# [Pw_forum] Quick question on coordinates

Paolo Giannozzi giannozz at nest.sns.it
Wed Feb 27 16:09:03 CET 2008

```Miguel Marti­nez wrote:

>    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?

what is right and what is a typo? As far as I know, the documentation
exactly reflects what the code does (see flib/latgen.f90) :

else if (ibrav == 12) then

sen=sqrt(1.d0-celldm(4)**2)
a1(1)=celldm(1)
a2(1)=celldm(1)*celldm(2)*celldm(4)
a2(2)=celldm(1)*celldm(2)*sen
a3(3)=celldm(1)*celldm(3)

else if (ibrav == 13) then

sen = sqrt( 1.d0 - celldm(4) ** 2 )
a1(1) = 0.5d0 * celldm(1)
a1(3) =-a1(1) * celldm(3)
a2(1) = celldm(1) * celldm(2) * celldm(4)
a2(2) = celldm(1) * celldm(2) * sen
a3(1) = a1(1)
a3(3) =-a1(3)

The case ibrav=13 has been corrected more than once by the way.
If you have evidence that there is something wrong above, please
explain what exactly and why

Paolo
--
Paolo Giannozzi, Democritos and University of Udine, Italy

```