How to pick up the right solution between these high M20 solutions?
No exact solution to that problem related to the "needle to be found in a hay bundle."
The ultimate proof that a solution is the right one is to solve the structure. Having only 31 hkls, this is not completely impossible with an organic compound if you already know the molecular formula. You "just have to" place that molecule inside of the cell by a direct space approach. If really you believe that the correct cell is in your list of possibilities, first eliminate of the list those cells leading to a strange Z value. Then guess a space group for every remaining cell, and apply either PSSP or FOX or ESPOIR or DASH or etc.
Now, if you think that the chance is to 100% waste a lot of time, I have a drawer almost full of such ambiguous cases. There remains a small place for your problem ;-).
Armel
