Displaying population analysis results
A CASTEP calculation can compute atomic charges, spins, and bond orders using Mulliken analysis, and atomic charges using Hirshfeld analysis.
CASTEP bond orders may be incorrect for small unit cells, when an atom is bonded to a number of periodic images of itself. For example, bond orders for the primitive cell of silicon are reported as 3.0, while the same calculation for the conventional cell gives 0.75. The factor of four appears because CASTEP sums up the bond orders for the four bonds that link the Si atom at the origin with the periodic images of itself in the primitive cell. A manual analysis of the multiplicities is required in such cases, specifically: determine the number of bonds to periodic images of the same atom and divide the bond order by that number. In cases where the bonds are not symmetry equivalent, you can only obtain an average bond order.
Displaying computed charges, spins, and bond orders
Mulliken charge analysis is one of the most common types of charge analysis. The spin density matrix and atomic overlap matrix are used to partition charges among the atoms. An alternative method of charge partitioning is based on deformation density and produces so-called Hirshfeld charges.
To import and display computed atomic charges, spins, and bond orders, you need to have the structure file (.xsd) and CASTEP
output file (.castep) in a folder within the current project.
To display atomic charges or spins
- Choose Modules | CASTEP | Analysis from the menu bar to display the CASTEP Analysis dialog.
- Select Population analysis from the list of properties at the top of the dialog.
- Make sure that the structure file from the CASTEP calculation is the active document.
- Select the type of charges (Mulliken or Hirshfeld) that you wish to apply from the dropdown list.
- Click the Assign charges to structure or Assign Mulliken spins to structure button, as appropriate, to import the charge or spin data into the structure.
- Right-click in the 3D Viewer and select Label from the shortcut menu to display the Label dialog.
- Set the Object type to Atom, select Charge or Spin, as appropriate, from the Properties list, and click the Apply button to display the charges or spins.
To display bond orders
- Choose Modules | CASTEP | Analysis from the menu bar to display the CASTEP Analysis dialog.
- Select Population analysis from the list of properties at the top of the dialog.
- Make sure that the structure file from the CASTEP calculation is the active document.
- Click the Assign Mulliken bond orders to structure button to import the bond order data into the structure.
- Right-click in the 3D Viewer and select Label from the shortcut menu to display the Label dialog.
- Set the Object type to Bond, select BondOrder from the Properties list, and click the Apply button to display the bond orders.