Moving a molecular fragment can seem simple—until you need to place it at a specific angle, keep the rest of the structure untouched, or repeat the same adjustment reliably. Freehand dragging is useful for a quick reposition, but it can be difficult to control when you need motion along a defined axis or in measured increments.
SAMSON offers several ways to handle this, from a quick camera-plane drag to local or global axis controls. The key is to choose the editor that matches the kind of movement you need, select the intended object first, and use snapping when precision matters.
Choose the right frame of reference
Before moving anything, decide what direction should stay meaningful during the move:
- Displacer (D): drag selected nodes in the plane of the screen. This is a convenient choice for a quick visual reposition.
- Local Move (M): translate and rotate along the selection’s principal axes. This is useful when the object’s own orientation is the natural guide—for example, when adjusting a molecular fragment relative to its local geometry.
- Global Move (K): translate and rotate along the global X, Y, and Z axes. Choose this when you need to relate a structure to the scene’s shared coordinate frame.
All three editors are available from the viewport’s left-side menu. In each case, select the object or atoms you intend to move before manipulating the controls. This simple check helps prevent an accidental move of a larger structure than intended.
Use the widget that matches the motion
In Local Move and Global Move, the central translation widget moves the selection in the camera plane. The straight arrows constrain translation to an axis, while the plane widgets constrain it to a plane. Curved arrows rotate around an axis; the spherical trackball allows free rotation.
The distinction between local and global controls matters most when a structure is already rotated. Local axes follow the selection’s principal axes; global axes remain aligned with the scene’s XYZ frame. If a direction feels unintuitive, check which frame the editor is using before dragging.
Make small adjustments repeatable
When a drag needs to land on a predictable increment, enable snapping in the editor’s quick-access menu at the top-left of the viewport. Snapping can also be configured under Interface > Preferences > Editors > Snapping. The documentation gives 0.5 Å as an example of a translational snapping value and 10° as an example of a rotational value; choose settings appropriate to the adjustment you are making.
For numerical control, right-click a move-editor widget, or hold Ctrl on Windows/Linux or Cmd on macOS while left-clicking it, to provide precise translation and rotation parameters. Right-clicking widgets also provides alignment commands. If a drag goes wrong, press Esc while moving to restore the previous position; completed move operations can be undone with Edit > Undo.
When the global frame is the goal
If your aim is not merely to move a structure, but to place it relative to the world axes or planes, use the structure’s context menu: right-click the structure or click the selection in the Document view, then choose Move selection. Options include centering on the origin and aligning with an axis or plane. Turning on the viewport grid can help you see the global XY plane at Z=0.
For rotations that need to be repeated around a chosen center, the Global Move workflow can use a specified pivot atom together with rotational snapping. That combination is useful when arranging copies around a structure: the pivot defines the rotational center, while snapping makes the angular steps predictable.
A practical sequence is: select only the intended atoms or object, choose local or global axes, use a constrained widget rather than free dragging, enable snapping if you need repeatable increments, and undo or cancel if the result is not right. For more options—including alignment, distribution, and editor details—see the SAMSON documentation on moving objects.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.