A molecular model can look convincing while its motion tells a different story. A gripper may close around a target in an animation, yet fail to grasp it once the system is simulated. One possible reason is that the imposed movement is too fast for the interaction to play out as intended.
This is a useful distinction for molecular modelers: an animation can define how atoms are positioned, while a simulation calculates how the system evolves from those positions. Getting the order and timing right helps make the simulated result easier to interpret.
Why animation order matters
In SAMSON, the Simulate animation performs multiple simulation steps at each frame. The Animator executes animations from top to bottom. That means the position-generating animation should come first, followed by Simulate, so the simulation starts from the positions created by the preceding animation.
To add it, double-click Simulate in the Animation panel of the Animator. Its keyframe is placed at the current frame; move it if needed. You can also reposition animation keyframes later, so you do not have to get the timeline perfect on the first try.
A practical way to think about the sequence is:
- Set up the motion. Place the animation that generates the starting positions before the simulation.
- Run the simulation. Put Simulate after that animation in the Animator.
- Review the resulting motion. If you want to save the trajectory, use Record path.
Timing can change the outcome
The documentation’s gripper example illustrates why speed is worth checking. The actuated part of a nano gripper moves down by 1.7 nm over 2.5 ps. Dividing distance by time gives an average speed of 0.68 nm/ps, or 680 m/s. In the example, the gripper moves too fast and fails to grasp a cylinder. The point is not that one speed is always correct for every model; it is that the imposed motion can affect what the simulation shows.
When a grasp or other interaction fails, check the motion as well as the structure. Ask: how far does the moving part travel, over how much time, and what happens to the target while it moves? Comparing the displacement and duration gives you a concrete way to describe the actuation rather than relying only on how fast it looks in the viewport. If the motion is too abrupt for the behavior you want to study, adjust the motion and compare the resulting trajectories.
Use simulation settings deliberately
The number of simulation steps per frame and the step size for the simulator’s state updater can be changed in the Inspector of the Simulate animation. These settings determine how the simulation is advanced as the animation plays. Change them intentionally, and keep track of the settings used when comparing runs; otherwise, it can be difficult to tell whether a different result came from the imposed motion or from the simulation setup.
A useful troubleshooting habit is to change one part of the setup at a time. First confirm that Simulate comes after the animation that establishes the positions. Then examine the imposed motion and its duration. Finally, review the simulation’s steps-per-frame and step-size settings. Saving a trajectory with Record path gives you a record to inspect or compare. SAMSON also provides Play path and Play reverse path animations for working with recorded paths.
The broader lesson is simple: a failed interaction is not always a sign that the molecular design alone needs changing. The order of animations and the timing of an imposed movement are also part of the experiment. Making those choices explicit can help you understand what a simulation is actually showing.
For setup details, see the Simulate animation documentation.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.