Properly setting up periodic boundary conditions and selecting the right unit cell are key to conducting efficient molecular simulations. For molecular modelers using GROMACS, understanding how to choose the most appropriate space-filling unit cell not only improves simulation accuracy but can also save computational resources. Here, we’ll explore the available unit cell shapes in SAMSON’s GROMACS Wizard and when to use them.
What Are Unit Cells and Why Should You Care?
When simulating molecules in periodic systems, unit cells are fundamental. They are essentially space-filling shapes that repeat in all directions under the periodic boundary condition. Their choice impacts how efficiently your system can be simulated. This is particularly important when modeling macromolecules like proteins in solution, where solvent molecules fill the voids.
The GROMACS Wizard supports five shapes for unit cells:
| Unit cell shape | Representation |
|---|---|
| Cubic |  |
| Orthorhombic |  |
| Triclinic |  |
| Rhombic dodecahedron |  |
| Truncated octahedron |  |
Which Unit Cell Should You Use?
Each unit cell shape has unique characteristics. For example, the cubic unit cell is simple and widely used, but it is less efficient for spherical macromolecules in solution because it requires more solvent molecules to fill the box.
If your simulation includes approximately spherical macromolecules, the rhombic dodecahedron and truncated octahedron are better choices. Both are closer to spherical shapes than a cube is, meaning fewer solvent molecules are required for the same minimum distance between macromolecular images. The rhombic dodecahedron, in particular, is the smallest and most regular space-filling unit cell, with a volume that is only 71% of a cube’s volume given the same image distance. This can save up to 29% of computational resources, reducing CPU time significantly for spherical or flexible molecules in solvent simulations.
Two Ways to Fit the Simulation Box
With the GROMACS Wizard, you can also control how to fit the system into a simulation box:
- Box lengths: Specify the dimensions of the box, which will fit tightly around your system. You may need to increase the size later to meet the minimum image convention. If you’re working with multiple conformations in a batch, this option ensures consistency across the batch, applying the same box size everywhere.
- Solute-box distance: Define the distance between the system and the box walls. At least 1 nm is recommended to avoid interactions between a solute and its periodic image. This is particularly important when the molecular system undergoes conformations. In batch simulations, box sizes are adjusted for each conformation individually.
The Takeaway
In molecular simulations, the shape and size of your unit cell can have a significant impact on computational efficiency and accuracy. Using advanced tools like SAMSON’s GROMACS Wizard, you can customize your settings to optimize simulations and dive deeper into molecular dynamics with ease. For more in-depth guidance on periodic boundary conditions and unit cell selection, head over to the official documentation.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. You can get SAMSON at SAMSON Connect.