This directory contains a program reading files OPTIC, POSCAR, and KPOINTS in
order to calculate the dielectric function on LDA level in the optical limit.
Currently only the tetrahedron method can be used for k-space integration and
you have to provide a pre-generated KPOINTS file with explicit k-point and
tetrahedron lists, automatic k-mesh generation is not supported by "optics"!

The program also reads a control file called "OPTCTR". As for INCAR the format
is completely free and defaults are defined for the parameters. You may specify:

OMMIN = minimum frequency to be considered (in equivalents of eV), default: 0
        (usually you will not touch this unless you want to resolve a certain
        energy range for Im(eps), the joint DOS or total DOS in very detail).
OMMAX = maximum frequency to be considered (in equivalents of eV), default: 20
NEDOS = number of data points on the frequency (= energy) axis, default: 4000
NBCON_USE = maximum number of conduction bands to be considered (usually the
            very highest bands are not so well converged and it may be useful
            to cancel them), default: about 0.85*NBCON (NBCON from OPTIC)
ISYMM = number of independent components of the dielectric tensot, it is 1 for
        isotropic (cubic) systems [(eps_xx+eps_yy+eps_zz)/3], 2 for uniaxial
        (tetragonal, hexagonal) systems [(eps_xx+eps_yy)/2 and eps_zz], 3 for
        biaxial systems (orthorhombic) [eps_xx, eps_yy and eps_zz], and 6 is
        taken in all other cases [not yet absolutely proper...], default: 1
LTET = logical flag switching on/off tetrahedron method, default: on
ISMEAR = smearing methods; -4/-5: tetrahedron method, <0: Lorentzian,
         0: Gauss, >0: Methfessel-Paxton (default: -1, Lorentzian smearing);
         WARNING: LTET must be EXPLICITLY set to .FALSE., LTET=.TRUE. (default)
         overrides anything (smearing would not work if only ISMEAR is set)!
SIGMA = smearing width (default: 0.1 eV)
LJDOS = logical flag switching on/off calculation of joint DOS, default: on
LDOS = logical flag switching on/off calculation of DOS, default: LDOS=LJDOS
LKRAMERS = logical flag switching on/off Kramers-Kronig transform for the
           calculation of the real part of the dielectric function, default: on
LEXTERNAL = logical flag signalling that a file with the imaginary part of the
            dielectric function already exists and that one likes to calculate
            the real part now (will force LKRAMERS=.TRUE.), default: .FALSE.
            If LEXTERNAL is set .TRUE. you have to provide an input file IMEPS.
LSEARCH = logical flag enabling a search/analysis of transition matrix elements
          as found on file OPTIC (default: off), result is written on standard
          output (header line: valence band, conduction band, spin component,
          which component of the dielectric tensor [1=xx, 2=yy, 3=zz, 4=xy,
          5=xz, 6=yz) , k point; second output line: transition energy, square
          of transition matrix elements in kinetic energy equivalents [eV],
          coordinates of k point; all records seperated by empty lines); for
          LSEARCH=.TRUE. no other action is taken (search only and exit)!
EMINSEARCH and EMAXSEARCH = energy window to be considered if LSEARCH=.TRUE.
                            (default: -1E30 eV to 1E30 eV, means "everything")
AMPMIN = minimum transition matrix element to be considered for printout
         if LSEARCH=.TRUE. (default: 0, means "everything"), allows for example
         to search for very strong transitions only
GAMMA = imaginary shift for Kramers-Kronig transform, default: 0.002
ETOP, EBOT or AZ = values determining the lower integration bound on the real
                   axis for Kramers-Kronig transform, condition: Im(eps) should
                   (still) be zero at omega=AZ otherwise you are in trouble,
                   defaults: EBOT=2. ; ETOP=0.05 ; AZ=0.5*(EBOT-ETOP)/(NEDOS-1)


The program will write following files:

JDOS  containing the joint DOS and the DOS (the energy scale of DOS is shifted,
      the shift is specified in the header of the file - this is necessary since
      the energy scale for joint DOS usually starts at zero but for DOS it can
      start anywhere, determined by the lowest eigenvalue which is usually <0).
      The format is   "Energy JDOS DOS"  and some column may contain zeros if
      one of the flags LJDOS or LDOS was set to .FALSE. and the other to .TRUE.

EPS   containing the real and imaginary part of the dielectric function. The
      format is   "Energy Re(eps) Im(eps)", depending on the setting of LKRAMERS
      the second column might contain zeros, in this case one may rename the
      file to IMEPS and can use it as input file together with LEXTERNAL=.TRUE.,
      reading from IMEPS in this case avoids accidental destruction of the input
      date in the case anything might go wrong (NFS problems, disk full, ...).
      In the header of this file you will also find a value for the effective
      plasma frequency im units of the expected one according to free electron
      gas theory, w_p,eff/w_p. This reflects the sum rule and should approach 1.