# [Pw_forum] Quick question on coordinates

Miguel Marti­nez miguel.martinez at ehu.es
Tue Feb 26 19:55:18 CET 2008

Dear colleages,

I'm currently doing some stuff on face-centered monoclinic structures, and
have found silly problems. For example, the documentation reads:

espresso/Doc/INPUT_PW:

monoclinic (p)
=============================
a1 = (a,0,0), a2= (b*sin(gamma), b*cos(gamma), 0),  a3 = (0, 0, c)
where gamma is the angle between axis a and b

base centered monoclinic
=============================
a1 = (  a/2,         0,                -c/2),
a2 = (b*cos(gamma), b*sin(gamma), 0),
a3 = (  a/2,         0,                  c/2),

¿Is it right? ¿Is it a typo? I fear it's a typo, because changing ibrav
from 12 to 13 decreases cell volume by exactly a factor of 2. What's more,
\vec{a}*\vec{b}=|a|*|b|*cos(ab),
while according to the basis in the monoclinic case we'd have
\vec{a}*\vec{b}=|a|*|b|*sin(ab)

Miguel

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Miguel Martínez Canales
Dto. Física de la Materia Condensada
UPV/EHU
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Tlf:  +34 94 601 5326
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