[QE-developers] ibrav = -3 vs. ibrav = 7

[Tone Kokalj]

Dear colleagues,

I spotted the following minor inconsistency between ibrav=-3 and
ibrav=7 primitive vectors:

ibrav = -3, Cubic I (bcc), (more symmetric axes):
    v1 = (a/2)(-1,1,1), v2 = (a/2)(1,-1,1),  v3 = (a/2)(1,1,-1)

ibrav = 7, Tetragonal I (bct)
    v1=(a/2)(1,-1,c/a),  v2=(a/2)(1,1,c/a),  v3=(a/2)(-1,-1,c/a)

This implies that -3 and 7 are mutually inconsistent. Perhaps, new
ibrav can be added as:

ibrav = -7 Tetragonal I (bct), (compatible with -3)
    v1=(a/2)(-1,1,c/a),  v2=(a/2)(1,-1,c/a),  v3=(a/2)(1,1,-c/a)

This new ibrav would aid at automatic conversion from, say, CIF or XSF
to pw.x input with ibrav, celldm(i) & ATOMIC_POSITIONS crystal.

Best regards,
Tone
-- 
Jožef Stefan Institute, Ljubljana, Slovenia