Partial geom opt of a linear combination of distances

How To
#1

Hi, first post here :raising_hand_man:

I wonder if the Qchem have the capability of performing partial geometry optimisation using linear combination of inter-atomic distances.

For example:

r12=distance between atom 1 and 2
r13=distance between atom 1 and 3
r14=distance between atom 1 and 4

And the partial geometry optimisation would keep frozen the value of r12+r13-r14 (not each distance at the time but the result of the sum).

Many thanks,
R

#2

I’m not sure there is anything that is quite what you want. The closest thing is one-dimensional restrained scan with harmonic bias potential, see
https://manual.q-chem.com/6.0/Ch9.S7.SS1.html
Between this and (possibly) fixed-atom constraints, you might be able to engineer something close to what you want.

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#3

This does not exist within Q-Chem in this way, but I think the geometric equivalence is to fix two of the angles of any of the 3 possible angles 2-3-4/ 3-4-2 / 4-2-3. But I would have to ponder this a bit longer to be 100% sure.

But @jherbert does probably provide the best solution which is a restrained scan.

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#4

If you want to break one bond and form another bond, then the r12mr34 option that John has mentioned works well. I don’t think you can do it for more complicated types of scans (but it would be nice if one could …).

#5

Yes, thanks, this will do the job, although it would be great to include more distances in the coordinate.

#6

Yes, I didn’t know about this choice in QChem, which is nice, but it would be great indeed to be able to include more distances in the reaction coordinate.
Thanks everyone!