[Pw_forum] Negative density of states

[stefano de gironcoli]

when using the smearing technique to compute the density of state you 
are broadening each energy level with an approximate delta function.
This can be a gaussian, the derivative of the fermi-dirac function or a 
gaussian multiplied by a polynomial.
In the first two cases the approximate delta function is positive 
definite, but for both methfessel-paxton and marzari-vanderbilt scheme 
the approximate delta function is not positive definite. (MV smearing is 
built to produce non-negative occupation numbers but it still can have 
negative contribution for its derivative).
This is the price to pay to have schemes for which the dependence of the 
total energy on the smearing width is quartic or cubic rather than 
quadratic as it would be otherwise.

In regions where the dos or ldos is non zero and if you're sampling the 
k-space sufficiently accurately this should not lead to negative values, 
but close to the band-edges it may
happen.

If you want to have positive definite dos/ldos use gaussian or FD smearings.

stefano


On 11/08/2015 00:47, Henry J Seeley wrote:
> Dear users,
>
> I have been using PWscf/Quantum Espresso for a couple months now, so
> although I'm familiar with the input/output formats and the parameters
> for calculations, I'm pretty oblivious to the inner workings of the
> program.
> I'm attempting to calculate the (local) density of states for lead
> sulfide slabs but I occasionally get negative density of states, which
> doesn't make any sense. I've used both dos.x and pp.x (ILDOS) and have
> gotten this result multiple times, sometimes only one and not the other
> for identical systems.
> Below is an input file, as well as a snippet of output data from pp.x
> (ILDOS).
>
>
> pp.x (ILDOS) output:
>
>           Position            ILDOS
> ...
>           2.9385797703        0.0000000145
>           2.9671096710        0.0000000685
>           2.9956395717        0.0000000096
>           3.0241694724        0.0000000231
>           3.0526993731       -0.0000000262    *negative?
>           3.0812292738        0.0000000213
>           3.1097591744       -0.0000000191    *negative?
>           3.1382890751        0.0000000124
>           3.1668189758        0.0000000510
>           3.1953488765        0.0000000245
>           3.2238787772        0.0000000443
> ...
>
>
>
> scf input:
>
> &CONTROL
>                          title = 'PbS.111thinPbline' ,
>                    calculation = 'scf' ,
>                         outdir =
> '/ibrix/users/home6/hseeley/HS2/8-4-2015/outdir/' ,
>                     pseudo_dir =
> '/ibrix/users/home6/hseeley/espresso-5.1.2/pseudo/' ,
>                         prefix = 'PbS.111thinPbline' ,
>    /
>    &SYSTEM
>                          ibrav = 0,
>                      celldm(1) = 11.19416,
>                            nat = 12,
>                           ntyp = 2,
>                        ecutwfc = 40 ,
>                    occupations = 'smearing' ,
>                        degauss = 0.02 ,
>                       smearing = 'marzari-vanderbilt' ,
>                           noncolin = .true. ,
>                           lspinorb = .true. ,
>    /
>    &ELECTRONS
>                    startingwfc = 'atomic+random' ,
>    /
>    &IONS
>    /
>
> CELL_PARAMETERS alat
>     -1.236988124   0.688397354   0.000000000
>     -0.608947267  -0.365438176   0.000000000
>     -1.198800945  -2.150150059   3.464101615
> ATOMIC_SPECIES
>      Pb  207.20000  Pb.rel-pbe-dn-kjpaw_psl.0.2.2.UPF
>       S   32.06500  S.rel-pbe-n-kjpaw_psl.0.1.UPF
> ATOMIC_POSITIONS alat
> S        1.195986189   2.158436994   6.057718495
> S        1.196613046   2.847559652   6.035203796
> S        1.607948658   2.157228920   6.639507369
> S        0.380505268   2.135992982   6.639522336
> S        0.802031048   1.425119068   7.221352165
> S        0.782293690   2.155298247   7.243857427
> Pb       1.004595715   2.477269862   5.817407381
> Pb       1.411636575   1.793701926   6.351977935
> Pb       1.404724785   2.500453276   6.338245575
> Pb       1.811166168   1.814015383   6.940832089
> Pb       0.586360087   1.789892412   6.927085220
> Pb       0.993402384   1.106318783   7.461627358
>
>
> K_POINTS automatic
>     10 10 5   0 0 0
>
>
>
> Any help would be much appreciated, thank you,
> Hank Seeley
> PhD Chemistry student
> University of Oregon
> hseeley at uoregon.edu
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