Author Topic: How to obtain the Eigenvectors of the hessian Vektors ?  (Read 9928 times)

Workingmarc

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How to obtain the Eigenvectors of the hessian Vektors ?
« on: July 14, 2008, 05:31:43 pm »
Hi, I am currently trying to determine the structure of the transition state of the following reaction as an exercise so as to get started:

F + H2 => FH + H

The linear transition state is already known and is given in the following document, whose contain is relatively secure:

www.uni-regensburg.de/.../nat_Fak_IV/Physikalische_Chemie/Schmeer/PDF_Files/Reaktionskinetik_Dynamik2.pdf

The internal coordinates are so described:

r2(FH)    1.602
r1(HH)  0.756

I have then simply created a file with cartesian coordinates in A corresponding to these values, and I have gotten:

   1   h           0.0000000000    0.0000000000    0.0000000000
     2   h           0.0000000000    0.0000000000   -1.4286329572
     3   f           0.0000000000    0.0000000000    3.0273412665

I have then sought the symmetry through desy and obtained C6v (which is not quite exact, but passable for my purposes here...)

Afterwards I wanted to determine automatically the internal coordinates trough iaut, but it only gave me the bond between the two H atoms, so I added manually the other coordinates H-F, and finally:

1k 1.000 stre    1h           2h                                    1.42863
    2k 1.000 stre    2h           3f                                    4.45597

I then went on in the next menus, always choosing the default options, and an EHT guess for the input occupations (by the way an open shell).

Afterwards, I have chosen a DFT optimization with the very efficient B3LYP, and then left Define.

To get an estimation of the hessian matrix, I have launched a aoforce calculation after a dscf:

but I found really discouraging eigenvalues:


       mode               1        2        3        4        5        6

     frequency        i890.17  i890.17  i150.04     0.00     0.00     0.00

mode               7        8        9

     frequency           0.00     0.00  4234.96

I returned afterwards in define, set itvc in the stp menu on the values 1,2 and 3 successively.

I have so launched three jobex -statpt calculations for the three eigenvalues.

For itvc = 1, I found exactly one imaginary eigenvalues for the egein-matrix, but the geometry was utterly wrong:

   0.00000000000000      0.00000000000000     -2.01745011056704      h
    0.00000000000000      0.00000000000000     -3.46188404445066      h
    0.00000000000000      0.00000000000000      5.47933415501772      f

After having written again the coordinates of the TST in define, I have launched jobex-statpt for itvc = 2 but gotten an error.

Now, I wonder if it is possible to know for each egeinvalues the expression of the corresponding eigenvector as function of the internal coordinates.

This way, I hope being able to identify the right one to vary.

Does any one of you know how to do it ?

Please, pardon my naivety and my lack of experience, since I am a newcomer in the fields of computational chemistry and geometry optimization (which I intend to use in order to get kinetic constants).


uwe

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Re: How to obtain the Eigenvectors of the hessian Vektors ?
« Reply #1 on: July 17, 2008, 11:40:41 am »
Hi,

there is a small chapter Analysis of Normal Modes in Terms of Internal Coordinates in the documentation (chapter 8.1) which might be helpful.

If you just want to 'see' the negative modes, you can use TmoleX to visualize the frequencies - also the negative ones. Or molden after running tm2molden, read in the generated molden.input file into molden.

Regards,

Uwe