Path orthogonal to the equipotential contours of a potential energy surface that connects the energy minima through a saddle point (transition state) from which it slopes downwards along the steepest descending lines in 3N-6 configurational space (N is the number of nuclei in the reacting system).
Note:
MERP calculation allows the investigation of detailed changes in molecular structure describing the @T06446@ of reactants to products.
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 370 (https://doi.org/10.1351/pac200779030293)