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hi Sam

Are the cell edges really different (i.e. did Mosflm round a,b and c down to 1 decimal place, or did the difference arise from some other source?)?

Assuming that the reported cell edges from Mosflm are really the same, solution 2 should be used because, with three obtuse angles, it is conventionally correct. So you'd cause fewer raised eyebrows with 2.

You could choose to use the cell with three acute angles and all your subsequent calculations would give you an identical model.

Technically, they are the same solution, just using different characteristic lattice transformations from the triclinic basis determined in the autoindexing (the first C.L. transformation from International Tables is the identity, so it looks as if the original cell reduction in Mosflm hasn't given the conventional cell).

Harry
--
Dr Harry Powell
Chairman of International Union of Crystallography Commission on Crystallographic Computing
Chairman of European Crystallographic Association SIG9 (Crystallographic Computing) 

On 25 Apr 2016, at 00:13, Sam Tang <[log in to unmask]> wrote:

Hello colleagues

I have a more or less conceptual question relating to unit cell parameters which arises from a little experiment with my P1 dataset (no twinning, no tNCS).  I would be glad if further discussion follows or perhaps any reading could be suggested on this?

So the story is, I first indexed my images in mosflm. Initially mosflm gave zero penalty for both of these two sets of unit cell parameters  
(i): 53.9  60.6  82.6  68.7  70.9  83.4
(ii): 53.98  60.66  82.69  106.73  109.04  96.56

At first I processed my dataset to 2.4 A with parameters (i). Pointless return (i) to be best solution.
Then I pushed to 2.3 A. With either (i) or (ii) Pointless returns (ii) to be best solution.
In all cases Phaser seem to give a reasonable MR solution.

So there come a few questions in my mind:
(1) Of course - which ones are the 'correct' unit cell parameters?
(2) If I integrate my data with (i), but Pointless returns (ii), should I re-do the indexing (still in P1) and integration with (ii) again?
(3) So if there is a 'right or wrong' here, how should I further look into Pointless results for the right unit cell?


Kind regards

Sam