Transition State Search

Transition-state (TS) search locates saddle points on the potential energy surface — structures with exactly one imaginary frequency. OpenQuantum provides three strategies for TS search:

• P-RFO (Partitioned Rational Function Optimization) — A trust-radius eigenvector- following method that walks uphill along the lowest Hessian mode and downhill in all other modes. Selected with Algorithm sella (or Sella task token) in the OPT section.
• Sella Optimizer — A saddle-point-aware IC optimizer with TS-BFGS Hessian update, P-RFO or Minimum Mode Following (MMF) step, sigma-based trust-radius schedule, and Davidson partial eigensolver for the lowest Hessian mode.
• IRC (Intrinsic Reaction Coordinate) — Path following from a TS to connected minima (reactant and product), using Gonzalez–Schlegel, mass-weighted, predictor-corrector, or bidirectional integration.
• Verification Workflows — Combined TS search → frequency verification → IRC path in a single input file using multiple task: sections.

Choose a strategy based on the quality of the initial guess and the curvature of the surface near the TS.