General Description
XLENS is a direct methods program that solves crystal structures from single-crystal (SC) or powder (PW) diffraction data. Ideally, the program works with the intensities of the unique reflections up to atomic resolution as well as with the approximate chemical composition of the compound. The program solves the crystal structure by optimising the phase values (generically denoted by Φ) of the large structure factors. In the solution of small crystal structures from SC data, this optimisation (phase refinement) is achieved by maximising the origin-free modulus sum function S(Φ) (Rius, 1993)
S(Φ) = < [ |E(H)| - |E|av ] · |E(H,Φ)| >H = max. !
In this expression |E(H)| are the moduli of the normalised structure factors of both large and weak reflections, |E|av is the average observed E value and |E(H,Φ)| are the moduli calculated in terms of the phases. The very robust phase refinement procedure implemented in this XLENS version uses the FFT algorithm and is called S-FFT (Rius et al., 2007).
In a typical default SC run, XLENS carries out 25 sets of phase refinements. Each set starts with random phase values for the large structure factors and ends with the refined phases Φref that are used to compute the figure of merit (FOM) of each phase refinement trial. The FOM is the correlation coefficient between observed |E(H)| and calculated |E(H,Φ)| moduli. After the phase refinement, the Fourier recycling part follows. This part ends with the calculation of a final R-value including all individual reflections. The structure solution process stops when at the end of the Fourier recycling the final R-value is lower that a preset value. Otherwise the next set of random phase values will be processed. XLENS can also develop and complete partial structures.
The PW case is normally more complicated. The PW peculiarities regarding file organisation (compatibility with the FULLPROF suite of programs) and specific strategies are described at the end of this WinHelp file.
Copyright © 2010, Jordi Rius & FullProf Suite Team