A molecular group can change its overall shape even when a single size measurement barely moves. That is a common source of confusion when reviewing a path or trajectory: the radius of gyration is useful, but by itself it can be too coarse to describe how spherical—or non-spherical—the group is.
Asphericity offers another view. In SAMSON’s Path Analyzer, it reports how far a selected group deviates from a spherical shape along a path. Rather than replacing size measurements, it helps answer a different question: is the group’s shape becoming more or less anisotropic as the path progresses?
Why look beyond one number?
Imagine comparing two regions of a trajectory using only a measure of overall size. Similar size values do not, on their own, tell you whether the selected group has a more elongated or otherwise less spherical shape. Asphericity adds a shape-focused descriptor to the analysis, making it easier to notice changes that a size-only view may not highlight.
This is particularly useful when you are trying to understand a structural transition, compare sections of a path, or identify whether a group’s global shape varies during the sampled motion. The descriptor does not explain the molecular cause of a change, but it can help point you to where a closer structural inspection may be useful.
Build a focused analysis
- Open Path Analyzer.
- Select Asphericity as the observable.
- Choose the path you want to inspect.
- Define an atom-containing group to analyze.
- Add either a time series or a histogram.
The group you select matters: the result describes that selection, not automatically the entire molecular system. For a useful comparison, keep the group definition consistent when examining different paths or regions. If you change what is included, a change in the plotted descriptor may reflect the changed selection as well as the molecular motion.
Choose the view that matches your question
A time series shows how the descriptor varies along the path. Use it when you want to locate when shape changes occur and relate those regions to frames or structural events you are investigating. A histogram summarizes the range of sampled values. It is useful for seeing how values are distributed overall, but it does not preserve their order along the path. If timing matters, inspect the time series as well.
Use complementary measurements
Asphericity is most informative as part of a small set of observations. Pair it with radius of gyration to consider shape alongside overall size, and with the shape parameter for an additional view of global shape changes. If one plot changes while another appears comparatively steady, that contrast can help refine the question you ask of the structures. Treat the plots as complementary descriptors, then inspect the relevant structures to understand what is changing.
A practical takeaway: when a size trace seems too simple to capture what you see in the structures, add a shape descriptor. Asphericity gives you a compact way to follow deviation from spherical shape, while the time-series and histogram views help you examine that change from different angles.
For details on inputs and plotting options, see the Asphericity documentation.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON at samson-connect.net.