A simulation box can look like a setup detail: choose a shape, add solvent, and move on. But if the box is too small, a molecule may interact with its own periodic image. That can make the forces in your simulation unreliable—and leave you troubleshooting results that were compromised before the run began.
Here’s a practical way to think about box shape and size when preparing a system in GROMACS Wizard: first decide how much empty space your solute needs, then choose a cell shape that fits the solute without adding unnecessary solvent.
Why periodic images matter
Periodic boundary conditions make a finite simulation box behave like a repeating array of boxes. When a particle approaches one side, its periodic counterpart appears on the opposite side. For short-range non-bonded interactions, GROMACS uses the minimum image convention: it considers the nearest image of each particle.
The practical consequence is easy to miss: your solute must not interact with its own periodic image. A useful rule of thumb is to leave at least 1.0 nm between the solute and the box boundary. That gives at least 2.0 nm between periodic images of the solute. Treat this as a starting point, and check the cutoff restrictions relevant to your simulation settings in the GROMACS Manual.
Choose a shape that matches the problem
GROMACS Wizard supports cubic, orthorhombic, triclinic, rhombic dodecahedron, and truncated octahedron unit cells. The simplest shape is not always the most economical one. For an approximately spherical macromolecule in solution, a rhombic dodecahedron or truncated octahedron can fit the solute more efficiently than a cube while maintaining a minimum distance between periodic images.
The rhombic dodecahedron is the smallest and most regular space-filling unit cell described in the tutorial. At the same image distance, its volume is about 71% of a cube’s—approximately 29% less volume to fill. For a spherical or flexible molecule in solvent, that can mean fewer solvent molecules and less simulation work. The savings depend on the system and setup; the important point is that box shape can affect how much solvent you need.
A cube remains a straightforward choice, and other shapes may suit systems that are not approximately spherical. The best choice is the one that accommodates your system and required image separation—not simply the shape with the most familiar name.
Two ways to size the box
During preparation, GROMACS Wizard offers two useful ways to define the initial fit:
- Box lengths: Set the box dimensions directly. The initial fit is tight, so increase the size as needed to maintain enough distance from periodic images. This option also lets you move the box to position the system. For a batch project, it can use a size based on all conformations or the whole path, which is useful when you need one box size throughout.
- Solute-box distance: Set the distance between the solute and the box. The tutorial recommends at least
1 nmas a practical way to help satisfy the minimum image convention. In a batch project, box sizes can differ between conformations or frames.
A quick pre-run check
Before starting a simulation, ask: Is the selected shape sensible for the solute? Is there enough space between the solute and the boundary? If this is a batch, should every conformation share one box size, or should each have its own? Answering these questions during preparation is easier than discovering later that a tight box affected the setup.
One detail about viewing results: GROMACS keeps particles in a brick-shaped volume for efficiency. When SAMSON loads GROMACS results, it tries to detect the unit-cell type, and you can adjust it in the trajectory importer if needed.
For the full explanation and related preparation guidance, visit GROMACS Wizard: Periodic boundary conditions.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.