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When the minimization is complete, reorient
the model so it appears as follows.
The conformation you converged to is not the
well-known chair conformation, which is the global
minimum. Instead, the model has converged on a
local minimum, the twisted-boat conformation.
This is the closest low-energy conformation to your
starting conformation.
Had you built this structure using substructures that
are already energy minimized, or the ChemDraw
panel, you would be close to the chair
conformation. The minimizer does not surmount
the saddle point to locate the global minimum, and
the closest minimum is sought.
The energy values in the Output window should be
approximately as follows:
For cyclohexane, there are six equivalent local
minima (twisted-boat), two equivalent global
minima (chair), and many transition states (one of
which is the boat conformation).
Locating the Global Minimum
Finding the global minimum is extremely
challenging for all but the most simple molecules. It
requires a starting conformation which is already in
the valley of the global minimum, not in a local
minimum valley. The case of cyclohexane is
straightforward because you already know that the
global minimum is either of the two possible chair
conformations. To obtain the new starting
conformation, change the dihedrals of the twisted
conformation so that they represent the potential
energy valley of the chair conformation.
The most precise way to alter a dihedral angle is to
change its Actual value in the Measurements table
when dihedral angles are displayed. An easier way to
alter an angle, especially when dealing with a ring, is
to move the atoms by dragging and then cleaning
up the resulting conformation.
To change a dihedral angle:
• Drag C1 below the plane of the ring, then drag
C4 above the plane of the ring.
The major contributions are from the 1,4 VDW and
Torsional aspects of the model.
ChemOffice 2005/Chem3D MM2 and MM3 Computations • 153
Minimize Energy