Conjugate Peak Refinement

Simulate > Conjugate Peak Refinement... finds a reaction path and its saddle point using the Conjugate Peak Refinement (CPR) method of Fischer and Karplus (1992). It needs only reactant and product structures, with no initial Hessian: an interpolated path between them is built first and then refined until its highest-energy point is a genuine saddle point. It complements the Nudged Elastic Band (NEB) tool, and its highest point is a good starting structure for a Transition state search (Baker).

The window (Figure 1)

• Reactants / Products: the two end-point structures, chosen among the coordinate sets of the system (they must belong to the same system, with atoms in the same order).

• Initial path images (default 8): the number of images of the interpolated starting path (built with the same path builder used by the NEB tool).

• RMS Grad. Tolerance (default 0.1): the convergence criterion of the refinement (kJ mol-1 Å-1).

• RMSD max. tolerance (default 1.5): the maximum RMSD allowed for a conjugate-direction line-search step.

• Steps per segment (default 3): how finely each segment of the path is scanned for a local energy maximum before refinement.

• Max. CPR runs (default 1000): the maximum number of CPR cycles.

• Beta type: the conjugate-gradient formula used in the minimizations between line maximizations: FR (Fletcher-Reeves) or HS (Hestenes-Stiefel).

• Initial orthogonal runs (default 0): the number of initial CPR cycles forced to use orthogonal search directions, which can help on difficult paths.

• Break if tau reached, Increase tau on futile loop and Final unrefinable refinement: options of pDynamo's CPR algorithm that control how the refinement treats segments that cannot be improved further.

• Trajectory Name / Folder: the refined path is always saved, since it is the result of the calculation.

Reading the result

The saddle point is a specific frame of the refined path, marked "saddle" in the Path Summary table of the job log; it is not necessarily the object created when the job finishes. Load the trajectory, go to that frame, and confirm it with Simulate > Normal Modes (one imaginary frequency) or refine it with the Baker search.

Reference: S. Fischer and M. Karplus, "Conjugate peak refinement: an algorithm for finding reaction paths and accurate transition states in systems with many degrees of freedom", Chem. Phys. Lett. 194, 252–261 (1992).