A molecular system can look perfectly isolated on screen and still interact with a copy of itself. That is the periodic-boundary-condition problem: the simulation box repeats in every direction, so a solute placed too close to its boundary may encounter its own periodic image. The resulting short-range forces can be wrong, even when the structure and preparation steps seem otherwise reasonable.
A useful way to prevent this is to treat box selection as two connected decisions: choose a shape that suits the solute, then leave enough space between the solute and its repeated copies.
First, think about the nearest image
With periodic boundary conditions, GROMACS uses the minimum image convention for short-range non-bonded interactions: it considers the nearest image of each particle. In practical terms, you want the solute to interact with its environment—not with a nearby copy of itself across a box boundary.
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. This is a starting point for box preparation, not a replacement for checking the cutoff restrictions relevant to your simulation. Long-range electrostatics also require special treatment; GROMACS supports lattice-sum methods such as Ewald, PME, and PPPM.
Match the box shape to the solute
A cubic box is familiar and straightforward, but it is not always the most economical choice. For an approximately spherical macromolecule in solvent, a rhombic dodecahedron or truncated octahedron can fit the system more efficiently while maintaining a minimum distance between periodic images.
The rhombic dodecahedron is the smallest and most regular space-filling unit cell described here. At the same image distance, its volume is about 71% of a cube’s—roughly 29% less volume to fill. That can mean fewer solvent molecules, and the documentation notes a corresponding CPU-time saving of about 29% for simulations of spherical or flexible molecules in solvent. The benefit depends on the system and setup; it is not a guarantee of the same speedup for every simulation.
GROMACS Wizard also supports cubic, orthorhombic, triclinic, and truncated-octahedron unit cells. The best choice depends on your solute’s shape and the geometry you need, rather than on a single shape being right for every model.
Choose how the box is fitted
During preparation, GROMACS Wizard offers two useful approaches:
- Box lengths: Set the box dimensions directly. The initial fit is tight, so increase the dimensions as needed to meet the minimum-image requirement. This can be useful for a batch project when you want one box size across all conformations or frames.
- Solute-box distance: Set the distance between the solute and the box. At least
1 nmis recommended as a practical way to provide space around the system. In a batch project, the box can then vary with each conformation or frame.
One quick planning check: if you choose a tight fit, remember to add room around the solute before accepting the box. If you are comparing many conformations, decide whether consistent box dimensions or a separately fitted box for each frame better suits your workflow.
Finally, box shape in a trajectory may not look exactly as expected. GROMACS keeps particles in a brick-shaped volume for efficiency; when importing results, SAMSON attempts to detect the unit-cell type, and the importer dialog lets you adjust it if needed.
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.