A small change to a molecular fragment can be surprisingly awkward: drag too freely and you may lose the orientation you were trying to achieve; move the wrong part and the rest of the structure shifts with it. In SAMSON, the Local Move editor offers a focused way to work on internal rotations, including editing a dihedral angle around a bond.
The key is to select the fragment you want to move and use the editor’s local controls. Unlike the Global Move editor, which uses the global X, Y, and Z axes, the Local Move editor works along the principal axes of the selected objects. That makes it useful when the direction you care about is defined by the molecule itself rather than by the scene.
Start with the right selection
Load a molecule, then select the atoms that form the fragment you want to rotate. If you are selecting several atoms, the rectangle selection editor can help. Selection matters: the move editor acts on the selected objects, so check that the intended fragment—not the whole molecule—is selected before you begin.
Activate the Local Move editor with the M shortcut. Its controls include translation arrows and planes, curved rotation arrows, and a trackball for free rotation. For a dihedral edit, you do not need to estimate the rotation by dragging one of the general-purpose controls: click the half of the bond corresponding to the fragment you want to rotate. A widget appears for editing the angle.
Use the widget deliberately
Before changing the angle, take a moment to inspect the selection and identify which side of the bond should move. Then click that half-bond and use the widget that appears. This is especially helpful when a molecule contains several nearby bonds: choosing the intended fragment first makes it clearer what your operation will affect.
For other kinds of local adjustment, the editor’s controls provide different constraints. The central translation widget moves the selection in the camera plane. Side translation arrows move it along the selection’s principal axes, while the side rotation widgets turn it around those axes. The trackball allows free rotation. These controls are useful for positioning a selection, but the bond-specific dihedral widget is the more direct choice when the goal is to adjust a torsion.
Make the adjustment repeatable
If you want a rotation to land on regular increments, enable rotational snapping from the viewport’s quick access menu. The documentation gives 10° as an example of a rotational snapping value. Snapping can make successive adjustments easier to compare, especially when you are arranging a structure by eye. If you need to inspect or change snapping preferences, they are also available under Interface > Preferences > Editors > Snapping.
For more precise translation or rotation parameters, right-click a move-editor widget, or hold Ctrl (or Cmd on macOS) while left-clicking it. This provides an alternative to relying only on mouse movement. Right-clicking widgets can also provide commands for aligning objects along global axes.
Keep an easy way back
During a drag, SAMSON shows the positional shift. If the preview is not what you intended, press Esc to cancel and restore the previous position. Once you release the mouse, the move is applied; move operations are undoable through Edit > Undo. That makes it practical to test an adjustment, assess the result, and reverse it if needed.
In short: select only the fragment you want to change, use the Local Move editor (M), and click the correct half of the bond to bring up the dihedral-angle control. Use snapping or the widget’s precise-parameter options when you need a more controlled adjustment.
Learn more in the SAMSON documentation on moving objects.
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