Efficiently Setting Up the Simulation Box for Pulling Simulations

Setting up a simulation box correctly is an essential but challenging step for molecular modelers, particularly when preparing for pulling simulations. A misconfigured box can lead to inaccurate results or require restarting the setup process. In this post, we’ll walk you through the step-by-step process of defining a simulation box in the SAMSON platform’s GROMACS Wizard, specifically for center-of-mass (COM) pulling experiments.

Why Does a Proper Simulation Box Matter?

The key to performing accurate molecular dynamics simulations is ensuring that the system’s configuration adheres to fundamental simulation conventions, such as the minimum image convention. When conducting a pulling simulation, inappropriate dimensions can result in overlapping molecules or failed periodic boundary conditions, skewing your data.

Steps to Define the Simulation Box

1. Account for the Pulling Distance

In a COM pulling simulation, the pulling direction and magnitude are the guiding factors in determining box dimensions. In our example, chain A is pulled away from chain B in the z-direction by 5 nm over the course of the simulation. To maintain simulation reliability, you should:

  • Set the box length in the pulling direction (z-axis) to at least twice the pulling distance, ensuring room for periodic boundary conditions.
  • Add additional allowance (e.g., 1 nm on either side) to avoid edge effects.

For this setup, the total box length in the z-direction is determined to be 12 nm.

2. Generate the Box in GROMACS Wizard

  1. Choose the Orthorhombic unit cell option and click Compute fitted box. This generates a box size tailored to your molecular system.
  2. Next, adjust the box dimensions manually using the Box lengths option. Set the box size to 6.5 nm x 4.5 nm x 12 nm.

Periodic box parameters

3. Fine-Tune the System’s Position

Centering the molecular system in the box is a common practice. However, for pulling simulations, it’s often necessary to offset the system along one axis for better control. In this case:

  • Deactivate the Center in box option.
  • Set the box center to 0.5 x 0.5 x 0.2 of the box size. This ensures that there is enough space in the pulling direction (z-direction).

You can visualize the adjusted simulation box directly in the Viewport. Below is an example of the box setup, showcasing the molecular system with appropriate spacing for the pulling simulation:

COM Pulling box in the Viewport

Tips for Troubleshooting

If your pulling direction is not aligned with the Cartesian axes, you can reorient the system easily in SAMSON:

  • Use the move editors or right-click on the structure and select Move selection > Align….
  • Choose an appropriate axis or plane for alignment.

This step ensures that the pulling vector is aligned with the z-axis for simplicity, as shown in the above example.

Conclusion

By carefully defining the box size and system position, you can ensure a successful COM pulling simulation with accurate results. This setup is foundational, and taking the time to refine it will save you significant troubleshooting down the road. To learn more about COM pulling and other simulation steps, visit the full documentation here.

SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON at https://www.samson-connect.net.

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