Group Modify
Group->Modify opens a dialog to modify: 1) the length between two atoms; 2) the angle between two directions defined by three atoms; 3) the angle between two planes defined by four atoms. All atoms in the current layer can be used in these modifications, even if they are unbonded or bonded to different systems.

In all cases, the second, third and fourth atoms indicated by the user remain unchanged, only the first atom is modified. All the other atoms bonded to the first atom will be in turn moved along the same direction or rotated along the same axis, as long as they are not bonded in any other way to the second, third or fourth atoms. This way, terminal chains are modified but ring chains are not, to prevent loop modifications where the overall result would be a translation or rotation of the whole bonded system.

### Length

To modify the length between two atoms, press the Length tag to select the length page (the default), then write the atoms identification in the entries 1 and 2 and the new length in the entry Value. Pressing Ok, if the atoms are recognized, the length between them changes to the new value. The direction atom1-atom2 is preserved.

An easy technique to select atom1, atom2 is to click the mouse sucessively over their visual representations. If Value already contains the new length, GAMGI changes it automatically after the second atom is selected.

### Angle

To modify the angle between two directions defined by three atoms, press the Angle tag to select the angle page, then write the atoms identification in the entries 1, 2 and 3, and the new angle in the entry Value. Pressing Ok, if the atoms are recognized, the angle between directions atom2-atom1 and atom2-atom3 changes to the new value. The plane atom1-atom2-atom3 and the lengths atom2-atom1 and atom2-atom3 are preserved. An error is shown when the three atoms are initially aligned along a common direction, because the plane atom1-atom2-atom3 is undefined.

An easy technique to select atom1, atom2, atom3 is to click the mouse sucessively over their visual representations. If Value already contains the new angle, GAMGI changes it automatically after the third atom is selected.

The angle signal convention is defined as follows: when vector atom2-atom3 transforms into vector atom2-atom1 by a direct, counter-clockwise rotation, then the angle is positive, otherwise is negative. In 3D space, the direction of this planar rotation depends of the side of the plane where the user is, so the signal of the angle depends of the user position relatively to the atoms.

### Torsion

To modify the angle between two planes defined by four atoms, press the Torsion tag to select the torsion page, then write the atoms identification in the entries 1, 2, 3 and 4, and the new angle in the entry Value. Pressing Ok, if the atoms are recognized, the angle between planes atom1-atom2-atom3 and atom2-atom3-atom4 changes to the new value. The angles atom1-atom2-atom3 and atom2-atom3-atom4, the lengths atom1-atom2, atom2-atom3 and atom3-atom4 and the direction atom2-atom3 are preserved. An error is shown when atom1, atom2, atom3 or atom2, atom3, atom4 are initially aligned along a common direction, because the plane atom1-atom2-atom3 or atom2-atom3-atom4 is undefined.

An easy technique to select atom1, atom2, atom3, atom4 is to click the mouse sucessively over their visual representations. If Value already contains the new angle, GAMGI changes it automatically after the fourth atom is selected.

The torsion angle signal convention is defined as follows: looking along the common direction atom2-atom3 (with atom1 near the user and atom4 away from the user), if vector common direction-atom4 transforms into vector common direction-atom1 by a direct, counter-clockwise rotation, the angle is positive, otherwise is negative. This angle is exactly the torsion angle written in the Value entry.

While the signal of the angle defined by three atoms depends of the user position, the signal of the torsion angle defined by four atoms is unique (because the four atoms define a 3D construction while the three atoms define only a 2D construction). If the user is looking from the other side, along the common direction atom3-atom2 (with atom4 near the user and atom1 away from the user), the user now sees the rotation in the opposite direction, which compensates the atom1, atom4 swap, so the signal of the torsion angle remains the same.
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