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Potential energy surface - saddle point – transition state

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The potential energy surface for a diatomic molecule.

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Relaxed-scan potential energy curves of dimers and ionic pairs at the B3LYP-D3/6-31+G* level, and the corresponding minimum structures. Color legend: C grey, H white, O red, and N blue.

PES of hydrogen bonds

The ionic pair has the deepest energy well (102.4 kcal mol−1) at R(H⋯O) = 1.86 Å and the well-width is relatively large.

The H2O dimer possesses a wide energy well with a depth of 7.2 kcal mol−1 (much smaller than the ionic pair) at R(H⋯O) = 1.90 Å.

The cationic dimer has an energy well of 2.4 kcal mol−1 at R(H⋯O) = 1.88 Å�

The anionic dimer has the smallest energy well of 1.3 kcal mol−1 at R(H⋯O) = 1.97 Å.

Due to the electrostatic repulsion, well depths of both cationic and anionic dimers are smaller than that of the H2O dimer.

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Dihedral scan of ethane.

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Scan along the S_N2 reaction coordinate

The scan starts with the fluorine too far from the carbon, so the first points decrease the energy. Around point #12 is the reactant complex, and from there the energy rises to a TS at around point #19. The product complex is around point #27.
Structure #19 is the guess structure for the TS, which will be submitted to a full optimization.

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The ozone/isoozone potential energy surface

a 2D surface in a 3D diagram. The dashed line on the surface is the reaction coordinate�(intrinsic reaction coordinate, IRC). A slice through the reaction coordinate gives a 1D “surface” in a 2D diagram. The diagram is not meant to be quantitatively accurate