Imaginary frequency elimitation,module $vibration work

[wuhen]

hello
After the step of $aoforce ,i get the file which used as infrared analysis, but there are some problems for 1 and 2
$vibrational spectrum
#  mode     symmetry     wave number   IR intensity    selection rules
#                         cm**(-1)        km/mol         IR     RAMAN
     1        a            -208.04         0.00000       YES     YES
     2        a            -185.28         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         0.13056        -       -
     8                        0.00         5.34576        -       -
May the 1 and 2 be imaginary frequency, but i don't know how to elimitate them?   There is a module $vibration ,how does it works?
 you can explain it in detail, many thanks 

[jan]

Hello

One possibility is to set your calculation to a higher energy convergence like 10^-8. If this does not
help your can use the script screwer. You use it by typing in "screwer" and than you have to select a mode
along which you want to distort the molecule. Choose on of the modes with an imaginary frequency. Than
you have to select a temperature. You have to play around with that value. If you choose a value too low you
end up in the minimum where you were before. If the value is too high the molecule will be too far from equilibrium
an it will most likely fail to converge. The screwer script writes new cartesian coordinates to the control file which
you find at the end of the file under the desciptor "distorted cartesian coordinates along mode ...". With this new
coordinates you can setup a new optimization than will hopefully converge to a PES minimum.
Since your imaginary frequencies a rather high i would stick to the screwer method.

 Regards

[wuhen]

many thanks , jan,   i think it should be a good answer.