Weird Behavior with New Geometry Optimizer

I recently tried using Q-Chem 6.0 to perform the geometry optimization of a complex. In particular, I began with a distorted geometry of the transition state and optimized towards the reactants as I was curious which specific reactant conformation is connected to this transition state. When I used the old geometry optimizer, everything works smoothly (see attached P-Pyridone_TS_Reverse_old_opt.out). However when I used the new geometry optimizer, the initial geometry optimization step seems totally crazy, especially since the exact Hessian is read in. The geometry optimization ultimately reaches a totally different local minimum (see P-Pyridone_TS_Reverse_3.out).

Is there a way to prevent such a crazy initial geometry optimization step?

Sample Output Files

@apetit sorry for the delay but I have had a chance to look over this.

The new optimizer tries to be more aggressive with it’s step generation, in the old optimizer is would limit the step to a norm of 0.3 while the new optimizer uses the rms of the step. In most of my testing this has proven to help speed up optimizations. In this case it is causing some issues, mainly if you look at the structure from initial structure to the second structure it causes the 2-hydroxypyridine to encroach on the other molecule. The new optimizer then guides the 2-hydroxypryidine away to a stable ground state, breaking the O H - N hydrogen bond. But with the old optimizer it takes a less agressive step which keeps the O H-N hydrogen bond intact. This can be easily observed by overlaying the two optimized structures into IQmol.

To restrict the step in the new optimizer use a smaller displacment value: max_displacement = 3.0e-1 (default value)