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Administrator<br />

modes and color settings are easy to control at a<br />

fine-grained level. Properties of atoms and bonds<br />

are easy to access and change.<br />

The Model Explorer is designed as a hierarchical<br />

tree control that can be expanded/collapsed as<br />

necessary to view whatever part of the model you<br />

wish. Changes are applied in a bottom-up manner,<br />

so that changes to atoms and bonds override<br />

changes at the chain or fragment level. You can<br />

show/hide/highlight features at any level. Hidden<br />

or changed features are marked in the hierarchical<br />

tree with colored icons, so you can easily keep track<br />

of your edits. See “Working With the Model<br />

Explorer” on page 107 for more information.<br />

Model Coordinates<br />

Each of the atoms in your model occupies a<br />

position in space. In most modeling applications,<br />

there are two ways of representing the position of<br />

each atom: internal coordinates and Cartesian<br />

coordinates. Chem3D establishes internal and<br />

Cartesian coordinates as you build a model.<br />

Z-matrix<br />

Internal coordinates for a model are often referred<br />

to as a Z-matrix (although not strictly correct), and<br />

are the most commonly used coordinates for<br />

preparing a model for further computation.<br />

Changing a Z-matrix allows you to enter relations<br />

between atoms by specifying angles and lengths.<br />

You display the Z-Matrix table by selecting it from<br />

the View menu. You can edit the values within the<br />

table, or move atoms within the model and use the<br />

Set Z-matrix submenu of the Structure menu. You<br />

can copy and paste tables to text (.txt) files or Excel<br />

spreadsheets using the commands in the context<br />

(right-click) menu.<br />

Below is an example of the internal coordinates<br />

(Z-matrix) for ethane:<br />

Cartesian Coordinates<br />

Cartesian coordinates are also commonly accepted<br />

as input to other computation packages. They<br />

describe atomic position in terms of X-, Y-, and<br />

Z-coordinates relative to an arbitrary origin. Often,<br />

the origin corresponds to the first atom drawn.<br />

However, you can set the origin using commands in<br />

the Model Position submenu of the Structure menu.<br />

Instead of editing the coordinates directly in this<br />

table, you can save the model using the Cartesian<br />

Coordinates file format (.cc1 or .cc2), and then edit<br />

that file with a text editor. You can also copy and<br />

paste the table into a text file or Excel worksheet<br />

using the commands in the context (right-click)<br />

menu.<br />

NOTE: If you do edit coordinates in the table, remember to<br />

turn off Rectify and Apply Standard Measurements in<br />

the Model Build panel of the Model Settings dialog while<br />

you edit so that other atoms are not affected.<br />

An example of the Cartesian coordinates for ethane<br />

is shown below.<br />

24 •Chem3D Basics <strong>CambridgeSoft</strong><br />

Model Building Basics

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