# Frequently Asked Questions¶

A list of common questions.

## What’s the best way to ask for help or submit a bug report?¶

If you have questions on the use of refnx please use the mailing list. If you find a bug in the code or documentation, use GitHub Issues.

## How should I cite refnx?¶

The full reference for the refnx paper is:

“Nelson, A.R.J. & Prescott, S.W. (2019). J. Appl. Cryst. 52, https://doi.org/10.1107/S1600576718017296.”

## How is instrumental resolution smearing handled?¶

There are a variety of ways that you can account for instrumental resolution smearing in refnx. The easiest is if the fractional instrumental resolution, $$\frac{dQ}{Q}$$, is constant. When setting up refnx.reflect.ReflectModel the fractional resolution can be specified, and the reflectivity that it calculates is automatically smeared. For a given $$Q$$ value the $$dQ$$ (found by multiplying the fractional resolution by $$Q$$) value refers to the Full Width at Half Maximum (FWHM) of a Gaussian approximation to the instrumental resolution. This Gaussian distribution is convolved with the unsmeared model to compare with the data. The second way of using the resolution function is for the $$dQ$$ values for each data point to be read in via from a data file (e.g. the 4th column of a text file). In this way point-by-point resolution smearing is achieved. The last way of specifying instrumental resolution is for a full resolution kernel to be provided for each data point. A resolution kernel is a probability distribution that describes the distribution of possible $$Q$$ vectors for each data point. The first two options are typically used, only more advanced users will ever need to apply the last option. For further details on instrumental resolution functions it’s a good idea to read the papers by van Well et al, and Nelson et al.

## What are the units of scattering length density?¶

If the scattering length density of a material is $$(124.88 + 12.85j)\times 10^{-6} A^{-2}$$ (the X-ray SLD for Au), then you would use 124.88 as the real part and 12.85 as the imaginary part.

## What are the ‘fronting’ and ‘backing’ media?¶

The ‘fronting’ and ‘backing’ media are infinite. The ‘fronting’ medium carries the incident beam of radiation, whilst the ‘backing’ medium will carry the transmitted beam away from the interface.