I've presumably found the TS that I am interested in. I always break down the IRC calculation into a separate reverse and forward calculation so I can run them in parallel.
My forward calculation is fine. It took a while to have the algorithm step along the energy landscape and it required the adjustment of stepsize=1. No other stepsize would initially work.
When I examine the forward IRC calculation the initial gradient is very small, for the first 30 or so steps i.e., a almost flat landscape. After that it slopes down quite nicely.
My problem comes when I try and calculate the reverse IRC. I have tried stepsizes from 1 (0.010 bohr) up to 25 (0.250 bohr) yet it still can't get past the first step. Yet my vibration is quite strong and visually (in GView) the motion of the particles would suggest that there should be a forward and reverse motion from the TS.
I have also tried irc=GS2 in an effort to use the old IRC algorithm. Still nothing.
Any thoughts on how to improve the IRC for the reverse pathway?
Dear Anthony,
I suggest to run the reverse pathway with the smallest increment you can, however, without using frequency. Once the calculations are completed to assign the small region with the highest energy and then to run frequency calculations for this very narrow range.
Rafik
Dear Anthony,
Did you find the TS by a reaction path or saddle-like procedure? Sometimes, calculated TS does not belong to the predicted reaction. IRC behavior might indicate this situation.
Wojciech
Dear Anthony,
It might worthy to try to increase the accuracy, sometime works in my case.
scf(conver=12) int(ecpacc=12,acc2e=12,SuperFineGrid)
Daniel
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