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Administrator<br />

staggered form. The majority of the energy<br />

contribution is from the torsional energy and the<br />

1,4 VDW interactions.<br />

NOTE: The values of the energy terms shown here are<br />

approximate and can vary slightly based on the type of<br />

processor used to calculate them.<br />

<strong>Com</strong>paring Two Stable<br />

Conformations of<br />

Cyclohexane<br />

In the following example you compare the<br />

cyclohexane twist-boat conformation and the chair<br />

global minimum.<br />

To build a model of cyclohexane:<br />

1. From the File menu, choose New.<br />

An empty model window appears.<br />

2. Select the Text Building tool.<br />

3. Click in the model window.<br />

A text box appears.<br />

4. Type CH2(CH2)5 and press the Enter key.<br />

The dihedral angle in the Actual column becomes 0,<br />

corresponding to the imposed constraint.<br />

The difference in energy between the global<br />

minimum (Total, previous calculation) and the<br />

transition state (Total, this calculation) is 2.73<br />

kcal/mole, which is in agreement with literature<br />

values.<br />

To further illustrate points about minimization:<br />

• Delete the value from the Optimal column for<br />

the dihedral angle and click the MM2 icon on<br />

the Calculation toolbar.<br />

After the minimization is complete, you are still at<br />

0 degrees. This is an important consideration for<br />

working with the MM2 minimizer. It uses first<br />

derivatives of energy to determine the next logical<br />

move to lower the energy. However, for saddle<br />

points (transition states), the region is fairly flat and<br />

the minimizer is satisfied that a minimum is<br />

reached. If you suspect your starting point is not a<br />

minimum, try setting the dihedral angle off by<br />

about 2 degrees and minimize again.<br />

CAUTION<br />

While there are other, perhaps easier, methods of creating<br />

a cyclohexane model, you should use the method described<br />

to follow this example.<br />

Before minimizing, it is wise to use the Clean Up<br />

Structure command to refine the model. This<br />

generally improves the ability of the Minimize<br />

Energy command to reach a minimum point.<br />

1. From the Edit menu, choose Select All.<br />

2. From the Structure menu, choose Clean Up.<br />

NOTE: The Clean Up command is very similar to the<br />

minimize energy command in that it is a preset, short<br />

minimization of the structure.<br />

To perform the minimization:<br />

• From the MM2 submenu of the Calculations<br />

menu, choose Minimize Energy, and click<br />

Run.<br />

152•MM2 and MM3 <strong>Com</strong>putations<br />

<strong>CambridgeSoft</strong><br />

Minimize Energy

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