A simulation box can look like a setup detail, but a poor choice can create a real modeling problem: under periodic boundary conditions, your solute is surrounded by copies of itself. If the box is too small, the molecule may interact with one of those periodic images, and the resulting forces may not represent the isolated system you intended to study.
Here is a practical way to think through box shape and spacing before starting a GROMACS simulation in SAMSON’s GROMACS Wizard.
Start with the distance, not just the shape
Periodic boundary conditions tile the simulation cell through space. GROMACS uses the minimum image convention for short-range non-bonded interactions: it considers the nearest image of a particle. The important practical goal is to prevent the solute from interacting with its own periodic copy.
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, not a substitute for checking the cutoff restrictions and settings relevant to your simulation.
Choose a cell that suits the solute
GROMACS Wizard supports cubic, orthorhombic, triclinic, rhombic dodecahedron, and truncated octahedron unit cells:
For an approximately spherical macromolecule in solvent, the rhombic dodecahedron or truncated octahedron can fit the system more efficiently than a cube. These shapes are closer to spherical, so they need fewer solvent molecules to maintain a given distance between periodic images. The rhombic dodecahedron is the smallest and most regular space-filling unit cell; at the same image distance, its volume is 71% of a cube’s. The documentation notes that this can save about 29% of CPU time for a spherical or flexible molecule in solvent.
That does not mean these shapes are always the right choice. The geometry should suit your system and setup, and no cell shape removes the need to provide adequate solute-to-boundary spacing.
Pick the fitting option that matches your workflow
When preparing the system, GROMACS Wizard offers two ways to set up the initial box:
- Box lengths: Set the box dimensions directly. The fit is tight, so increase the size as needed to satisfy the minimum image convention. This option also lets you move the box around the system. For a batch, the initial size is based on all conformations or the full path, making it useful when you want one box size throughout.
- Solute-box distance: Set the distance between the solute and the box. The recommended minimum is 1 nm. For a batch, the box sizes can differ across conformations or frames.
In other words, choose box lengths when consistent dimensions across a batch matter; choose solute-box distance when each structure should be fitted with its own spacing. In either case, verify that the final geometry leaves enough room around the solute.
A quick pre-run check
- Is the selected cell shape suitable for the overall shape of the solute?
- Is there at least 1 nm between the solute and the box boundary as a practical starting point?
- Have you checked the relevant GROMACS cutoff restrictions for your simulation?
- If this is a batch, do you want one shared box size or separate fitted boxes?
One detail that can be surprising when inspecting results: GROMACS keeps particles in a brick-shaped volume for efficiency. When SAMSON loads GROMACS results, it tries to detect the unit-cell type; you can also change it in the trajectory importer dialog.
For the full explanation and related setup steps, see the GROMACS Wizard guide to periodic boundary conditions.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.