ECCE-QM IRC: both directions end in the same valley (SN2 backward branch reaches the product complex); false 'minimum reached'

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#764 2 comments 0 reactions 1 assignee View on GitHub

@ohlincha is already working on this.

Since Nov 1, 2026.

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Description

SN2 Cl⁻ + CH₃Br, B3LYP/aug-cc-pVDZ, charge −1, from the ORCA TS geometry (radium, ecce-qm from main, 2026-11-01).

  • irc_step 0.1 (default), irc_max_points 40: both directions run to the 40-point limit; backward at s = −1.98 E = −3074.14836, s = −2.98 E = −3074.15365, s = −3.88 E = −3074.15546 Eh, still descending.
  • irc_step 0.2, irc_max_points 80: ends with 'forward 32 points, gradient below threshold: minimum reached; backward 21 points, gradient below threshold: minimum reached'. Backward at s = −1.99 E = −3074.14576, s = −2.98 E = −3074.14774, last point s = −4.08 E = −3074.14833 Eh, still descending by ~5e-5 Eh per step.

So with the larger step the backward branch is already 2.6 mEh above the step-0.1 path at s = −2, flattens, and is reported as a minimum about 7 mEh above where the step-0.1 run already was. Forward looks consistent.

To find: whether the corrector step loses the path at 0.2 (step-size control, Hessian update), and why the minimum test fires while the energy still decreases (gradient threshold alone; add an energy-change criterion or an optimisation of the end point). A false 'minimum reached' must not be reported.

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