A molecular structure can become more extended, less extended, or change its overall shape in ways that are not easy to summarize with a single measure. This creates a familiar analysis problem: a trajectory may show a change in compactness, but that alone does not tell you whether the global shape is also changing.
The Shape parameter in SAMSON’s Path Analyzer is a compact descriptor of global shape. It complements radius of gyration and asphericity, helping distinguish different anisotropic shape regimes along a path or trajectory. It is most useful when the question is not just “How compact is the structure?” but also “How does its overall shape vary?”
From a path to a shape profile
To inspect shape changes, open Path Analyzer, choose Shape parameter under Observable, select the Path, and define the Group to analyze. The group must be an atom-containing selection. Then choose Add Time Series to follow the descriptor along the path, or Add Histogram to inspect which shape regimes are preferred across the data.
These two views answer different questions. A time series keeps the order of the path visible: it can help you locate where the descriptor changes or stays relatively consistent. A histogram summarizes the distribution, making it useful for seeing which values or regimes occur frequently. Because the histogram does not show when a value occurred, use the time series when sequence and progression matter.
Make the comparison meaningful
The group selection is part of the analysis, not a minor setup detail. Before comparing paths or interpreting a change, check that you are analyzing the intended atoms. If the group changes between analyses, the resulting descriptors may not be describing the same part of the molecular system. Keep the selection consistent when you want a like-for-like comparison.
It also helps to decide what you want to learn before choosing a view. Are you looking for a change at a particular point along a path? Start with the time series. Are you interested in the range of shape regimes represented overall? Look at the histogram. When a pattern catches your attention, return to the path and inspect the corresponding structures rather than treating a descriptor as a complete explanation by itself.
Read shape alongside compactness
Shape parameter is intended to complement, not replace, other structural descriptors. Radius of gyration and asphericity provide related perspectives, so considering them together can give a more useful account of global structural trends than relying on any one value alone. For example, if one descriptor changes, the others can help you ask whether the observation concerns compactness, anisotropy, or a broader change in shape.
A practical habit is to avoid assigning a physical meaning to a value in isolation. First identify the path and atom group, then look at the trend or distribution, and finally compare it with asphericity and radius of gyration. This keeps the interpretation grounded in the data you actually analyzed and makes it easier to communicate what the plot does—and does not—show.
For setup details, see the Shape parameter documentation.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.