Requesting electron density difference analysis

Materials Studio provides the option of generating the electron density difference in two different ways:

• Electron density difference with respect to a linear combination of the atomic densities.
• Electron density difference with respect to a linear combination of the densities of sets of atoms contained in the model.

The former option produces a density difference field which shows the changes in the electron distribution that are due to formation of all the bonds in the system. This object is appropriate for bulk solids to illustrate the charge redistribution due to chemical bonding.

The latter option generates a density difference field which effectively corresponds only to the formation of bonds between atoms in different sets and to charge redistribution within the sets that is due to the presence of other sets. This property is useful to describe such processes as bonding of molecule(s) to external and internal (for example zeolites) surfaces, or creation of large molecules from smaller fragments.

Materials Studio supports the calculation of the density difference with respect to the densities of sets of atoms only for tasks that do not involve changes in atomic geometry. For Geometry Optimization and Dynamics values of the Task property, you can request this type of density difference after the run is complete by using the Properties task.

To perform an electron density difference analysis

1. Choose Modules | CASTEP | Calculation from the menu bar to open the CASTEP Calculation dialog.
2. Select the Properties tab.
3. Check the Electron density difference checkbox on the properties list.
4. Use the radio buttons to select the appropriate type of density difference to be created.
5. When density difference with respect to sets of atoms is requested, it is necessary to create at least one set of atoms in the model. There is a naming convention for sets that are taken into account: they should have "DensityDifference" as part of the set name. In order to create the sets:
   1. Choose Edit | Edit Sets from the menu bar.
   2. Make sure that the correct 3D structure document is active.
   3. Select the atoms that will constitute the first set.
   4. Click the New... button to create a new set.
   5. Give the set a name, for example DensityDifference1.
   6. Repeat the above steps to create further sets if required.

Multiple sets should be mutually exclusive.

When only one set with the correct name is present in the structure, Materials Studio assumes that the rest of the atoms should be treated as a second set. Thus, in the case of a molecule-surface interaction, it is sufficient to create a single set that comprises all the atoms in the molecule.

See Also:

CASTEP Calculation dialog