Crystallization screens and methods
Because it is not generally possible to rationally predict crystallization conditions, screens have been developed which sample a wide range of crystallization space in order to identify preliminary crystallization condititions.
Many such screens are commercially available; they fall into several categories:
1. Sparse-matrix screens, which contain conditions that have previously proved successful.
2. Grid screens, in which specific crystallization conditions are sampled in a systematic way.
3. Imcomplete factorial screens, in which individual components of crystallization conditions are varied in a systematic way.
The PACT screen
The PACT screen was developed by Janet Newman in the lab of Tassos Perrakis. This systematic screen, a pH-, anion- and cation-testing (PACT) screen, aims to decouple the components of each condition and to provide information about the protein, even in the absence of crystals, rather than cover a wide crystallization space. The PACT screen is available from Molecular Dimensions and Qiagen  under license, and by Jena Biosciences.
The exact formulations and all information for how to make the PACT screen are available as supplementary material of the original article by Newman et al and are available under Open Access policy.
Qiagen(Nextal) vs Hampton screens
Nextal Classics Suite is almost exactly the same as the Hampton Screen I and II (except that it's 96 conditions, whereas two Hampton Screens amount to 98; also, Nextal's screen is reorganized to bring closer the similar conditions for easier observation). Nextal Classics II Suite is almost exact replica of Hampton's Index Screen (even numbering is identical). One exception is the proprietary (!) Tacsimate in Hampton's Index, which in NExtal's screen is replaced (somewhat inappropriately) by sodium citrate.
What is Tacsimate?
"Tacsimate contains 1.36 M Malonic acid, 0.25 M Ammonium citrate tribasic, 0.12 M Succinic acid, 0.3 M DL-Malic acid, 0.4 M Sodium acetate, 0.5 M Sodium formate, and 0.16 M Ammonium tartrate dibasic."
What is Tacsimate? or go to the source
Searching for silver bullets: An alternative strategy for crystallizing macromolecules. Alexander McPherson and Bob Cudney. Journal of Structural Biology 156 (2006) 387–406.