A familiar modeling frustration: you try to reposition one part of a molecule, but the wrong atoms move—or a small mouse gesture gives you a rotation that is hard to reproduce. The key is to separate the task into three decisions: what is selected, which reference frame you need, and how much snapping or precision control to use.
In SAMSON, move editors act on selected objects. Before dragging, select the atoms or other movable nodes you intend to change. For a molecular fragment, the rectangle selection editor can help select a group of atoms. This simple check is especially useful when working inside a larger structure: a move editor cannot infer which part you meant to isolate.
Choose a frame that matches the task
The Local Move editor (M) translates and rotates along the selection’s principal axes. It is a natural choice when the movement should follow the orientation of the selected fragment—for example, when adjusting a piece of a molecule relative to its own geometry. Its colored widgets identify the local axes: light blue for the first, mid blue for the second, and dark blue for the third.
The Global Move editor (K) instead uses the global X, Y, and Z axes. Choose it when you need motion aligned with the scene’s reference frame, rather than with the fragment. The Displacer (D) is a simpler option for moving objects in the camera plane. These editors are available in the viewport’s left-side menu.
A controlled move, one operation at a time
- Select the intended atoms or object. Confirm that the selection is limited to the part you want to reposition.
- Activate the appropriate editor. Use Local Move for axes tied to the selection, Global Move for the global XYZ frame, or Displacer for a camera-plane shift.
- Choose the widget for the operation. The central translation widget moves in the camera plane. Side arrows translate along axes, plane widgets constrain translation to a plane, and curved arrows rotate around an axis. The sphere-shaped trackball supports free rotation.
- Drag and inspect before committing. Press and hold the relevant widget, move the mouse, then release to apply the change. If the preview is not what you intended, press Esc during the operation to cancel and restore the previous position.
Breaking a complex adjustment into separate translations and rotations makes it easier to see which action caused an unwanted change. It also helps you choose the right constraint: an axis widget is more controlled than a free rotation when you already know the axis you need.
Make movements repeatable with snapping
When approximate mouse control is not enough, enable translational or rotational snapping from the editor’s quick access menu at the top-left of the viewport. The documentation gives examples of a 0.5 Å translation increment and a 10° rotation increment. These are examples, not universal settings: choose an increment suited to the adjustment you are making. Snapping can also be configured in Interface > Preferences > Editors > Snapping.
For numerical control, right-click a move-editor widget, or use Ctrl/Cmd with a left-click on a widget to provide precise translation and rotation parameters. Right-clicking widgets also provides alignment options along global axes. For a local structural adjustment, the Local Move editor can additionally help edit a dihedral angle: click the half of the bond corresponding to the fragment you want to rotate, then use the widget that appears.
Finally, remember that move operations are undoable through Edit > Undo. That makes it practical to test a movement, inspect the result, and revert if the change is not useful. For more on alignment, distribution, snapping, and move editors, see the SAMSON documentation on moving objects.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.