UnitCell : least squares cell refinement program with regression diagnostics
UnitCell is a program which runs on Macintosh computers and under Windows or linux. It refines cell parameters from powder diffraction data. Unlike most existing cell refinement programs it does not require initial estimates for cell constants; it uses a non-linear least squares method, which allows the refinement to be carried out on the actual observed data (e.g. two-theta in the case of angle-dispersive monochromatic diffraction, energy in the case of energy-dispersive diffraction). Furthermore, the program incorporates regression diagnostics, particularly deletion diagnostics, to aid in the detection of outliers and influential data which could be deleterious to the regressed results. These diagnostic tools are particularly valuable in assessing the sensitivity of the calculated results to the observations of individual Bragg peaks.
The method and implementation are described in T J B
Holland and S A T
Redfern (1997) "Unit cell refinement from powder
diffraction data: the use of regression diagnostics". Mineralogical
Magazine 61: 65-77.
UnitCell provides a graphical interface by which
various options and modes of operation may be easily selected,
with data input via standard text files.
Earlier versions of the program used weighted least squares, using a default uncertainty of 0.005 deg 2theta. However, in many cases this is too small, particularly with poorly crystalline or extremely fine-grained samples and values of sigmafit much greater than unity are calculated. In the past the user was advised to scale the uncertainties based on the value of sigmafit, but the current implementation is much more transparent in allowing the user to enter their best estimate of uncertainty in advance. If this has been estimated well then sigmafite should be close to 1.0 and no corrections are needed for the calculated uncertainties. It is the user's responsibility to interpret the uncertainties in the light of the value of sigmafit returned by the program, as indicated in the UnitCellInfo.pdf.
* Updated (Mar 2021) OSX version is now 64bit.
The executables, sample input files, and an info file explaining the use of the program and the deletion diagnostics are available here:
download: mac OSX
download: Windows
download: Linux
File: UnitCellInfo.pdf