Molecular modelers focusing on pulling simulations encounter a frequent challenge: ensuring adequate box dimensions to avoid artifacts and maintain accuracy during center-of-mass (COM) pulling. Setting inappropriate box dimensions often results in simulation artifacts due to the periodic boundary conditions. This post explores how to properly define the simulation box for a COM pulling workflow using the GROMACS Wizard in SAMSON.
Why Box Dimensions Matter in COM Pulling
In molecular dynamics simulations, particularly for COM pulling, it is essential to define a simulation box that accounts for the pull’s direction and ensures compatibility with the minimum image convention. If elements involved in pulling cross the periodic boundaries improperly, the simulation could lead to incorrect results. Proper preparation mitigates these risks, and the GROMACS Wizard offers tools to set up the box correctly.
Step-by-Step Guide to Defining the Box for COM Pulling
The guiding principle when defining a box for pulling is leaving sufficient space in the pulling direction, considering the total pull distance and some additional buffer space for stability. Here’s a step-by-step approach as demonstrated in the GROMACS Wizard tutorial:
1. Compute a Fitted Box
After loading the molecular system (e.g., 2BEG), choose the Orthorhombic unit cell and let the GROMACS Wizard compute a fitted box for the system. Once the initial dimensions are set, you can adjust the box to match the pulling parameters.
2. Account for Pull Distance
If the pulling process involves a displacement of, say, 5 nm in the z-direction, the box should be at least twice the pull distance along the z-axis. To follow the minimum image convention, an additional 1 nm buffer space on either side should be included. For the 2BEG system, the z-axis length is calculated as:
5 nm (pull distance) × 2 + 1 nm × 2 (buffer spaces) = 12 nm.
3. Customize the Box Dimensions
Manually set the box size to 6.5 nm × 4.5 nm × 12 nm to accommodate the pulling direction while keeping realistic lateral dimensions. This ensures stability and prevents undesired interactions across periodic boundaries.
4. Center the System
Deactivate the Center in box option and set the center of mass position to 0.5 × 0.5 × 0.2 of the box size. This asymmetric placement leaves sufficient space along the z-direction for the pull to occur without interference.
Visualizing the Setup
Once the box parameters are applied, visualize the simulation box directly in SAMSON’s Viewport. Ensure the box’s alignment matches the pulling direction. Below is an example of the 2BEG system with the adjusted box:
Tips for Success
If your pulling direction isn’t aligned with standard axes, reorient the structure in SAMSON using either the move editors or the context menu (Move selection > Align…). Choose the appropriate axis or plane of alignment to simplify your box setup.
Conclusion
Correct definition of box dimensions in the GROMACS Wizard is an integral part of ensuring the success of your COM pulling simulations. By carefully calculating and visualizing the box setup, you can avoid issues caused by periodic boundary conditions and proceed confidently with your molecular dynamics experiments. To explore this in greater detail, refer to the full documentation at this link.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Download SAMSON at SAMSON Connect.