A simulation box can look like a simple setup choice—until it causes two common headaches: using more solvent than necessary, or allowing a molecule to interact with its own periodic image. The right unit-cell shape and enough space around the solute help address both.
Here’s a practical way to think through the choice when preparing a system in GROMACS Wizard.
Why box shape matters
Periodic boundary conditions make a finite simulation box behave like a repeating arrangement of copies. For short-range non-bonded interactions, GROMACS uses the minimum image convention: it considers the nearest image of each particle. If the box is too tight, a solute may interact with a periodic copy of itself, making the modeled environment different from the one you intended.
The usual cube is not the only option. GROMACS Wizard supports cubic, orthorhombic, triclinic, rhombic dodecahedron, and truncated octahedron unit cells:
For an approximately spherical macromolecule in solution, the rhombic dodecahedron and truncated octahedron can fit the solute more efficiently than a cube. The rhombic dodecahedron is the smallest and most regular space-filling option described here. At the same image distance, its volume is about 71% of a cube’s—so it can require fewer solvent molecules. The documentation notes that this can save about 29% of CPU time for spherical or flexible molecules in solvent.
That does not mean a non-cubic cell is always the right choice. The useful question is whether its shape suits your solute and whether it leaves enough room around it.
A practical spacing rule
As a rule of thumb, 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. The purpose is to reduce the risk of the solute interacting with its own copy under periodic boundary conditions.
Think of this as a starting point for box setup, not a substitute for checking the interaction cutoffs and restrictions relevant to your simulation. For those details, consult the GROMACS cut-off restrictions.
Choose how to fit the box
- Box lengths: Set the box dimensions directly. The initial fit is tight, so increase the size as needed to leave adequate distance from periodic images. This can be useful for a batch project when you want one box size across all conformations or the whole path.
- Solute-box distance: Set the distance between the solute and the box. The documentation recommends at least 1 nm. In a batch project, this choice gives each conformation or path frame its own box size.
A quick decision sequence is: identify the solute’s overall shape; select a cell that fits it sensibly; choose whether consistent box dimensions or a consistent solute-to-box distance matters more for your batch; then check that the resulting spacing is adequate.
One detail when viewing results
GROMACS keeps particles in a brick-shaped volume for computational efficiency. When you load trajectories, SAMSON tries to detect the unit-cell type, and you can change it in the importer dialog if needed. So a brick-shaped representation in loaded results does not necessarily mean that the original simulation used a cubic unit cell.
For the full explanation and setup context, see the GROMACS Wizard guide to periodic boundary conditions.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.