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GRID is used to switch on and off a hexagonal grid of points which facilitates the building of "tidy" diagrams.
Suppose that the drawing area is blank.
A faint grid of points appears in the drawing area.
The grid of points vanishes.
Thus we can toggle between grid and non-grid.
With the grid in place then when a new position is selected the new active atom will be placed at the nearest grid position.
You should note that if the drawing area already contains a fragment when GRID is selected then an attempt is made to construct a grid which will best superpose the existing fragment. Thus the grid size will be the average bond length in the fragment.
If the program state is DRAW the grid will be placed on the active atom.
If the program state is MOVE the grid will be placed on the atom added last.
The GRID option in the BUILD menu, in its most basic form, can be used to place a hexagonal grid on an empty drawing area - see (i) below.
However, with a little understanding of the way a grid is placed in the BUILD menu, you can tailor the grid in a number of ways to suit your diagram.
The average bond length of the fragment (if any) determines the spacing of the grid points.
If you know the size of the grid you require, then you should generate one bond, of the correct length, then select GRID.
The grid spacing will then be equal to the length of the bond - see (ii).
If the program state is DRAW the grid will lie exactly on the active atom.
Thus, if you have generated a 5 -membered ring and want to ensure that the grid is superimposed exactly upon one particular atom, then you should select that atom to invoke the DRAW state, then select GRID - see (iii) below.
If the program state is MOVE the grid will be superimposed on the last atom generated ie. the highest-numbered atom - see (iv), where the last atom generated was the top-left C atom.
By default, on a blank screen, the grid will be generated vertically:
Suppose you wish the grid to look like:
When the grid is superimposed on an atom the angle is determined as follows:
in DRAW, by the angle to the first atom bonded to the active atom
in MOVE, by the angle to the last atom.
The angle from the horizontal is rounded to the nearest 15° increment.
Thus, to generate a "horizontal" grid, build a horizontal bond involving two carbon atoms - see (v) below.
With the right-hand C atom active, select GRID.
The grid is then placed as shown in (v).
It does not matter if the starting bond is not perfectly horizontal.
After the grid is in place you can delete the atom which is not exactly on a grid point - using DELATM. Alternatively you could use MOVE ATOM in the EDIT menu to move the atom to a grid point.
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Volume 2 Chapter 1 Build menu GROUPS.