Measure **Area** properties involving planes.
### Quantity

The supported **Area** properties are listed below. Planes
and other objects selected may belong to different structural systems
(cells, molecules, etc.), bonded or not. Only objects in the current
layer can be chosen.
### Value

After selecting a quantity, 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.

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.

**Plane Node Area**

The area per node in a crystallographic plane. The plane parent must be a cell object.

The area per node is calculated for the selected plane object, dividing
the volume per node by the distance between planes: **A = V / d**.
To ensure correct results, the real distance between adjacent planes
must be used. GAMGI uses the following algorithm: 1) determine
the smaller possible indices from the plane indices. For primitive
lattices or primitive vectors these are always the true Miller indices.

2) for **C**, **I**, **F**, **R** centered lattices, with
conventional vectors, the structure factor might be zero for these
indices. In this case, there are equivalent planes at 1/2 (base-centered,
body-centered and face-centered lattices) or 1/3 (rombohedral) of
the distance to the origin. In this case, use the following rules
to obtain the true Miller indices:

C: unless h+k == 2n, (h k l) becomes (2h 2k 2l)I: unless h+k+l = 2n, (h k l) becomes (2h 2k 2l)F: unless h,k,l have same parity, (h k l) becomes (2h 2k 2l)R: unless -h+k+l = 3n, (h k l) becomes (3h 3k 3l)

For example, conventional planes (100) in cI and hR lattices are in fact
(200) and (300) planes respectively, so the true distance between adjacent
planes is actually d/2 and d/3, where d is the uncorrected distance.
This correction is not taken into account when determining the **Plane
Separation**, only when calculating the **Plane Node Area** and the
**Plane Node Density**.

3) if the plane indices are smaller or equal to the Miller indices, the correct distance between planes is the distance for planes with Miller indices, as planes with smaller indices must be multiples of Miller planes, with the same area/node. If the plane indices are higher than the Miller indices, then the planes are either Laue planes or multiples of Laue planes, and in both cases an error message is issued, because the area/node is not uniquely defined for these planes.

For example, conventional planes (100) and (200) in cI have an area/node equal to a**2, but planes (300) and (400) have no unique area/node, varying between 0 and a**2. The Miller indices are (200).

**Plane Node Density**

The number of nodes per unit area in a crystallographic plane (the inverse of the previous quantity). The plane parent must be a cell object.

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 quantity or page is chosen, previous data is automatically removed.