The hidden cost of a too-tight simulation box

A molecular system can look perfectly prepared on screen and still have a box-sizing problem: under periodic boundary conditions, the solute may interact with an image of itself. That can distort the forces in your simulation. At the same time, making the box unnecessarily large means filling and simulating more solvent than needed.

The goal is not simply “make the box bigger.” It is to choose a box shape and size that avoid unwanted interactions between periodic images without wasting space. Here’s a practical way to think through that choice when preparing a system in GROMACS Wizard.

First, understand what repeats

Periodic boundary conditions treat the simulation box as part of a repeating lattice: copies of the box surround the one you are viewing. For short-range non-bonded interactions, GROMACS uses the minimum-image convention, considering the nearest image of each particle. If the solute is too close to a boundary, it may be too close to 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 practical starting point, not a substitute for checking the cutoff restrictions relevant to your simulation. The GROMACS manual explains those restrictions in more detail.

Choose a shape that fits the problem

A cubic box is easy to picture, but it is not always the most space-efficient choice. GROMACS Wizard also supports 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 shape more closely than a cube. That means less solvent is needed for a given minimum distance between macromolecular 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. A smaller solvent-filled volume can reduce simulation cost, though the best choice depends on the system and setup.

Rhombic dodecahedron unit cell
A rhombic dodecahedron is one option for fitting an approximately spherical solute efficiently.

Truncated octahedron unit cell
The truncated octahedron is another space-filling option available in GROMACS Wizard.

Pick the fitting method that matches your workflow

  • Box lengths: The box is fitted tightly to the system, so you can set its dimensions and position. Then increase its size as needed to satisfy the minimum-image requirement. For a batch project, the initial size is based on all conformations or the whole path, which is useful when you want one box size throughout.
  • Solute-box distance: Set the distance from the solute to the box instead of specifying the box dimensions directly. At least 1 nm is recommended for the minimum-image convention. In a batch project, each conformation or path frame can have a different box size.

Choosing a unit cell during system preparation
Choose the unit-cell shape and its size while preparing the system.

A quick pre-run check

  1. Consider the solute’s overall shape: is it approximately spherical, or does another box geometry suit it better?
  2. Choose whether you need consistent box dimensions across a batch or a consistent solute-to-box distance.
  3. Check that the solute has adequate clearance from the boundary. Use the recommended 1 nm distance as a starting point, and verify the relevant GROMACS cutoff restrictions.

One detail can be surprising when inspecting results: GROMACS keeps particles in a brick-shaped volume for efficiency. When SAMSON loads GROMACS results, it attempts to detect the unit-cell type, and the importer dialog lets you modify it if needed.

For the full explanation and links to related preparation tutorials, see GROMACS Wizard: Periodic boundary conditions.

Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.

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