If you find that certain, high-angle (low d-spacing) reflexions are missing from your simulated powder pattern, don't despair: chances are the program can easily be set to show as much (or as little) data as you require.

Let's start by checking what you can't change: this is the minimum d-spacing allowed by the Bragg equation, which relates the inter-planar spacings, d to the Bragg angle, θ, for a given wavelength, λ:

Now, for the standard Cu Kα wavelength of 1.54 Å, the minimum d-spacing you can hope to record is 0.77 Å. That's a simple fact of life: you can't record reflexions with 2θ angles higher than 180° ! (But you can select a smaller wavelength via the Diffract > Wavelength command - if only it were so easy in real life!).

Back to CrystalDiffract:

• By default, CrystalDiffract is configured to work with an arbitrary number of reflexions; the default setting is 9000 - but the program can handle far more, if required.

• There is also the option of curtailing any reflexions below a certain, minimum d-spacing. The default cut-off is 1 Å.

These default values were chosen to provide quick simulation for the commonly-used range of angles for most materials. You can easily override them, as follows:

1. Choose the Preferences command from the Edit menu (Windows version), or the Application menu (Mac version). The CrystalDiffract Preferences dialog appears.

2. Click the Profile tab.

3. In the Reflexions group, choose a lower value for the Minimum d-spacing (or zero if you want the lot!). You can also turn off the Limit number to option, to ensure that the program will generate everything.

4. Click the Save button. These will now be your default settings, ready for new diffraction windows.

5. The new settings will apply to any new windows that you option in the program; existing windows remain unchanged. So, to generate a new diffraction pattern with the new settings, you'll need to open a new diffraction window and load your structure.

To summarize then: it's important to recognize that the wavelength of radiation imposes a limit on how much of the diffraction pattern can be recorded.

CrystalDiffract also has some presets which you may wish to override if you need to work at very-high angles. These are easy to adjust using the Preferences command.