A common source of confusion in molecular modeling is that the same nanosystem can have several different kinds of models associated with it. A protein may look correct on screen, yet that does not tell you which data define its geometry, which objects control its motion, or which components calculate its energy. When a result is unexpected, separating these roles can make the next step much clearer.
In SAMSON, models are organized by what they describe—not simply by how they look in the Document view. The platform uses five model categories, while state updaters play a separate role in advancing states during minimization and simulations. Thinking through these roles is a practical way to diagnose what you are seeing.
Start with the structure
Structural models describe geometry and topology. They typically contain atoms and bonds, and may include higher-level nodes such as molecules, chains, and residues. If your concern is whether the system has the expected components or organization, start by checking its structural nodes. A representation on screen can be useful, but the geometry and topology belong to the structural model.
Separate appearance from physical behavior
Visual models provide graphical representations: for example, a protein’s secondary structure, a Gaussian surface, an electron-density isosurface, or a volumetric electrostatic field. Changing a visual model changes how information is represented; it does not, by itself, define the forces used in a simulation.
This distinction helps with a familiar puzzle: “Why did changing the display not change the simulation?” The two operations concern different model categories. Visual models can be applied to selected nodes—or to the whole document when nothing is selected—through the selection’s context toolbar, Visualization > Add > Visual model, or the Ctrl/Cmd + Shift + V shortcut. Some visual models expose parameters in the Inspector: select the visual model in the Document view and choose Inspect from its context menu.
SAMSON’s default visual models can also support selecting and highlighting atoms, residues, or chains directly on surfaces, depending on the active selection filter. So a display is not only for presentation; it can help you inspect and work with parts of a structure.
Ask what controls motion and forces
Dynamical models indicate where the degrees of freedom are in structural nodes. For a particle system, for example, a dynamical model can assign three translational degrees of freedom to each atom. It describes the dynamical state and stores positions, momenta, and masses.
Interaction models represent energies and forces in a dynamical model. Examples include Lennard-Jones, spring, elastic network, and universal force field models. An interaction model provides forces corresponding to the degrees of freedom; for a particle system, it can provide the total energy and the force on each particle.
That gives a useful troubleshooting sequence: if the geometry is wrong, inspect the structural model; if the view is unclear, inspect the visual model; if motion or stored dynamical quantities are at issue, consider the dynamical model; and if energies or forces are at issue, look to the interaction model.
Remember the fifth category—and the updater
Property models represent properties not already covered by the other four categories. These can include a scalar value such as a protein’s radius of gyration, a scalar or vector field, or a function. They provide a place for additional information without confusing it with geometry, visualization, dynamics, or interactions.
State updaters are not a sixth model category. They implement methods for advancing states during simulations, such as minimization algorithms, Monte Carlo methods, or molecular dynamics algorithms. In short: models describe aspects of the system, while a state updater specifies a method for changing its state.
Keeping these roles distinct turns “the model isn’t working” into a more specific question: Is the structure correct? Is the visual representation appropriate? Are the degrees of freedom defined? Are the intended energies and forces available? Or is the chosen updater responsible for advancing the state? For definitions and related workflows, see the SAMSON model reference.
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