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<div style="direction: ltr;font-family: Tahoma;color: #000000;font-size: 10pt;">Dear all,<br>
<br>
I'm relaxing the structure of rutile TiO2 using ultrasoft pseudopotentials for Ti and O and PBEsol + U with U=5 eV on Ti. For that purpose I use occupations = 'fixed'. But I found that one of the Ti ions has negative occupations. To me this looks a bit strange
since I'm not using any smearing. The same result can be obtained by another set of PP. Improving the cutoffs or the k-point sampling does not resolve the problem. Any thoughts about the reason and possible remedies are appreciated. I copy below the occupation
matrices for the two Ti ions in the cell and the input file.<br>
<br>
<br>
Thank you,<br>
Mostafa Youssef<br>
MIT<br>
<br>
The matrices are:<br>
<br>
atom 1 Tr[ns(na)] = 2.70769<br>
eigenvalues: <br>
0.142 0.177 0.182 0.417 0.436<br>
eigenvectors:<br>
0.675 0.000 0.000 0.325 0.000<br>
0.000 0.000 0.500 0.000 0.500<br>
0.000 0.000 0.500 0.000 0.500<br>
0.000 1.000 0.000 0.000 0.000<br>
0.325 0.000 0.000 0.675 0.000<br>
occupations:<br>
0.231 0.000 0.000 0.000 -0.129<br>
0.000 0.309 -0.127 0.000 0.000<br>
0.000 -0.127 0.309 0.000 0.000<br>
0.000 0.000 0.000 0.177 0.000<br>
-0.129 0.000 0.000 0.000 0.328<br>
atom 2 Tr[ns(na)] = 2.70768<br>
eigenvalues: <br>
0.142 0.177 0.182 0.417 0.436<br>
eigenvectors:<br>
0.675 0.000 0.000 0.325 0.000<br>
0.000 0.000 0.500 0.000 0.500<br>
0.000 0.000 0.500 0.000 0.500<br>
0.000 1.000 0.000 0.000 0.000<br>
0.325 0.000 0.000 0.675 0.000<br>
occupations:<br>
0.231 0.000 0.000 0.000 0.129<br>
0.000 0.309 0.127 0.000 0.000<br>
0.000 0.127 0.309 0.000 0.000<br>
0.000 0.000 0.000 0.177 0.000<br>
0.129 0.000 0.000 0.000 0.328<br>
N of occupied +U levels = 5.415371<br>
<br>
The input file:<br>
&CONTROL<br>
calculation = 'vc-relax' ,<br>
verbosity = 'high' ,<br>
restart_mode = 'from_scratch' ,<br>
outdir='./' ,<br>
pseudo_dir = './' ,<br>
tstress = .true. ,<br>
tprnfor= .true. ,<br>
nstep = 2000 ,<br>
etot_conv_thr = 1.0D-7 ,<br>
forc_conv_thr = 4.0D-5 ,<br>
/<br>
&SYSTEM<br>
ibrav = 0 ,<br>
nat = 6 , <br>
ntyp = 2 , <br>
nbnd = 48 ,<br>
ecutwfc = 45 ,<br>
ecutrho = 360 , <br>
nosym =.true.<br>
occupations = 'fixed',<br>
lda_plus_u = .true.,<br>
Hubbard_U(1) = 5.0 ,<br>
Hubbard_U(2) = 0.0 ,<br>
/<br>
&ELECTRONS<br>
diagonalization='david',<br>
mixing_mode = 'plain' ,<br>
mixing_beta = 0.7,<br>
startingwfc = 'random',<br>
conv_thr = 1.0d-8 , <br>
/<br>
&ions<br>
ion_dynamics='bfgs'<br>
/<br>
&CELL<br>
cell_dynamics = 'bfgs' ,<br>
cell_dofree = 'shape',<br>
press_conv_thr = 0.5 ,<br>
/<br>
CELL_PARAMETERS angstrom<br>
4.6320252669841118 0.0000000000000000 0.0000000000000000<br>
0.0000000000000000 4.6320252669841118 0.0000000000000000<br>
0.0000000000000000 0.0000000000000000 2.9608930000414442<br>
ATOMIC_SPECIES<br>
Ti 47.867 ti_pbesol_v1.4.uspp.F.UPF<br>
O 15.9994 o_pbesol_v1.2.uspp.F.UPF<br>
ATOMIC_POSITIONS crystal <br>
Ti 0.0000000000000000 0.0000000000000000 0.0000000000000000<br>
Ti 0.5000000000000000 0.5000000000000000 0.5000000000000000<br>
O 0.3048741584772117 0.3048741584772117 0.0000000000000000<br>
O 0.6951258415227883 0.6951258415227883 0.0000000000000000<br>
O 0.8048741584772117 0.1951258415227883 0.5000000000000000<br>
O 0.1951258415227883 0.8048741584772117 0.5000000000000000<br>
K_POINTS automatic <br>
4 4 6 1 1 1<br>
<br>
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