[Q-e-developers] ibrav = +/- 9

[Lorenzo Paulatto]

Dear All,
there is a small bug or a problem with the documentation of ibrav= -9


The Doc says:

9          Orthorhombic base-centered(bco) celldm(2)=b/a
                                           celldm(3)=c/a   
    v1 = (a/2, b/2,0),  v2 = (-a/2,b/2,0),  v3 = (0,0,c)
-9          as 9, alternate description
    v1 = (a/2,-b/2,0),  v2 = (a/2,-b/2,0),  v3 = (0,0,c)


However the code says:

     IF ( ibrav == 9 ) THEN
        !   old PWscf description
        a1(1) = 0.5d0 * celldm(1)
        a1(2) = a1(1) * celldm(2)
        a2(1) = - a1(1)
        a2(2) = a1(2)
     ELSE
        !   alternate description
        a1(1) = 0.5d0 * celldm(1)
        a1(2) =-a1(1) * celldm(2)
        a2(1) = a1(1)
        a2(2) =-a1(2)
     END IF

Which means:
-9          as 9, alternate description
    v1 = (a/2,-b/2,0),  v2 = (a/2,b/2,0),  v3 = (0,0,c)


I.e the sign of the y-component of v2 is the opposite. 	

I'm writing here because I do not know which of the two is correct. Thanks to 
G Radtke for bringing this to my attention (He thinks that the Doc is correct 
and the code is wrong)

LP


-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC -- CNRS & Université Paris 6
+33 (0)1 44 275 084 / skype: paulatz
http://www.impmc.upmc.fr/~paulatto/
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