Procedure

This experiment focuses on constructing a unit cell of a lattice. You will work with four different lattices: Simple Cubic (SCP), Face-Centered Cubic (FCC), Body-Centered Cubic (BCC), and Hexagonal Close-Packed (HCP). The goal is to build the correct unit cell by selecting the appropriate atoms and verifying the construction.

Steps for the Experiment

• Interface Overview:
  • The interface provides a dropdown menu to switch between the four lattice types.
  • A toggle switch allows you to enter "Select Atoms" mode for choosing atoms within the unit cell.
  • The select region feature enables the selection of atoms within a specified 3D region.
  • There are also buttons for clearing selections and checking the correctness of your constructed unit cell.
• Building the Unit Cell: First, you can add atoms to the unit cell using features like
  • Add Atom at Coordinates: This allows you to place an atom at specific coordinates within the lattice.
  • Repeat Selected Pattern: You can select a pattern of atoms and repeat it once along a given vector direction.
  • Translate Pattern: This feature allows repeating the selected pattern multiple times in the specified direction (N times).
  • Move Selected Pattern: Shift a group of selected atoms in a specific direction to refine the unit cell construction.
• Using Dummy Atoms for Verification:
  • Use the Add Dummy Atom at Coordinate feature to insert a temporary atom at a specific location. This helps you visually check the validity of your unit cell before finalizing it.
  • Note that dummy atoms are not considered when you use the Check Packing button to verify the correctness of your unit cell.
• Checking the Unit Cell:
  • Once you’ve constructed the unit cell, select the atoms that make up the unit cell, choose the corresponding lattice type from the dropdown, and then click the Check Packing button.
  • This will evaluate the construction and display whether the unit cell is correct.
• Selecting Atoms in Complex Lattices:
  • Some lattices, particularly those with atoms located in difficult-to-select positions (e.g., inner atoms in FCC or BCC lattices), may require you to use the Select Region feature.
  • Select Region allows you to select all atoms inside a defined 3D region enclosed by selected atoms. It also selects atoms whose centers are located on the surface of the polyhedron formed by these selected atoms.
• Handling Irrational Coordinates:
  • In some cases, you may observe that the atom coordinates are irrational (e.g., in FCC or HCP lattices). In these cases, you are expected to input coordinates with at least 3 digits of precision to ensure accurate evaluation of the unit cell.

Simulation Tips

Pay close attention to the atom placements and the relationships between them in the context of the unit cell’s geometry. Accurate placement is essential for building a correct unit cell.

Note

Some lattices are presented with an atom radius smaller than the mathematically calculated value to make the lattice points more visible and easier to work with during the experiment.