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Hello Dr Powell and Dom Thanks for your prompt feedbacks. Yes indeed the cell edges are the same. (The difference simply arose during copy-and-paste.) And in fact mosflm consistently returns the conventional cell preferentially. But is there a reason why people would choose to use the acute angles for calculations? *PS - in my case, my lab-mate and I were simply trying to explore the different settings of mosflm out of curiosity. So we are indeed sorry if we have asked something dump here...* Sam On 25 April 2016 at 08:22, Bellini, Dom <[log in to unmask]> wrote: > In fact I guessed wrong! > > > As Harry just pointed out to me: > > > "Unfortunately, it's not quite that simple... > > squashing a parallelpiped (by squeezing two opposite vertices together) > gives you six faces, each of which has two obtuse and two acute angles, but > there are only two (opposite) vertices that have all three obtuse; the six > "equatorial" vertices of the parallelpiped each have two acute and one > obtuse angle." > > D > > > > ------------------------------ > *From:* CCP4 bulletin board <[log in to unmask]> on behalf of Bellini, > Dom <[log in to unmask]> > *Sent:* 25 April 2016 00:40 > > *To:* [log in to unmask] > *Subject:* Re: [ccp4bb] Unit cell parameters > > > Hi Sam, > > > They could probably be representing the same cell, representing the two > different types of corners of cell. Imagine you squash/deform > a parallelepiped into a P1 cell, you would create 4 of the corners to have > 3 obtuse angles and the other 4 corners to have 3 acute angles. > > > I guess ... > > > D > > > > > > ------------------------------ > *From:* CCP4 bulletin board <[log in to unmask]> on behalf of Harry > Powell <[log in to unmask]> > *Sent:* 25 April 2016 00:34 > *To:* [log in to unmask] > *Subject:* Re: [ccp4bb] Unit cell parameters > > 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] > <[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 > > >