[CPMD-list] Monoclinic symmetry !!

[Belabbas Imad]

Dear CPMD users,

I am doing some calculations on molecular crystals with a monoclinic symmetry.

Let start with a comment :

To make CPMD able to recognize a monoclinic space group, one has to adopt the following  orientation (mentioned in the subroutine symmetry.f):
 
     * The principle axis of symmetry has to be along the Z direction.
     * One of the two other axes has to be along the X direction.
     * The last axis has to be in the plane perpendicular to the principle axis. 

Moreover, In CPMD, for the monoclinic crystal system one has to provide the cell parameters in the following way (Cell absolute degree) (this is indicated in sysin.f) :

                                             a  b  c  d  90 90

at this stage I have a small remark, if one consider the first convention, related to the space group, the angle that should be provided by the user is that between a and b which is defined in the general conventions of crystallography as gamma !!

So, if one want to be consistent with the general conventions of crystallography, the cell parameters of a monoclinic crystal should be something like:

                                   a  b  c  90  90  d   ( d is gamma)

Anyway this is not a fundamental issue.


Let move now to my problem !!! :

I have performed single point calculations (wave function optimization) by specifying the cell parameters in two different ways :

1-  CELL 
     7.980 0.51504 1.03634 -0.18052 0.00 0.00

2-   CELL ABSOLUTE DEGREE 
      7.980 4.110 8.270 100.4 90.00 90.00

Having adopted the required orientation  for the space group to be recognized, I expect that the translation vectors of the unit cell to be something like:  

A1 :   *     0.0     0.0
A2:    *      *       0.0
A3:    0.0  0.0     *

( where ' * ' represents a non null component )

In the first case, I have obtained as output :

            TOTAL ENERGY =          -92.20038879 A.U

************************** SUPERCELL ***************************
 SYMMETRY:                                             MONOCLINIC
 POINT GROUP :                           [MONOCLINIC]    2/m(c2h)
 LATTICE CONSTANT(a.u.):                                 15.08001
 CELL DIMENSION:  15.0800  0.5150  1.0363 -0.1805  0.0000  0.0000
 VOLUME(OMEGA IN BOHR^3):                              1800.33886
 LATTICE VECTOR A1(BOHR):           15.0800     0.0000     0.0000
 LATTICE VECTOR A2(BOHR):           -1.4021     7.6392     0.0000
 LATTICE VECTOR A3(BOHR):            0.0000     0.0000    15.6280
 RECIP. LAT. VEC. B1(2Pi/BOHR):      0.0663     0.0122     0.0000
 RECIP. LAT. VEC. B2(2Pi/BOHR):      0.0000     0.1309     0.0000
 RECIP. LAT. VEC. B3(2Pi/BOHR):      0.0000     0.0000     0.0640

 ****************************************************************

The translation vectors look fine !!

In the second case, I have obtained:

                  TOTAL ENERGY =          -92.20038437 A.U

************************** SUPERCELL ***************************
 SYMMETRY:                                             MONOCLINIC
 POINT GROUP :                           [MONOCLINIC]    2/m(c2h)
 LATTICE CONSTANT(a.u.):                                 15.08001
 CELL DIMENSION:  15.0800  0.5150  1.0363 -0.1805  0.0000  0.0000
 VOLUME(OMEGA IN BOHR^3):                              1800.33222
 LATTICE VECTOR A1(BOHR):           15.0800     0.0000     0.0000
 LATTICE VECTOR A2(BOHR):           -1.4021     7.6392     0.0000
 LATTICE VECTOR A3(BOHR):            0.0000     0.0000    15.6280
 RECIP. LAT. VEC. B1(2Pi/BOHR):      0.0663     0.0122     0.0000
 RECIP. LAT. VEC. B2(2Pi/BOHR):      0.0000     0.1309     0.0000
 RECIP. LAT. VEC. B3(2Pi/BOHR):      0.0000     0.0000     0.0640
****************************************************************

In this case the printed Lattice vectors are wrong !!!!

The two cases led to a small energy variation in the energy of the unit cell !!

Please can  someone comment on this issue !!


Ps. I am attaching the used input and obtained output files  


Best wishes and happy new year,

Imad





             
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