Hello,

I would like to ask about this - we can approximate the PES in the direction of the imaginary freq. mode as

E(x) = E_0 - 1/2 kf x^2,

where E_0 is some constant, kf is a positive constant related to freq. as -i omega = sqrt(kf/m), where m is the reduced mass.

I wonder that the condition

E_0 - E(x) = kb T (b for Boltzmann)

has two solutions for the new coordinates (here represented by the x).

x_1 = x + deltax (elongation)

x_2 = x - deltax (contraction)

(deltax = sqrt(2 kb T / kf), as was well derived and numerically tested above)

The screwer produce just the elongation one.

I am working on the SO4(2-).12H2O molecule (triplets of waters oriented around each sulfate oxygen, in the directions of lone electron pairs) - energy minimization by relax module and no symmetry considered.

I have got first time 4 rather small but imaginary frequencies, did screwing along each of the coordinates independently and the results had then just one or two im.freq., so I screwed them again, but even after iterating this procedure I still have one im.freq. left.

I have tried several temperature factors (from 50K to 350K), but I think the problem might that the mode is not as simple as for e.g. biatomic and the second solution may play a role.

I can calculate it by hand, by the "screwer 2.0" should include the possibility for contraction calculation as well, I would suggest.

And, please would you suggest for my optimization problem?

I am using DFT-D3/B3LYP, def-SVP for all atoms (later I plan to use def-TZVPP for the sulfate part in order to better describe the least bonded electrons). My goal is to estimate (sulfate) complexation gibbs functions (and therefore stability constants) for several cations and lewis acids in general (with explicit hydration) - what do you think about this choice, please?

Best regards,

Jakub