative eigenvalue means you found a true TS. Running an IRC calculation will connect the TS to the reactants and products. The reactants and products themselves should have only positive eigenvalues. It is necessary to validate these critical points using vibrational calculations in order to verify their point on the potential energy surface (PES). However, the intermediate points along the PES which connect the critical points will not yield those characteristic of critical points, because they are not on a place on the PES where the derivative of the energy with respect to geometry is zero. Therefore, these points will not have only positive eigenvalues. Unless you use a more sophisticated method, the vibrational calculations will yield no insight. The IRC is something of a constrained geometry optimization. Do not bother yourself with frequency calculations on the intermediate points, as they are not critical points. Typically, the IRC calculation is used to verify that the transition structure you found is indeed the one which connects your (expected) products and reactants. If you have verified your reactants, TS, and products as critical points, and have verified that you've indeed found that TS that connects your reactants and products, you've successfully solved your problem. Kind regards, Billy Wayne McCann, Ph.D. Oak Ridge National Lab Chemical Sciences Division Chemical Separations Group On Wed, Aug 28, 2013 at 10:32 AM, Close, David M. CLOSED=mail.etsu.edu < owner-chemistry]=[ccl.net> wrote: > > Sent to CCL by: "Close, David M." [CLOSED[*]mail.etsu.edu] > Ichraf: > You are right. But since you have more than on negative eigenvalue, you > are not on a true TS. You are on a saddle point. You are however close to > the answer. You have to cut out the coordinates and displace the geometry > in the direction of the TS. Redo the calculation and you should arrive at > the TS. There is a simple fortran program to perform this procedure. I > can send it to you if you like. > Regards, Dave Close. > > -----Original Message----- > > From: owner-chemistry+closed==etsu.edu..ccl.net [mailto: > owner-chemistry+closed==etsu.edu..ccl.net] On Behalf Of Ichraf Oueslati > ichraf.oueslati(_)obspm.fr > Sent: Wednesday, August 28, 2013 8:57 AM > To: Close, David M. > Subject: CCL: More than one imaginary frequencies > > > Sent to CCL by: "Ichraf Oueslati" [ichraf.oueslati : obspm.fr] Hi all, > > In order to find a Minimum Energy Path, I run an IRC calculation with G09. > When the frequencies of the points along the path were computed, I found > more than one imaginary frequencies for each one. > > I already started IRC calculation with an optimized TS which has one and > only imaginary frequency. For optimisation, I started by using B3LYP method > then MP2 method with ts and noeigen keyword. > This optimized TS connects to the reactant and product (I checked the > geometries). I thought that the points along the MEP must have only > positive frequencies. Am I wrong ?? > > Did the reaction path founded by IRC algorithm is correct? Why did I get > those negative frequencies for the points along the MEP? what should I do > to fix this problem?http://www.ccl.net/chemistry/sub_unsub.shtmlhttp://www.ccl.net/spammers.txt> > > --047d7ba97f3a2b3d3804e5158fba Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Ichraf,=C2=A0

That you found a transiti= on structure with one and only one negative eigenvalue means you found a tr= ue TS. =C2=A0Running an IRC calculation will connect the TS to the reactant= s and products. =C2=A0The reactants and products themselves should have onl= y positive eigenvalues. =C2=A0It is necessary to validate these critical po= ints using vibrational calculations in order to verify their point on the p= otential energy surface (PES). =C2=A0

However, the intermediate points along the PES which co= nnect the critical points will not yield those characteristic of critical p= oints, because they are not on a place on the PES where the derivative of t= he energy with respect to geometry is zero. =C2=A0Therefore, these points w= ill not have only positive eigenvalues. =C2=A0Unless you use a more sophist= icated method, the vibrational calculations will yield no insight. =C2=A0

The IRC is something of a constrained geometry optimiza= tion. =C2=A0Do not bother yourself with frequency calculations on the inter= mediate points, as they are not critical points. =C2=A0Typically, the IRC c= alculation is used to verify that the transition structure you found is ind= eed the one which connects your (expected) products and reactants. =C2=A0

If you have verified your reactants, TS, and products a= s critical points, and have verified that you've indeed found that TS t= hat connects your reactants and products, you've successfully solved yo= ur problem. =C2=A0

Kind regards,

Billy Wayne McCa= nn, Ph.D.
Oak Ridge National Lab
Chemical Sciences Divi= sion
Chemical Separations Group


On Wed, Aug 28, 2013 at 10:32 AM, Close,= David M. CLOSED=3Dmail.etsu.edu <owner-chemistry]=[ccl.net> wrote:

Sent to CCL by: "Close, David M." [CLOSED[*]mail.etsu.edu]
Ichraf:
=C2=A0 You are right. =C2=A0But since you have more than on negative eigenv= alue, you are not on a true TS. =C2=A0You are on a saddle point. =C2=A0You = are however close to the answer. =C2=A0You have to cut out the coordinates = and displace the geometry in the direction of the TS. =C2=A0Redo the calcul= ation and you should arrive at the TS. =C2=A0There is a simple fortran prog= ram to perform this procedure. =C2=A0I can send it to you if you like.
=C2=A0 Regards, Dave Close.

-----Original Message-----
> From: owner-chemistry+closed=3D=3Detsu.edu= ..ccl.net [mailto:owner-chemistry+closed=3D=3Detsu.edu..ccl.net] On Behalf Of Ichraf Oueslati ichraf.oueslati(_)obspm.fr
Sent: Wednesday, August 28, 2013 8:57 AM
To: Close, David M.
Subject: CCL: More than one imaginary frequencies


Sent to CCL by: "Ichraf =C2=A0Oueslati" [ichraf.oueslati : obspm.fr] Hi all,

In order to find a Minimum Energy Path, I run an IRC calculation with G09. = When the frequencies of the points along the path were computed, I found mo= re than one imaginary frequencies for each one.

I already started IRC calculation with an optimized TS which has one and on= ly imaginary frequency. For optimisation, I started by using B3LYP method t= hen MP2 method with ts and noeigen keyword.
This optimized TS connects to the reactant and product (I checked the geome= tries). I thought that the points along the MEP must have only positive fre= quencies. Am I wrong ??

Did the reaction path founded by IRC algorithm is correct? Why did I = get those negative frequencies for the points along the MEP? what should I = do to fix this problem?http://www.ccl.net/cgi-bin/ccl/send_ccl_messageht= tp://www.ccl.net/chemistry/sub_unsub.shtmlhttp://www.ccl.net/spammers.txt


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