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ChemOffice.Com - CambridgeSoft

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The Output window displays intermediate<br />

messages about the status of the minimization. A<br />

message appears if the minimization terminates<br />

abnormally, usually due to a poor starting<br />

conformation.<br />

The following contains keywords automatically<br />

sent to MOPAC and some additional keywords you<br />

can use to affect convergence.<br />

Keyword<br />

Description<br />

Keyword<br />

LET<br />

RECALC=5<br />

Description<br />

Overrides safety checks to<br />

make the job run faster (or<br />

further).<br />

Use this keyword if the<br />

optimization has trouble<br />

converging to a transition state.<br />

EF<br />

GEO-OK<br />

MMOK<br />

RMAX=n.nn<br />

RMIN=n.nn<br />

PRECISE<br />

Automatically sent to MOPAC<br />

to specify the use of the<br />

Eigenvector Following<br />

minimizer.<br />

Automatically sent to MOPAC<br />

to override checking of the<br />

Z-matrix.<br />

Automatically sent to MOPAC<br />

to specify Molecular Mechanics<br />

correction for amide bonds.<br />

Use the additional keyword<br />

NOMM to turn this keyword<br />

off.<br />

The calculated/predicted<br />

energy change must be less<br />

than n.nn. The default is 4.0.<br />

the calculated/predicted energy<br />

change must be more than n.n.<br />

The default value is 0.000.<br />

Runs the SCF calculations<br />

using a higher precision so that<br />

values do not fluctuate from<br />

run to run.<br />

For descriptions of error messages reported by<br />

MOPAC see Chapter 11, pages 325–331, in the<br />

MOPAC manual.<br />

To interrupt a minimization that is in progress:<br />

• Click Stop in the Movie Controller.<br />

Example:<br />

Locating the Eclipsed<br />

Transition State of Ethane<br />

Build a model of ethane:<br />

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

2. Double-click in the model window.<br />

A text box appears.<br />

3. Type CH3CH3 and press the Enter key.<br />

A model of ethane appears.<br />

4. Select the Rotation tool.<br />

5. Click the arrow next to the Rotation tool, and<br />

drag down the Rotation dial.<br />

click here to open the<br />

Rotation dial<br />

dihedral rotators<br />

<strong>ChemOffice</strong> 2005/Chem3D MOPAC <strong>Com</strong>putations • 179<br />

Optimize to Transition State

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