### Author Topic: Transition state search: what number should I use for two negative frequencies?  (Read 1228 times)

#### baishaotao

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##### Transition state search: what number should I use for two negative frequencies?
« on: May 16, 2020, 07:22:38 PM »
Dear friends,

I have found one structure that should close to the transition state structure in woelfling-job calculations. Aoforce calculations show two negative frequencies as shown below. The mode 1 with frequency of -527.58 cm -1 is the imaginary frequency along the reaction path. I have setted itrvec  1 for transition state search, which leads to the structure that actually follows mode 2. Thus, the problem is what number should I set for this job? Another general question: how can I know I follow the right transition vector for transition state search?

\$vibrational spectrum
#  mode     symmetry     wave number   IR intensity    selection rules
#                         cm**(-1)        km/mol         IR     RAMAN
1        a            -527.58         0.00000       YES     YES
2        a             -78.66         0.00000       YES     YES
3                       -0.00         0.00000        -       -
4                       -0.00         0.00000        -       -
5                       -0.00         0.00000        -       -
6                       -0.00         0.00000        -       -
7                        0.00         0.00000        -       -
8                        0.00         0.00000        -       -
9        a              12.93         0.00928       YES     YES
10        a              25.73         0.00596       YES     YES
11        a              31.46         0.26516       YES     YES
12        a              35.84         0.23318       YES     YES
13        a              44.13         0.02108       YES     YES
14        a              45.94         0.08892       YES     YES
Many thanks in advance!

Best regards,
Dr. Shaotao Bai
KU Leuven

#### uwe

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##### Re: Transition state search: what number should I use for two negative frequencies?
« Reply #1 on: May 19, 2020, 04:08:27 PM »
Hello,

the transition state search by default follows the mode which is closest to the initial transition vector. The statpt module calculates the overlap of the initial mode with those of each step and then follows the one which has the largest overlap.

First thing to check before running a transition state search after a frequency calculation is to make sure not having any approximate hessian files in your job directory - only the newly generated hessian file. So remove the file hessapprox if it is present in the job directory.

To always follow the mode with the lowest eigenvalue, try the keeptmode option in the \$statpt section (see manual for details, search for keeptmode).

If the two lowest modes come get very close to each other, such that the overlaps of the initial eigenvector with both modes are similar, it might help to run a full Hessian calculation from time to time. An automated way can be activated by adding the hssfreq <number> keyword to \$statpt, and the full Hessian will be calculated every N'th step. This can be very time consuming, though.

Good luck!

Uwe

#### baishaotao

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##### Re: Transition state search: what number should I use for two negative frequencies?
« Reply #2 on: May 19, 2020, 04:25:17 PM »
Dear Uwe,

Thanks a lot!

I have tried full Hessian calculated every N'th step by hssfreq 1. It is indeed very time consuming. I checked the vibration modes so far. It is still good. By that I mean I can see the vibration along the reaction coordinates.

I will see if it works out. Otherwise, I will try to use keeptmode option in the \$statpt section.

Best regards,
Dr. Shaotao Bai
KU Leuven