[CPMD-list] lattice constant of magnesium-oxide

[Nico.Zobel at TU-Berlin.DE]

Dear CPMD-community,

I want to evaluate an USPP of Mg (which I generated using Vanderbilt's
code) by calculating the lattice constant of MgO (the crystal I'm actually
interested in, it should be 4,21 angstroem). The results I get are the
following:

lattice constant 			total energy
in angstroem				in eV

4.14					    	-1092.00484447
4.15    					-1092.01032496
4.16    					-1091.62546596
4.17   					-1092.01537806
4.18    					-1092.01386046
4.19    					-1092.01213459
4.20    					-1092.00728452

The value for 4.16 Angstroem is somewhat strange and I don't know why. In
all cases I used the same input-file (except the value for the lattice
constant). The important parts of the input file is provided below.

I also encountered this penomenon when I calculated the lattice constant
of Li2O. I usually was able to get a better accordance of the results by
changing the cutoff-energy slightly (1 to 5 Ry).
But I'm not sure whether this procedure is really correct. That's why I
would like to know if this is a common behaviour of CPMD or if this is due
to unrealistic parameters of the USPP I use (please see appendix) ?

Thank you very much for your time.

Nico Zobel.

P.S.:
I used both of the CPMD-patches provided by Axel Kohlmeyer in this
mailing-list on Feb 25th.

P.S. to Axel Kohlmeyer:
My Mg-USPP was CPMD-compatible, my Li-USPP was not.

---------------------------input-file---------------------------------

&CPMD
  OPTIMIZE WAVEFUNCTION
  PCG MINIMIZE
  CONVERGENCE ORBITALS
    1.d-6
&END

&DFT
  FUNCTIONAL PBE
&END

&SYSTEM
  ANGSTROM
  SYMMETRY
    8
  CELL
    8.4 1.0 2.0  0  0  0
  CUTOFF
    40.0
  TESR
    4
  SCALE
  DUAL
    5.0
&END

&ATOMS
*012-Mg.uspp BINARY NEWF
  LMAX=D
   64
....
*O_VDB_PBE.psp BINARY NEWF
  LMAX=D
   64
....
&END

--------------------------mg-pseudopotential-------------------------

   ============================================================
   |  pseudopotential report: version  7.3.5 date  3-10-2005  |
   ------------------------------------------------------------
   |  mg (Vol06)           PBE - GGA           exchange-corr  |
   |  z =  12.00  zv =   2.00  exfact =   5.00000             |
   |                            etot  =  -1.65416             |
   |  index    orbital      occupation    energy              |
   |    1        300           2.00        -.35               |
   |  keyps = 3     ifpcor = 0                                |
   |  rinner =      1.43     for L=    1                      |
   |  rinner =      1.43     for L=    2                      |
   |  rinner =      1.43     for L=    3                      |
   |    new generation scheme:                                |
   |    nbeta =  4     kkbeta =  583     rcloc =    3.1000    |
   |    ibeta     l     epsilon   rcut iptype                 |
   |       1      0        .00   3.00      2                  |
   |       2      0       -.35   3.00      2                  |
   |       3      1        .00   3.00      2                  |
   |       4      1       -.35   3.00      2                  |
   |  npf    = 6  ptryc =   8.000                             |
   |  lloc   = 2  eloc   =    .000                            |
   |  ifqopt = 3  nqf    = 6  qtryc =   8.000                 |
   |  all electron calculation used koelling-harmon equation  |
   |         ************logarithmic mesh************         |
   ============================================================


-- 
Dipl.Ing. Nico Zobel

Technische Universität Berlin  
Institut für Energietechnik  
Fachgebiet Energieverfahrenstechnik und  
Umwandlungstechniken regenerativer Energien (EVUR)

Fon:   +49 30 314 24381
Fax:   +49 30 314 22157              

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