Dear readers,

I am using OpenMx 3.8 compiled with option -O2. The test runs showed that the compilation is successful with this flag. I am running a graphene system with 4 carbon atoms in a unit cell. In accordance with this unit cell, the Dirac point lies at P-point in reciprocal space. To do band calculations, if i use an E-cut of 250Ry (which i assume is already quite high) and grid size of (13 13 1) with C6-s2p1 PAO's, i get a band gap of about 0.001ev with cohesive energy 8.1ev. My question is why i am not getting a zero-band gap (-O2 flag was used for compilation) ? Please suggest

This is the input file that i am using for my calculations

#

File Name

#

System.CurrrentDirectory ./ # default=./
System.Name graphene
DATA.PATH ../../../../DFT_DATA13
level.of.stdout 1 # default=1 (1-3)
level.of.fileout 1 # default=1 (0-2)

#

Definition of Atomic Species

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Species.Number 1

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Atoms

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Atoms.Number 4
Atoms.SpeciesAndCoordinates.Unit Ang # Ang|AU

Atoms.UnitVectors.Unit Ang # Ang|AU

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SCF or Electronic System

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scf.XcType GGA-PBE # LDA|LSDA-CA|LSDA-PW|GGA-PBE
scf.SpinPolarization on # On|Off|NC
scf.ElectronicTemperature 300.0 # default=300 (K)scf.energycutoff 250.0 # default=150 (Ry)
scf.maxIter 200 # default=40
scf.EigenvalueSolver Band # DC|GDC|Cluster|Band
scf.lapack.dste dstevx # dstegr|dstedc|dstevx, default=dstegr
scf.Kgrid 13 13 1 # means n1 x n2 x n3
scf.Mixing.Type rmm-diisk # Simple|Rmm-Diis|Gr-Pulay|Kerker|Rmm-Diisk
scf.Init.Mixing.Weight 0.30 # default=0.30
scf.Min.Mixing.Weight 0.001 # default=0.001
scf.Max.Mixing.Weight 0.40 # default=0.40
scf.Mixing.History 5 # default=5
scf.Mixing.StartPulay 6 # default=6
scf.Mixing.EveryPulay 6 # default=6
scf.criterion 1.0e-10 # default=1.0e-6 (Hartree)

#

MD or Geometry Optimization

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MD.Type Nomd # Opt|EF|BFGS|RF|DIIS
MD.Opt.DIIS.History 6 # default=3
MD.Opt.StartDIIS 7 # default=5
MD.Opt.EveryDIIS 6 # default=10
MD.maxIter 200 #
MD.Opt.criterion 1.0e-4 # default=1.0e-4 (a.u.)

#

Band dispersion

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Band.dispersion on # on|off, default=off

if

#

Dos.fileout off # on|off, default=off
Dos.Erange -15.0 25.0 # default = -20 20
Dos.Kgrid 300 300 1 # default = Kgrid1 Kgrid2 Kgrid3

DosGauss.fileout off
DosGauss.Num.Mesh 4000
DosGauss.Width 0.01