[Pw_forum] Proper Symmetry Representation

[Dr. Robert Molt Jr.]

Good evening,

This is my first solid-state electronic structure calculation; please 
forgive if my questions are obvious. I am not confident in the proper 
symmetry representation of my system. I attach my input file and 
associated questions. Don't worry on my value of ecutwfc; I am trying to 
do cheap calculations at the moment for pedagogy, and do not care how 
quality a value this is right now. Same goes for my k space points.

&CONTROL
     calculation  = "vd-relax"
     outdir='/p/work1/workspace/molt/Espresso_Test',
     pseudo_dir = '/p/home/molt/Espresso_PP',
     restart_mode = "from_scratch"
/

&SYSTEM
     celldm(1)                 =  1.3454e+01
     ecutwfc                   =  4.00000e+01
     ibrav                     = 2
     nat                       = 74
     nspin                     = 1
     ntyp                      = 2
/

&ELECTRONS
     diagonalization  = "david"
     electron_maxstep = 100
     mixing_beta      =  7.00000e-01
     mixing_mode      = "plain"
     startingpot      = "atomic"
     startingwfc      = "atomic+random"
/

&IONS

/

&CELL
  cell_dynamics=bfgs
/

K_POINTS {automatic}
  2  2  2  0 0 0

ATOMIC_SPECIES
Si      28.085 Si.pbe-n-kjpaw_psl.0.1.UPF.TXT
O      15.999 O.pbe-n-kjpaw_psl.0.1.UPF.TXT

ATOMIC_POSITIONS {angstrom}
Si    0.000000    0.000000    0.000000
Si    0.000000    0.000000    7.120000
Si    0.000000    7.120000    0.000000
Si    0.000000    7.120000    7.120000
Si    7.120000    0.000000    0.000000
Si    7.120000    0.000000    7.120000
Si    7.120000    7.120000    0.000000
Si    7.120000    7.120000    7.120000
Si    0.000000    3.560000    3.560000
Si    7.120000    3.560000    3.560000
Si    3.560000    3.560000    0.000000
Si    3.560000    3.560000    7.120000
Si    3.560000    0.000000    3.560000
Si    3.560000    7.120000    3.560000
Si    5.340000    1.780000    5.340000
Si    1.780000    1.780000    1.780000
Si    1.780000    5.340000    5.340000
Si    5.340000    5.340000    1.780000
  O    0.890000    0.890000    0.890000
  O    6.230000    2.670000    4.450000
  O    2.670000    4.450000    6.230000
  O    4.450000    6.230000    2.670000
  O    0.890000    4.450000    4.450000
  O    6.230000    6.230000    0.890000
  O    2.670000    0.890000    2.670000
  O    4.450000    2.670000    6.230000
  O    4.450000    0.890000    4.450000
  O    2.670000    2.670000    0.890000
  O    6.230000    4.450000    2.670000
  O    0.890000    6.230000    6.230000
  O    4.450000    4.450000    0.890000
  O    2.670000    6.230000    4.450000
  O    6.230000    0.890000    6.230000
  O    0.890000    2.670000    2.670000
  O   -0.890000   -0.890000    0.890000
  O   -0.890000    0.890000   -0.890000
  O    0.890000   -0.890000   -0.890000
  O   -0.890000   -0.890000    8.010000
  O   -0.890000    0.890000    6.230000
  O    0.890000   -0.890000    6.230000
  O    0.890000    0.890000    8.010000
  O   -0.890000    6.230000    0.890000
  O   -0.890000    8.010000   -0.890000
  O    0.890000    6.230000   -0.890000
  O    0.890000    8.010000    0.890000
  O   -0.890000    6.230000    8.010000
  O   -0.890000    8.010000    6.230000
  O    0.890000    8.010000    8.010000
  O    6.230000   -0.890000    0.890000
  O    6.230000    0.890000   -0.890000
  O    8.010000   -0.890000   -0.890000
  O    8.010000    0.890000    0.890000
  O    6.230000   -0.890000    8.010000
  O    8.010000   -0.890000    6.230000
  O    8.010000    0.890000    8.010000
  O    6.230000    8.010000   -0.890000
  O    8.010000    6.230000   -0.890000
  O    8.010000    8.010000    0.890000
  O    6.230000    6.230000    8.010000
  O    6.230000    8.010000    6.230000
  O    8.010000    6.230000    6.230000
  O    8.010000    8.010000    8.010000
  O   -0.890000    2.670000    4.450000
  O   -0.890000    4.450000    2.670000
  O    8.010000    2.670000    2.670000
  O    8.010000    4.450000    4.450000
  O    2.670000    4.450000   -0.890000
  O    4.450000    2.670000   -0.890000
  O    2.670000    2.670000    8.010000
  O    4.450000    4.450000    8.010000
  O    2.670000   -0.890000    4.450000
  O    4.450000   -0.890000    2.670000
  O    2.670000    8.010000    2.670000
  O    4.450000    8.010000    4.450000

1.) As far as I can tell, SiO2 is a cubic fcc lattice. The Espresso 
website (http://www.quantum-espresso.org/faq/input-data/#3.1) states 
that Espresso uses the primitive, rather than conventional, unit cell, 
unless one specifies otherwise. As I have elected ibrav=2, I infer I am 
now in the latter category. The directions also state that an all-atom, 
rather than symmetry-unique atom, representation is chosen. Would I thus 
specify the Cartesians of 18 Silicon atoms, for an fcc lattice? The code 
is such that it can recognize the symmetry of the atoms put in, parse 
out which ones are the unique atoms, and use that symmetry to not 
re-calculate the same integrals over-and-over, etc.? If it's helpful to 
visualize, I pulled this structure from a cif file from 
http://www.crystallography.net/cod/1010954.html

2.) I have specified the lattice constant as celldm(1)=13.454. I assume 
that the default, Bohr radius, is not affected by selecting the 
"Angstrom" flag later on.

3.) nat, according the manual, specifies the number of atoms in the unit 
cell. Does this mean the total number of atoms, i.e., 18+56 for SiO2 if 
I have selected ibrav=2, or does the code expect the primitive cell atom 
number since that's the Espresso default?

Thank you for your time,

Dr. Robert Molt
Indiana-University Perdue University
ENSCO