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["Chun Li " <chun.li@mf.mpg.de>]

Dear Abiniters,

 

Thanks a lot. But I am still not so clear about the last question: Does the unit cell will be expanded infinitely automatically in all of the primitive vector directions (without any definition?)? If I only want to make it periodically in one direction, how to set the parameters?

 

One more question: After a calculation of bend structure, there is a COMMENT in the log file:

 

symanal : COMMENT -

  The Bravais lattice determined only from the primitive

  vectors, bravais(1)=  6, is more symmetric

  than the real one, iholohedry=  2, obtained by taking into

  account the atomic positions.

 symspgr : the symmetry operation no.   1 is the identity

 symplanes : the symmetry operation no.   2 is a mirror plane

 symspgr : spgroup=   6  Pm   (=Cs^1)

 inkpts: Sum of    1 k point weights is    1.000000

 

Does it mean the cartesian coordinates I defined in the input file have not been used in the calculation (I defined the coordinates using xangst according to the three directions of the primitive vectors)? But why?

 

Can you give me detailed explanations? Thank you very much.

 

Chun Li

 

-----Original Message-----
From: Wu Rongqin [mailto:g0203654@nus.edu.sg]
Sent: 2004年7月11日 4:09
To: forum@abinit.org
Subject: RE: [abinit-forum] some basic questions

 

DEAR chUN Li

 

 

 

 

 

I have some basic questions about the input variables:

 

1.       Is there certain physics meaning about acell? Many examples use the value of the crystal constant. However, if there are two crystal constants (a and c), which one should we use?

Acell:     acell1(a), acell2(b), acell3(c)  this is the format.

 

2.       If I have defined the xcart, Must I also provide xangst, acell, and rprim?

 

Xcart,xangst and xred define the atomic coordinates, either one works. No need to imput all

And should the origin of the coordinates I defined be the same as the origin of the primitive vectors?

                               

No need. But if consistent, it more clear

 

That is, should the values of xcart be relative with the values of rprim (if rprim should also be defined)? I almost confused about the following variables: xcart, xangst, xred, acell, rprim, and rprimd.

                               

 

In simple words, the first three give out the atom coordinates in the cell, acell defines the three lattice constant. rprim and pprimd define the lattice type

3.        

4.       If the crystal is composed of two kinds of atom, I want to define a unit cell containing 20 atoms, then how can I setup the periodical boundary conditions to perform simulations? I have not found relative example in the tutorial.

 

 

You can set up your cell with any number atoms as you like!. Your cell will be expanded infinitely according to the lattice vectors. So the pbc is not a problem.