[Pw_forum] vc-relax - negative and non-zero cell parameters

[Christopher Heard]

Hi, 
I have a question regarding vc-relaxation in relative coordinates.
If I input a cubic lattice and allow the program to relax the cell parameters, and print as relative coordinates of the original cell parameters (alat), I dont understand how off diagonal terms can be non-zero (what is 0 x 0.9 if not 0?) or how negative values have physical meaning.

Perhaps I am not understanding the method by which the calculation is made, but I couldn't get any understanding from the INPUT_PW.txt file or the manual, so I am asking the list.

Cheers,
Chris


========================================================
INPUT FILE

&CONTROL
  calculation  = "vc-relax",
  prefix       = "QETEST",
  pseudo_dir   = " /home/heard/pseudo ",
  outdir       = ".",
  nstep = 10,
/
&SYSTEM
  ibrav     = 0,
  nat       = 3,
  ntyp      = 2,
  ecutwfc   = 20.D0,
  occupations = 'smearing'
  smearing = 'mp'
  degauss = 0.06
/
&ELECTRONS
  electron_maxstep =  1000,
  conv_thr    = 1.D-6,
  mixing_beta = 0.3D0,
/
&IONS
/
&CELL
  cell_dynamics = 'bfgs',
/
CELL_PARAMETERS cubic
30.23  0.0  0.0
 0.0 30.23  0.0
 0.0  0.0 30.23
ATOMIC_SPECIES
Cu 63.5 Cu.pbe-d-rrkjus.upf
Ag 107.9 Ag.pbe-d-rrkjus.upf

ATOMIC_POSITIONS {crystal}

Cu  0.9110342051985  0.5412603911172  0.9652262133233
Cu  0.3046428810907  0.3592799679414  0.0488897975312
Ag  0.868139803195   0.7597487846068  0.8302658063157
K_POINTS {Gamma}


======================================================
START OF OUTPUT FILE

   bravais-lattice index     =            0
     lattice parameter (a_0)   =      30.2300  a.u.
     unit-cell volume          =   27625.7732 (a.u.)^3
     number of atoms/cell      =            3
     number of atomic types    =            2
     number of electrons       =        33.00
     number of Kohn-Sham states=           21
     kinetic-energy cutoff     =      20.0000  Ry
     charge density cutoff     =      80.0000  Ry
     convergence threshold     =      1.0E-06
     mixing beta               =       0.3000
     number of iterations used =            8  plain     mixing
     Exchange-correlation      =  SLA  PW   PBE  PBE (1434)
     EXX-fraction              =        0.00
     nstep                     =           10


     celldm(1)=  30.230000  celldm(2)=   0.000000  celldm(3)=   0.000000
     celldm(4)=   0.000000  celldm(5)=   0.000000  celldm(6)=   0.000000

     crystal axes: (cart. coord. in units of a_0)
               a(1) = (   1.000000   0.000000   0.000000 )
               a(2) = (   0.000000   1.000000   0.000000 )
               a(3) = (   0.000000   0.000000   1.000000 )

===========================================================
OUTPUT COORDINATES

     new unit-cell volume =  27844.52160 a.u.^3 (  4126.13235 Ang^3 )

CELL_PARAMETERS (alat= 30.23000000)
   1.003485881   0.000341145  -0.000166335
   0.000341140   1.002476151   0.000847099
  -0.000166325   0.000846982   1.001936910

ATOMIC_POSITIONS (crystal)
Cu       0.911117504   0.540084682   0.965438693
Cu       0.305212112   0.357448827   0.044305096
Ag       0.867487274   0.762755634   0.834638028
===========================================================