Subject: Re: CCL:TRANSITION STATE AND IMAGINARY..., ANSWER

From: Per-Ola Norrby <[email-masked]>
Date: Sat, 27 Nov 1999 08:45:13 +0100

>Hello, Vladimir Basiuk asked:
>Dear CCLers,
>
>Please could you help me to clarify the following point.
>Is the presence of one imaginary frequency the necessary and sufficient
>criterion to consider the structure as TS?
>Thank you and best regards,
>Vladimir
> ***
>
>The answer is yes. A structure is a TS if and only if it has one
>imaginary frequency. Of course
>(1)this alone does not prove it is the TS you are looking for, and
>(2) at another level of calculation the TS may become a minimum or
>hilltop.
>
>E Lewars

	A further clarification: I have frequently seen that the
APPROXIMATE Hessian is used for this purpose, that is, the Hessian that has
been built up from gradient information at various points during the
optimization.  Some programs will report the eigenvalues for this Hessian.
However, many updating algorithms will make the approximate Hessian more
"positive" than the real Hessian.  The signs of eigenvalues of the
approximate Hessian CANNOT be used for validation of ANYTHING.

	In addition, the real Hessian can also display negative eigenvalues
for other reasons, especially if the gradient is not very close to zero.
It is always a good idea to compare the frequencies before and after
projection (Gaussian, for example, will allow this).  The projection will
remove the eigenvalues corresponding to translation and rotation.  Even
before projection, the corresponding frequencies should be very close to
zero as long as the gradient is low (say, absolute values less than 10-20
cm-1).  The remaining frequencies should not shift by more than 1-2 cm-1
after projection.  If the shift is larger, a small negative eigenvalue may
not be significant.

	Per-Ola Norrby

* Per-Ola Norrby, Dept. of Med. Chem., Danish School of Pharm.
* [email-masked], http://compchem.dfh.dk/PeO/
* tel. +45-35306506, fax +45-35306040