UT theoretical chemistry code forum

Hello all. I've been using NEB to fine the minimum energy paths between configurations of my transition metal/absorbate systems, but when I look at the geometry at the peak of the barrier I oftentimes find more than one imaginary frequency. Even after increasing the number of beads and zooming in to the transition state, I get very similar geometries and very similar multiple imaginary frequencies. Is there something I'm missing here about what NEB is giving me, or is there something further I must do to find the first order saddle point/transition state?

Thanks for any help you can give me.

I also have these proplems. Pls give some suggestions! Thanks very much!

If you use the climbing-image and it is converging to a saddle, there is no need to use more images. Converging to a low force criterion is important, but adding images will not make a difference.

There can be a few reasons for multiple negative modes:

1. You have not converged well-enough to the saddle.

2. Make sure that your frequency calculation is converged. You will typically have to use accurate forces (ediff = 1e-7 or 1e-8). Then, test convergence by changing your finite difference step in the range of 0.01 to 0.001 Ang. Only if the modes do not change, can you trust them.

3. Be aware of any zero modes in your system. Rotations and translations can appear as small (positive or negative) frequencies.

4. There are rare cases in which the NEB can converge to a second order saddle. If there are no problems with 1-3, I would be interested in looking at the calculation, and suggesting ways around the problem.

After neb and then frequency calculations, I saw the vibration mode of imaginary frequency, and there are not any vibration mode for the transition state. Pls give some suggestion, thanks!

hi,
Prof. graeme, you said "Then, test convergence by changing your finite difference step in the range of 0.01 to 0.001 Ang. Only if the modes do not change, can you trust them."
I don't understand "the modes do not change", how should I confirm that the mode keep unchanged? Thanks!

By the value of the frequencies.