[Pw_forum] bug in data input?

[Carlo Nervi]

Dear Miguel,
thank you for the reply. Indeed also I come to the conclusion to perform 
a permutaion (instead of 3,1,2 I did 1,3,2, which seems to me equivalent 
after looking at the XCrysden screen).

What is confusing, for a end-user like me, is apparently the possibility 
in the classical approach "A, B, C, cosAB, cosAC, cosBC" (even though I 
would like more the a,b,c, alfa,beta,gamma way of input) to define all 
these quantities without any restrictions. But this is not the case as I 
discovered later.
The input of the "A, B, C, cosAB, cosAC, cosBC" mode works exactly like 
the celldm() way. This was not clear to me reading the 
INPUT_PW.html#id3667896 and INPUT_PW.html#id3668028 documents.

Thank you,
	Carlo

Miguel Martínez ha scritto:
> Dear Carlo,
> 
> You are using the standard setting, taking the B axis as the monoclinic
> one. Looking at Doc/INPUT_PW.html you will see that quantum espresso
> takes *C* as the monoclinic axis. For simple monoclinic cells, you just
> have to perform the permutation 1,2,3 to 3,1,2.
> 
> I've lost all hope of understanding base-centered monoclinic systems,
> and I resort to ibrav=0 in such cases. 
> 
> Regards,
> 
> Miguel
> 
> 
> On Wed, 24 Feb 2010 18:52:43 +0100 Carlo Nervi <carlo.nervi at unito.it>
> wrote:
>> Dear all,
>> I did some geometry optimizations with pw.x starting from a
>> monoclininc cell, using Angstrom and the "A, B, C, cosAB, cosAC,
>> cosBC" input methods, for example:
>>
>>   &SYSTEM
>>                         ibrav = 12,
>>                             A = 8.0828 ,
>>                             B = 12.5828 ,
>>                             C = 9.7644 ,
>>                         cosAB = 0.0d0 ,
>>                         cosAC =
>> -0.10713175431387366149628407325294d0 , cosBC = 0.0d0 ,
>>                           nat = 104,
>>                          ntyp = 4,
>>                       ecutwfc = 120 ,
>>   /
>>
>> The input cif file contains the usual a, b, c, and alfa, beta=96.150, 
>> gamma. Therefore, I think, it should be correct to set cosAC as shown 
>> above. However, after open the input file with XCrysden, I realized
>> that the cell was 90-90-90 (a parallelepiped).
>> Where I am wrong?
>>
>> Playing a little bit with cosAB, cosAC and cosBC I understood that
>> only cosAB makes the correct angle (96 degrees) to appears in
>> XCrysden. Therefore I tried another approach: switch b and c, and to
>> use the celldm() approach. Of course I had to use crystallographic
>> fractional coordinates and to exchange y with z in the Atomic
>> coordinates. I'm not sure this is the best approach, but apparently
>> it works. I would appreciate any suggestions from the community.
>>
>>   &SYSTEM
>>                         ibrav = 12,
>>                     celldm(1) = 15.274655166 ,
>>                     celldm(2) = 1.20797 ,
>>                     celldm(3) = 1.5567 ,
>>                     celldm(4) = -0.107131784 ,
>>                           nat = 104,
>>                          ntyp = 4,
>>                       ecutwfc = 120 ,
>>   /
>>
>> Is it normal that pw.x behaves in this way? In other words, are there 
>> any restrictions using the "A, B, C, cosAB, cosAC, cosBC" input
>> method?
>>
>> Thank you in advance,
>> 	Carlo
>>
> 
> 


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Carlo Nervi carlo.nervi at unito.it Tel:+39 011 6707507/8
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