Many properties are estimated by group contribution techniques. In these techniques, each group, or fragment of a molecular structure, is assigned a value. A chemical’s molecular structure is first dissected into a set of groups and then the contributions of all groups are combined into an estimate. Some estimation techniques collect groups into categories. UNIFAC is one such group contribution technique, collecting groups into main groups and subgroups. This section contains one field for managing the categorization of groups. The Group Categories section contains one field that enables you to assign each group to a category.

1	Groups will be used to dissect molecular structures.
2	Categories will be used to lookup contributions.

Estimation technique codes that use group categories typically contain two parts. The first part dissects a molecular structure into a set of groups.

The Groups function retrieves the array of groups used by the specified technique. Note that these groups are compiled from the Group Contributions Section not the Group Categories Section.

The Dissect function finds the occurrence of each group within a chemical's molecular structure, e.g. comps[i] in the example code above.

Once the set of dissection groups and occurences have been determined, the category of each group is then retrieved.

For example, for the group categories shown in the image at the top of this page, the GroupCategory function would return "main 46" for the group "-CO-N(CH2-)-CH2-". You would now use this group category to lookup a contribution value.

Note that the same functionality can be accomplished by simply duplicating group contribution values. For example, a technique may have the same group contribution value for all acylic alkyl groups:

-CH3

-CH2-

>CH-

>C<

These four groups could be assigned to a group category, e.g., main01, and a single value for main01 could be entered into the Group Contribution Section.

Another approach is to simply add the four groups to the Group Contribution Section all having the same group contribution value.

When practical, we recommend using duplicate group contributions. The intent is clear and it provides a very easy format for future updates, e.g., when you find more data and wish to update the contribution for the -CH2- group.

However, when your group contribution entries contain many interactions between groups, then using group categories becomes a much better approach for keeping your values organized and easily updated.

1. Open the MKS Core Knowledge Base document. Create a working copy of the document (see here) to use with this example.
2. Change to the document's Techniques chapter and navigate to a technique having group contribution values.
3. Scroll down to the Group Categories section and click the left mouse button in the field’s table. The application activates the Group Categories edit dialog. (See documentation on the Group Categories Dialog for details.)
4. Select an empty row and press the dialog’s Edit button. The application activates the Group Category edit dialog. (See documentation on the Group Categories Edit Dialog for details.)
5. Enter a group or press the List button to the right of the Group control. The application will display a list of all groups in the current technique.
6. Enter a value for the Category or press the List button to the right of the Category control to choose a category from one of the categories already entered for this technique.
7. Finally add Source Information and press the dialog's OK button. The application adds the values to the Group Categories dialog and displays the group and category in the dialog's table control.
8. Repeat the above procedure as needed to add additional groups to this or other categories.

Topic	Description
Estimating Chemical Properties	a short video demonstrating how to estimate physical properties using either Synapse or Cranium.
Estimating Mixture Properties	a short video demonstrating how to estimate physical properties using either Synapse or Cranium.
Getting Started using Cranium	provides a quick tour of Cranium's capabilities including a discussion of structure editing.
Techniques Chapter	the techniques chapter contains numerous sections for entering and displaying the code and parameter values needed to generate physical property estimates.