A simulated mechanism can fail for a reason that is easy to overlook: the parts may be moving on a timescale that leaves too little opportunity for the interaction you want to observe. When a nanoscale gripper closes too quickly, for example, it may pass or push away its target instead of grasping it. Before changing the structure, it can be useful to inspect how the motion and the simulation are being scheduled.
In SAMSON, the Simulate animation performs multiple simulation steps at each animation frame. It can be combined with animations that control atom positions, allowing you to run a simulation while parts of a system are being moved. This is useful for testing a guided or constrained motion, but the order of those animations matters.
Start with the motion timescale
In one documented nano-gripper example, the actuated blue part moves 1.7 nm in 2.5 ps. That corresponds to an average speed of about 680 m/s. In that example, the gripper fails to grasp the cylinder. The calculation is simple:
speed = displacement / time
Estimating a motion’s speed does not, by itself, prove why an interaction failed. It does give you a useful diagnostic. If a component is being driven rapidly, test whether the interaction changes when its motion is paced differently. Keep track of what you changed so you can distinguish the effect of the motion schedule from changes to the molecular model or simulation settings.
Put the simulation after the starting-position animations
The Animator executes animations from top to bottom. Therefore, place Simulate after the animations that generate the positions from which the simulation should begin. If the order is wrong, the simulation may not be acting on the intended starting configuration at that point in the animation.
To add it, double-click Simulate in the Animation panel. Its keyframe is placed at the current frame; move the keyframe if the simulation should start elsewhere. Keyframes can be moved later, so you can adjust the sequence as you refine the setup.
Separate motion control from simulation stepping
The Inspector for the Simulate animation lets you change the number of steps per frame and the step size for the simulator’s state updater. These settings control how the simulation advances within each animation frame. They are distinct from the animations that control atom positions: changing simulation steps is not the same as changing how quickly a driven component moves.
A practical troubleshooting pass is to check the animation order, confirm where the simulation keyframe begins, estimate the driven part’s displacement over the relevant time, and then vary one setting or motion schedule at a time. This makes it easier to understand whether a failed grasp is associated with the imposed motion or with another part of the setup. Avoid assuming that a visually smoother animation necessarily means the underlying simulation has changed in the same way; inspect the simulation settings and resulting trajectory.
Keep the trajectory for review
If you want to examine what happened, the Record path animation can save the trajectory generated by the simulation. The resulting path can also be used with Play path or Play reverse path animations. Reviewing a saved trajectory can help you see when the target and gripper diverge, rather than relying only on the final frame.
The central lesson is modest but useful: when a nanoscale mechanism misses its target, check both the sequence of animations and the timescale of the imposed motion before concluding that the design itself is at fault.
Learn more about the Simulate animation in the SAMSON documentation.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.