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## Description of the command structure

The input command file consists of separate lines each containing

-
  a label
-
  a label followed by a character, integer and/or fix-point or
-
  a character, integer and/or fix-point constant.

Note that

-
  labels can be in upper or lower case,

-
  compulsory labels, i.e. these that must be included in the input command file, are marked ``'' and the optional ones ``'',

-
  the compulsory labels must follow the order given below; the optional ones can be inserted anywhere between title and stop labels,

-
  optional parameters are enclosed in square brackets,

-
  denotes a fix-point constant, - an integer, - a string of characters,

-
  exclamation mark placed anywhere in an input line starts a comment; what follows "!" is ignored.

The following labels can be used:

$&bull#bullet;$
  TITLE
  Format:
    title
    is any string of up to 80 characters describing the current case. This string is used as a header of disk output files containing orbitals and potentials.

$&bull#bullet;$
  METHOD
  Format:
    method
    Select the type of calculation.
    :
      HF - Hartree-Fock method
    :
      HFS - Hartree-Fock-Slater method
    :
      OED - One Electron Diatomic states

$&bull#bullet;$
  NUCLEI
  Format:
    nuclei
    Set the nuclei charges and the bond length.
    :
      nuclear charge of centre A (integer)
    :
      nuclear charge of centre B (integer)
    :
      bond length (real)
    :
      - the internuclear separation can be given in angstrom units if this string is included (the conversion factor 0.529177249 is used)
    If then the molecule is considered to be a homonuclear one (this threshold can be changed by redefining HOMOLEVL variable in blk_data.inc.raw).

  HOMO
  Format:
    homo
    This label is used to impose explicitly symmetry upon orbitals of homonuclear molecules in order to improve scf/sor convergence.

  BREAK
  Format:
    break
    When this label is present homonuclear molecules are calculated in symmetry and the symmetry labels (u or g) are superfluous (see below).

$&bull#bullet;$
  CONFIG
  Format:
    config
    :
      the total charge of a system

  The following cards define molecular orbitals and their occupation. Note that the last orbital description card must contain an extra label .

  The possible formats are:

  Format:
    :
      number of fully occupied orbitals of a given irreducible representation (irrep) of the group; 2 electrons make orbitals fully occupied and 4 electrons are needed for filling the orbitals belonging to the other irreps
    :
      symbol of the irrep to which the orbitals belong (sigma, pi, delta or phi)

  Format:
    :
      number of fully occupied orbitals of a given irrep of the group
    :
      symbol of the irrep to which the orbitals belong (sigma, pi, delta or phi)
    :
      symbol for the inversion symmetry of the irreps (u or g)
    Use this format for a homonuclear molecule unless break command is included.

  Format:
    :
      number of orbitals of a given irrep of the group
    :
      symbol for the irreps to which the orbitals belong (sigma, pi, delta, phi)
    -:
      or . (a dot); denotes spin up/down electron and . denotes an unoccupied spin orbital

  Format:
    :
      number of orbitals of a given irrep of the group
    :
      symbol for the irrep to which the orbitals belong (sigma, pi, delta, phi)
    :
      symbol for the inversion symmetry of the irrep (u or g)
    -:
      or . (a dot); denotes spin up/down electron and . denotes an unoccupied spin orbital

  GRID

  Two possible formats are (the second one is retained for the backward compatibility):

  Format:
    grid
    An integer and a real define a single 2d grid.
    :
      the number of grid points in variable
    :
      the practical infinity
    is calculated so as to make the step size in variable equal to the stepsize in variable. and have to meet special conditions. If the conditions are not fulfilled the nearest (but smaller) appropriate values are used.

  Format:
    grid
    Two integers and one real define a single 2d grid.
    :
      the number of grid points in variable
    :
      the number of grid points in variable
    :
      the practical infinity
    and have to meet special conditions. If the conditions are not fulfilled the nearest (but smaller) appropriate values are used.

  SUBGRID
  Format:
    subgrid
    Define up to 3 subgrids.
    :
      number of subgrids (integer)
    :
      the number of grid points in variable (integer)
    :
      number of points in the direction for each of the subgrids (integers)
    :
      the step size in variable for the first subgrid (real)
    :
      step sizes (reals) for other subgrids are defined as ,

  Note that either GRID of SUBGRID label must be included in the input command file.

  INTERP
  Format:
    interp
    Use this label to change the grid between separate runs of the program. The restriction is that only the number of grid points in one of the variables or can be changed at a time.

$&bull#bullet;$
  INITIAL
  Format:
    initial
    :
      determine the initial source of orbitals and potentials:
      - - molecular orbitals are formed as a linear combination of hydrogenic functions on centres and ; in the case of HF or HFS calculations Coulomb (exchange) potentials are approximated as a linear combination of Thomas-Fermi () potentials at the two centres; if method OED is chosen the potential function is approximated as a linear combination of and terms and the exchange potentials are set to zero

      - - GAUSSIAN94 output is used to retrieve exponents and expansion coefficients of (uncontracted) molecular orbitals (it is assumed that the output is contained in gauss94.out and gauss94.pun files) and Coulomb and exchange potentials are initialized as in case; see routine PREPG94 for more details

      - - GAUSSIAN94 modified output is used to retrieve exponents and expansion coefficients of molecular orbitals (it is assumed that the output is contained in gauss94l.out file) and Coulomb and exchange potentials are initialized as in case; see routine PREPG94L for details

      - - initial orbitals and potentials are retrieved from disk files created in a previous run

      - - orbitals and Coulomb potentials are retrieved from disk files and exchange potentials are initialized as in case (convenient when going from HFS to HF calculations)

    :
      specifies how exchange potentials will be read/written and manipulated (stored in memory). The program always keeps all orbitals and Coulomb potentials in core. The exchange potentials can also be all kept in core (if there is enough memory). However, during a relaxation of a particular orbital only a fraction of them is in fact needed. Thus all exchange potentials can be kept on disk as separate files (named fort.31, fort.32, ... during a run) and only relevant ones are being retrieved when necessary.1
      - - read exchange potentials as separate files and write them back as separate files
      - - read all exchange potentials in a file but write them out as separate files
      - - read all exchange potentials separately but write them out as a single file
      - - read and write exchange potentials in the form of a single file

    :
      if then this parameter must be set to 1 or 2. In such a case the initialization of each of the orbitals has to be defined in terms of the linear combination of atom centered hydrogen-like functions. For each orbital include a card of the following format (the order of orbitals should match the order specified under the config label):
      Format:
        where
          - mixing coefficient for a hydrogenic orbital on the centre (real),
          - its principle quantum number (integer)
          - its orbital quantum number (integer)
          - the effective nuclear charge if or a screening parameter if (real)
          - mixing coefficient for a hydrogenic orbital on the centre (real),
          - its principle quantum number (integer)
          - its orbital quantum number (integer)
          - the effective nuclear charge if or a screening parameter if (real)
          - set to 1 to freeze the orbital during scf; otherwise set to 0 (integer)
          - a number of successive over-relaxations for a given orbital (integer); if omitted is set to 10

  FEFIELD
  Format:
    fefield
    :
      a strength of an external static electric field directed along the internuclear axis (in atomic units)

  MULTIPOL
  Format:
    multipol
    :
      if multipole moment expansion coefficients are recalculated when the maximum error in orbital energy is reduced by (the default value is 1.15). To suppress recalculation of the coefficients set to a negative real number. This is useful when generating potentials from a set of fixed orbitals, e.g from GAUSSIAN94 orbitals.

  SCF
  Format:
    scf
    :
      maximum number of scf iterations (default 1000); to skip the scf step set to a negative integer,
    :
      every scf iterations orbitals and potentials are saved on disk (default 100). If functions are saved on disk upon completion of the scf process. If functions are never written to disk,
    :
      if the maximum error in orbital energy is less than than the scf process is terminated (the default value is 10),
    :
      if the maximum error in orbital norm is less than than scf process is terminated (the default is 10),
    :
      the level of output during scf process
      - - the orbital convergence rate, orbital energy and normalization of every orbital is printed in every scf iteration
      - - the orbital convergence rate, orbital energy and normalization of the worst converged orbital is printed in every scf iteration (default)
      - - the orbital convergence rate, orbital energy and normalization of the worst converged orbital is printed every iterations. Printing of ``... multipole moment expansion coefficients (re)calculated ...'' communique is suppressed
    Total energy is printed every iterations.

  FIX
  Format:
    fix
    If , or are set to 1 then orbitals, Coulomb potentials or exchange potentials, respectively, are kept frozen during the scf/sor process (the respective default values are 0, 0 and 2). If then exchange potentials are relaxed only once during an scf cycle. and cannot be set to 1 if hydrogenic orbitals are used to initiate the orbitals.

  XALPHA
  Format:
    xalpha
    If this label is present the Slater exchange approximation, i.e. , is used and the parameter can be modified. This approximation is useful to quickly improve the initial HF orbitals. is a real number and 0.7 is its default value.

  SOR
  Format:
    sor
    Change default values of sor parameters.
    :
      the number of (MC)SOR iterations (over each subgrid) for a given function being relaxed in a single SCF cycle (the default value is 10)

    :
      a scaling factor used to change when relaxing potentials (see RELCOUL1/2 and RELEXCH1/2). The new value of (MC)SOR iterations is calculalated as /; the default value of is 1.

    :
      if SOR (MCSOR) method is used to solve Poisson equations for orbitals and potentials (the default value is 1); if SOR method is used to solve Poisson equations for orbitals and MCSOR - for potentials

$&bull#bullet;$
  OMEGA
  Format:
    omega
    Up to three real numbers in each line setting over-relaxation parameter for relaxation of orbitals and potentials for each subgrid. If the second or the third parameter is omitted, the value for the first subgrid is used.

    Note that a semiempirical formula can be used to calculate a near-optimal value of by specifying as a negative real number.

  ORDER
  Format:
    order
    Up to three integers defining the ordering of mesh points on subgrids.
    :
      natural ordering
    :
      'middle' type of sweep (the default)

  FERMI
  Format:
    fermi
    When this label is present, the Fermi nuclear charge distribution is used. and define the atomic masses (in amu) of nuclei A and B.

  GAUSS
  Format:
    gauss
    When this label is present, the Gauss nuclear charge distribution is used. and define the atomic masses (in amu) of nuclei A and B.

  DEBUG
  Format:
    debug
    Up to 20 different debug flags can be set at a time. If the integer is encountered the debug flag is set, i.e. idbg .
$&bull#bullet;$
  STOP
  Format:
    stop
    This label indicates the end of input data.

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