Minimum energy path calculations

The Geometry Optimizations page and the Saddlepoint optimization tutorial already shows how simple saddle-points can be located via the combination of relaxed surface scans and eigenvalue-following algorithm (OptTS) in ORCA. The relaxed surface scan also gives a crude approximation to the minimum energy path connecting 2 local minima. Occasionally one encounters more difficult saddlepoints that are not easily found by simply scanning a simple internal coordinate; or one is interested in locating the true minimum energy path that connects 2 local minima.

Commonly used minimum energy path methods are:

• - Nudged elastic band method (requires only gradients)

• - Growing string method (requires only gradients)

• - Synchronous Transit-Guided Quasi-Newton (requires the Hessian)

• - Intrinsic Reaction Coordinate (requires first locating the saddlepoint)

ORCA 4.1 and beyond features the nudged elastic band method (NEB) and the intrinsic reaction coordinate (IRC) methods.

It is also possible to interface ORCA to other programs that can perform these calculations while ORCA remains responsible for evaluating the energy+gradient.