Recent results in these two areas are presented. For the characterization and optimization of the overall conformation of chain and (macro)ring molecules collective coordinates are introduced. These are generalizations of our natural internal coordinates. They account for the fact that longer chains and rings generally possess soft delocalized deformational modes, even if they are fairly rigid along local distortions.<br/>To locate transition states, we add external forces to the calculated ones. These have been used for some time to correct for systematic errors and to characterize bond strengths but their use for reactive potential surfaces started by several groups simultaneously in 2009. For isomerization reactions, our method uses forces determined from local cubic approximations of the potential energy surface to push the system toward a transition state between two minima. This is superior to coordinate driving which, as pointed out by Schlegel, frequently ends up in blind valleys. For addition/dissociation reactions, predefined fragments, based on chemical reasoning, are pushed together or pulled apart.

Methods for the determination of equilibrium and transition state geometries

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ACS Fall 2022

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