Dear Christopher,<br><br>1) the structure, in the input you provided, looks weird to me. Are you sure it is really what you intend to simulate?<br>2) The vc-relaxation process with symmetry conservation is driven by crystal symmetry. That is to say: cubic unit<br>
cell does not necessary mean cubic symmetry. Coming back to your structure, by visual inspection one can see<br>that it does not possess cubic symmetry (it is P1, most probably). Therefore off-diagonal terms of Bravais matrix<br>
evolved during cell-shape relaxation.<br><br>Best regards,<br>Maxim.<br><br><div class="gmail_quote">2011/5/20 Christopher Heard <span dir="ltr"><<a href="mailto:CJH085@bham.ac.uk">CJH085@bham.ac.uk</a>></span><br><blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
Hi,<br>
I have a question regarding vc-relaxation in relative coordinates.<br>
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.<br>
<br>
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.<br>
<br>
Cheers,<br>
Chris<br>
<br>
<br>
========================================================<br>
INPUT FILE<br>
<br>
&CONTROL<br>
calculation = "vc-relax",<br>
prefix = "QETEST",<br>
pseudo_dir = " /home/heard/pseudo ",<br>
outdir = ".",<br>
nstep = 10,<br>
/<br>
&SYSTEM<br>
ibrav = 0,<br>
nat = 3,<br>
ntyp = 2,<br>
ecutwfc = 20.D0,<br>
occupations = 'smearing'<br>
smearing = 'mp'<br>
degauss = 0.06<br>
/<br>
&ELECTRONS<br>
electron_maxstep = 1000,<br>
conv_thr = 1.D-6,<br>
mixing_beta = 0.3D0,<br>
/<br>
&IONS<br>
/<br>
&CELL<br>
cell_dynamics = 'bfgs',<br>
/<br>
CELL_PARAMETERS cubic<br>
30.23 0.0 0.0<br>
0.0 30.23 0.0<br>
0.0 0.0 30.23<br>
ATOMIC_SPECIES<br>
Cu 63.5 Cu.pbe-d-rrkjus.upf<br>
Ag 107.9 Ag.pbe-d-rrkjus.upf<br>
<br>
ATOMIC_POSITIONS {crystal}<br>
<br>
Cu 0.9110342051985 0.5412603911172 0.9652262133233<br>
Cu 0.3046428810907 0.3592799679414 0.0488897975312<br>
Ag 0.868139803195 0.7597487846068 0.8302658063157<br>
K_POINTS {Gamma}<br>
<br>
<br>
======================================================<br>
START OF OUTPUT FILE<br>
<br>
bravais-lattice index = 0<br>
lattice parameter (a_0) = 30.2300 a.u.<br>
unit-cell volume = 27625.7732 (a.u.)^3<br>
number of atoms/cell = 3<br>
number of atomic types = 2<br>
number of electrons = 33.00<br>
number of Kohn-Sham states= 21<br>
kinetic-energy cutoff = 20.0000 Ry<br>
charge density cutoff = 80.0000 Ry<br>
convergence threshold = 1.0E-06<br>
mixing beta = 0.3000<br>
number of iterations used = 8 plain mixing<br>
Exchange-correlation = SLA PW PBE PBE (1434)<br>
EXX-fraction = 0.00<br>
nstep = 10<br>
<br>
<br>
celldm(1)= 30.230000 celldm(2)= 0.000000 celldm(3)= 0.000000<br>
celldm(4)= 0.000000 celldm(5)= 0.000000 celldm(6)= 0.000000<br>
<br>
crystal axes: (cart. coord. in units of a_0)<br>
a(1) = ( 1.000000 0.000000 0.000000 )<br>
a(2) = ( 0.000000 1.000000 0.000000 )<br>
a(3) = ( 0.000000 0.000000 1.000000 )<br>
<br>
===========================================================<br>
OUTPUT COORDINATES<br>
<br>
new unit-cell volume = 27844.52160 a.u.^3 ( 4126.13235 Ang^3 )<br>
<br>
CELL_PARAMETERS (alat= 30.23000000)<br>
1.003485881 0.000341145 -0.000166335<br>
0.000341140 1.002476151 0.000847099<br>
-0.000166325 0.000846982 1.001936910<br>
<br>
ATOMIC_POSITIONS (crystal)<br>
Cu 0.911117504 0.540084682 0.965438693<br>
Cu 0.305212112 0.357448827 0.044305096<br>
Ag 0.867487274 0.762755634 0.834638028<br>
===========================================================<br>
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</blockquote></div><br><br clear="all"><br>-- <br>Best regards, Max Popov<br>Ph.D. student<br>Materials center Leoben (MCL), Leoben, Austria.<br>