A method to obtain a near-minimal-volume molecular simulation of a macromolecule, using periodic boundary conditions and rotational constraints.

If the rotational motion of a single macromolecule is constrained during a molecular dynamics simulation with periodic boundary conditions it is possible to perform such simulations in a computational box with a minimal amount of solvent. In this article we describe a method to construct such a box, and test the approach on a number of macromolecules, randomly chosen from the protein databank. The essence of the method is that the molecule is first dilated with a layer of at least half the cut-off radius. For the enlarged molecule a near-densest lattice packing is calculated. From this packing the simulation box is derived. On average, the volume of the resulting box proves to be about 50% of the volume of standard boxes. In test simulations this yields on average a factor of about two in simulation speed.