A simulation box can look comfortably large and still create a problem: periodic boundary conditions make copies of the system repeat in every direction. If the solute gets too close to one of those copies, it may interact with its own periodic image. The resulting forces can be wrong, even though the molecule itself looks perfectly reasonable in the viewer.
Choosing a box is therefore a balance: leave enough space to avoid unwanted image interactions, but do not fill unnecessary volume with solvent. Here is a practical way to think through that choice when preparing a system in GROMACS Wizard.
First, picture the repeating system
Periodic boundary conditions tile the unit cell through space. Under the minimum image convention, only the nearest image of each particle is considered for short-range non-bonded interactions. Long-range electrostatics need additional methods, such as PME, but they do not remove the need to choose sensible box dimensions.
The key question is not simply, “Is the box bigger than my molecule?” It is, “Can the solute interact with a periodic copy of itself?” A practical 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 useful starting point, and check that your setup also satisfies the cut-off restrictions for the simulation.
Match the box shape to the solute
A cubic box is familiar and straightforward, but it is not always the most economical shape. For an approximately spherical macromolecule in solution, a rhombic dodecahedron or truncated octahedron can fit the solute more closely while preserving the required image distance. Less box volume means fewer solvent molecules to simulate.
The rhombic dodecahedron is especially compact: the documentation notes that its volume is about 71% of a cube with the same image distance. For a spherical or flexible molecule in solvent, that can reduce the amount of solvent—and associated simulation work—compared with using that cube. The best shape still depends on the system; a compact cell is useful only if it leaves adequate clearance around the solute.
GROMACS Wizard also supports cubic, orthorhombic, triclinic, and truncated-octahedron unit cells. The available shapes are illustrated below. Consider the geometry of your solute and the space it needs in each direction rather than choosing a shape by habit.
Choose how to size the box
- Box lengths: Set the box dimensions directly. The initial fit is tight, so increase the dimensions as needed to maintain enough separation from periodic images. This option also lets you move the box to position the system. For a batch project, it can use dimensions based on all conformations or the whole path, keeping the box size consistent across the batch.
- Solute-box distance: Set the distance between the solute and the box. At least 1 nm is recommended as a practical way to help satisfy the minimum image convention. In a batch project, each conformation or path frame can have a different box size.
A useful preparation check is to identify the solute’s outermost extent, decide how much clearance it needs, and then inspect the resulting box—not just the displayed molecule. For a batch, also consider whether comparable box dimensions across frames matter for your workflow; the two fitting options handle batches differently.
One detail when viewing results
GROMACS keeps particles in a brick-shaped volume for computational efficiency. When you load GROMACS results, SAMSON attempts to detect the unit-cell type, and the importer dialog lets you modify it if needed. So the shape you see in a trajectory view may not be a simple visual cue for the original cell geometry.
Before starting a run, use the minimum-image rule as a practical sanity check, choose a cell shape suited to the solute, and verify that the relevant cut-off restrictions are met. For details on the box options in context, see Step 1: Prepare. For the full explanation of unit cells and periodic boundaries, read the GROMACS Wizard periodic boundary conditions tutorial.
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