When setting up pulling simulations in molecular modeling, one frequently overlooked yet critical step is defining an appropriate simulation box. The size of the box directly influences the accuracy of the results, particularly in simulations with periodic boundary conditions (PBC). Preparing the box correctly ensures that chains being pulled remain free from unintended interactions due to periodicity constraints.
Why Getting the Box Dimensions Right Matters
In molecular dynamics, periodic boundary conditions replicate the system across space, so molecules at the edge of the box interact with replicas of their neighbors. To avoid artifacts, it’s crucial to ensure that the pulling distance never exceeds half the box length in any direction. Overlapping interactions between periodic images can skew results and reduce the reliability of simulations.
Setting Up the Box in SAMSON
For a pulling simulation, leaving enough space in the pulling direction is essential. In this example, we pull chain A from chain B by 5 nanometers in the z-direction over the course of the simulation. To accommodate this, the box length along the z-axis needs to be sufficient to avoid overlapping interactions.
Here are the steps to define the box:
- Choose the Orthorhombic unit cell and click Compute fitted box. This computation sets default dimensions based on the overall structure of the system.
- Switch to the Box lengths option, which allows you to define custom dimensions. Taking into account the pulling distance, set the box dimensions as
6.5 nm x 4.5 nm x 12 nm. This ensures the z-dimension is twice the pulling distance (10 nm) plus additional space (1 nm on each side). - Deactivate the Center in box option and manually set the center of the system to
0.5 x 0.5 x 0.2. These values correspond to fractions of the box size and help better align the system for the simulation.
Below, you can see the parameters used for defining the box in this tutorial:
Visualizing the Box
One of the advantages of using SAMSON is its ability to visualize the box directly in the Viewport. Once the box is configured, toggle the visualization to confirm that the system fits comfortably within the defined space. Adjustments can then be quickly made if needed.
Critical Reminders
When working with simulations where pulling is involved, always ensure the following:
- The box length in the pulling direction is sufficient to prevent interactions beyond the minimum image convention.
- System centering is appropriate for the pulling direction and aligns well with the defined box dimensions.
- You visualize the box and corresponding system in the software to confirm there are no overlaps or unexpected setups.
With these steps, you’ll set the foundation for a successful pulling simulation, minimizing artifacts and ensuring high-quality results.
For further details, including comprehensive tutorials on this process, visit the full documentation page here.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. You can download SAMSON at SAMSON Connect.