A common modeling frustration: you need to reposition one part of a molecule, but a freehand drag can send it in an unexpected direction or leave its orientation difficult to reproduce. The key is to choose the movement reference that matches the task. In SAMSON, the Local Move and Global Move editors provide different coordinate frames, while snapping can make translations and rotations more controlled.
Start with the right selection
Before moving anything, select the atoms or other movable nodes you intend to reposition. If you select a molecular fragment, only that selection is the target of the move. For a group of atoms, a selection editor such as rectangle selection can help define the fragment. Take a moment to check the selection before dragging: it is much easier to correct the target now than to undo an unintended movement later.
Choose local or global axes
Use the Local Move editor when movement should follow the selected structure’s principal axes. This can feel natural when adjusting a fragment according to its own orientation. Press M to activate it. Its colored translation and rotation widgets represent the selection’s local axes; the central translation widget moves in the camera plane, and the trackball allows free rotation.
Use the Global Move editor when you want movement tied to the scene’s fixed X, Y, and Z axes, regardless of how the selected fragment is oriented. Press K to activate it. Its side arrows and planes translate along global axes, while its curved arrows rotate around them. This is often the clearer choice when placing structures relative to the viewport’s global reference frame.
Make movement repeatable with snapping
When approximate dragging is not enough, turn on translational or rotational snapping from the editor’s quick access menu at the top-left of the viewport. You can also adjust snapping preferences through Interface > Preferences > Editors > Snapping. For example, a translation increment of 0.5 Å or a rotation increment of 10° gives you a consistent step size while you work. These are example values, not universal settings: choose increments that suit the scale and purpose of your model.
Snapping is especially useful when arranging repeated copies or making a series of deliberate rotations. The documentation’s example uses the Global Move editor to position copies around a structure by specifying a pivot atom and rotational snapping. A pivot matters because it determines the center of rotation: rotating around a suitable atom can preserve the placement you care about, while an unsuitable center may send the fragment along an unintended arc.
A careful movement workflow
- Select the intended atoms or nodes and verify the selection.
- Choose Local Move for selection-oriented axes, or Global Move for fixed scene axes.
- Enable snapping if you need controlled increments.
- Drag the appropriate translation or rotation widget. The editor previews the movement before you release.
- If the preview is not right, press Esc to cancel and restore the previous position. Once applied, the move can also be undone with Edit > Undo.
For precise translation or rotation parameters, right-click a move-editor widget, or use Ctrl/Cmd plus a left-click on it. You can also right-click widgets to access alignment options. These controls help turn a visually guided adjustment into a more deliberate operation.
The practical takeaway: decide what should stay fixed, select only what should move, and then pick the coordinate frame and snapping increment that support that goal. For more details and additional movement methods, see the SAMSON documentation on moving objects.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.