# Atom Measure

- Length
- Angle

Measure Angle properties involving atoms.

### Property

The supported Angle properties are listed below. Atoms and other objects selected may belong to different structural systems (clusters, molecules, etc.), bonded or not. Only objects in the current layer can be chosen.

An easy technique to select the objects is to click the mouse sucessively over their visual representations, in the sequence shown in the dialog. After the last needed object is selected, GAMGI automatically starts the calculation.

3-Atom Angle

The valence angle between directions Atom2-Atom1 and Atom2-Atom3.

The signal convention is: when vector Atom2-Atom1 is rotated from vector Atom2-Atom3 through a direct, counter-clockwise rotation, 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 angle signal depends of the user position relatively to the atoms.

4-Atom Angle

The torsion angle between two planes, defined by Atom1-Atom2-Atom3 and Atom2-Atom3-Atom4. 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.

The signal convention is: looking along vector Atom2-Atom3 (with Atom1 near the user and Atom4 away from the user), when vector common direction-Atom1 is rotated from vector common direction-Atom4 by a direct, counter-clockwise rotation, the angle is positive, otherwise is negative.

While the signal of the 3-atom angle depends of the user position, the signal of the 4-atom angle 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.

### Value

After selecting a property, enter the name of the objects and press Ok. If all objects are recognized, GAMGI does the calculation and shows the result in the Value entry.

When a calculation ends, GAMGI keeps the objects identification. To start a new calculation, just click the mouse over a new object: GAMGI automatically cleans the previous data and inserts the new object identification. When a new property or page is chosen, previous data is automatically removed.
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