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Administrator<br />

Dipole Moment<br />

The dipole moment is the first derivative of the<br />

energy with respect to an applied electric field. It<br />

measures the asymmetry in the molecular charge<br />

distribution and is reported as a vector in three<br />

dimensions.<br />

The dipole value will differ when you choose<br />

Mulliken Charges, Wang-Ford Charges or<br />

Electrostatic Potential, as a different density matrix<br />

is used in each computation.<br />

Mulliken Charges<br />

This property provides a set of charges on an atom<br />

basis derived by reworking the density matrix from<br />

the SCF calculation. Unlike the Wang-Ford charges<br />

utilized in the previous example, Mulliken charges<br />

give a quick survey of charge distribution in a<br />

molecule.<br />

NOTE: For more information, see the MOPAC online<br />

manual, page 41 and 121.<br />

NOTE: For more information see the MOPAC manual,<br />

page 119.<br />

The following table contains the keywords<br />

automatically sent to MOPAC.<br />

Keyword<br />

GEO-OK<br />

MMOK<br />

Description<br />

Automatically sent to MOPAC to<br />

override checking of the Z-matrix.<br />

Automatically sent to MOPAC to<br />

specify Molecular Mechanics<br />

correction for amide bonds. Use the<br />

additional keyword NOMM to turn<br />

this keyword off.<br />

Charges<br />

The property, Charges, determines the atomic<br />

charges using a variety of techniques discussed in<br />

the following sections. In this example the charges<br />

are the electrostatic potential derived charges from<br />

Wang-Ford, because Wang-Ford charges give<br />

useful information about chemical stability<br />

(reactivity).<br />

The following table contains the keywords<br />

automatically sent to MOPAC.<br />

Keyword<br />

MULLIK<br />

GEO-OK<br />

MMOK<br />

Description<br />

Automatically sent to MOPAC to<br />

generate the Mulliken Population<br />

Analysis.<br />

Automatically sent to MOPAC to<br />

override checking of the Z-matrix.<br />

Automatically sent to MOPAC to<br />

specify Molecular Mechanics<br />

correction for amide bonds. Use<br />

the additional keyword NOMM to<br />

turn this keyword off.<br />

Charges From an Electrostatic Potential<br />

The charges derived from an electrostatic potential<br />

computation give useful information about<br />

chemical reactivity.<br />

The electrostatic potential is computed by creating<br />

an electrostatic potential grid. Chem3D reports the<br />

point charges derived from such a grid.<br />

In general, these atomic point charges give a better<br />

indication of likely sites of attack when compared to<br />

atomic charges derived from the Coulson density<br />

182•MOPAC <strong>Com</strong>putations<br />

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

<strong>Com</strong>puting Properties

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