Optical Properies for the case of NANOSHEETS

[giacomo giorgi]

Dear All,
I am not able to figure out the reason of such behaviour.
I calculated LEPSILON of a transition metal Nanosheet from this my INCAR:

SYSTEM = Si_Vac_Td

# xc
GGA = PE
VOSKOWN = 1
ISTART = 1
LREAL = .False.
PREC= High
ISMEAR = -5
SIGMA = 0.05


#electronic RELAXATION
NELMIN = 4
NELM = 200

ISPIN=2
MAGMOM=4*3.0 8*0.

IBRION = 1

EDIFF = 1.E-5
EDIFFG = -0.005 ! inizialmente -0.004

NGX=44
NGY=44
NGZ=88

NSW = 0

AMIX = 0.2
BMIX = 0.00001
AMIX_MAG = 0.8
BMIX_MAG = 0.00001

LORBIT = 11
ALGO= Fast

ICHARG = 2
LEPSILON= .TRUE.
LRPA= .TRUE.

Obtaining this:

HEAD OF MICROSCOPIC STATIC DIELECTRIC TENSOR (independent particle, excluding Hartree and local field effects)
------------------------------------------------------
5.754697 0.004359 -0.076709
0.004359 5.750777 0.127533
-0.076712 0.127540 3.245107
------------------------------------------------------


THEN, I want to calculate the number of empty bands I need for LOPTICS calculation.
My system has 72 electrons.I used this INCAR file for LOPTICS calculation


SYSTEM = Si_Vac_Td

GGA = PE
VOSKOWN = 1
ISTART = 1

LREAL = Auto
PREC= High

ISMEAR = -5
SIGMA = 0.05

#electronic RELAXATION
NELMIN = 4
NELM = 200

ISPIN=2
MAGMOM=4*3.0 8*0.

IBRION = 1

EDIFF = 1.E-5
EDIFFG = -0.005 ! inizialmente -0.004

NGX=44
NGY=44
NGZ=88

#ionic steps
NSW = 0

AMIX = 0.2
BMIX = 0.00001
AMIX_MAG = 0.8
BMIX_MAG = 0.00001

LORBIT = 11
ALGO= Fast

ICHARG = 11
NEDOS = 2000
NBANDS = 216
LOPTICS=.TRUE.

NPAR=1


and I got this TOTALLY inconsistent result

frequency dependent IMAGINARY DIELECTRIC FUNCTION (independent particle, no local field effects)
E(ev) X Y Z XY YZ ZX
--------------------------------------------------------------------------------------
0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000
0.060473 234.119842 234.925528 0.855976 -0.119609 11.929655 -7.117063
0.120947 343.260688 346.185785 1.288430 -0.791147 17.599514 -10.470146
......
......


frequency dependent REAL DIELECTRIC FUNCTION (independent particle, no local field effects)
E(ev) X Y Z XY YZ ZX
--------------------------------------------------------------------------------------
0.000000 542.767235 542.109189 5.858583 -0.727117 27.386045 -16.431252
0.060473 455.924465 455.697459 5.562982 -0.949792 23.007099 -13.806761
0.120947 274.937030 272.941386 4.941564 -0.956293 13.732510 -8.313677
0.181420 114.117603 108.135992 4.382102 -0.246997 5.349250 -3.393894
0.241893 0.999424 -5.076931 4.000804 0.350195 -0.412819 0.077756
0.302367 -60.275526 -62.318263 3.824766 0.392349 -3.327931 1.961089
....
.....


Results form omega-->0 and from the static dielectric constant calculations are neither qualitatively comparable......

I wonder if the 2D (Nanosheet) system's optical properties can be calculated with VASP.

Thanks a lot for any possible suggestion!

Very best regards,
Giacomo

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