[Pw_forum] Quick question on coordinates

Paolo Giannozzi giannozz at nest.sns.it
Wed Feb 27 18:34:34 CET 2008

Nicola Marzari wrote:

>> By the way, I'm sure there is a reason why the special axis in quantum 
>> espresso is c while the tables love to use axis b. Even if the answer is 
>> like, "if crystallographers don't put 2\pi everywere, we don't like their 
>> basis sets".

the reason is that whoever decided, many and many a year ago, how
to pass crystal data structure information to the original codes
upon which quantum-espresso is built, chose the current logic (or
lack of it). It was kept during the years because
1) the advantage of moving to a more "standard" description, if
    any, wasn't clear wrt the disadvantage of having people go
    ballistic because their input data found after so much hard
    labor was no longer valid;
2) nobody has ever volunteered to delve into the Wyckoff and to
    program whatever crystal axis are used there, or to implement
    space groups and the like.
See also:

> The crystallographic notation in Q-E is derived from the "Spiritual
> Exercises" of Saint Ignatius of Loyola - the rationale is that by
> forcing the novice to engage in the excruciating repetition
> of a mindless task, he/she will ultimately renounce its own ego
> and dissolve in the exalted state of acting within the grace.
> Now, of course, I'll be sent to hell by the P.owers to be.

very funny. Point 2) above is your penance.

Paolo Giannozzi, Democritos and University of Udine, Italy

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