Specification of the system


The input of the initial structure can be given with the following key System. Apart from cases in which the system is retrieved by restarting a previous calculations this specification is always required.

        Atom Coords

    {Charge NetQ}


         eps1 value
         eps2 value
         eps3 value
         eps4 value
         eps5 value
         eps6 value


      Atom Mass

The System key accepts a set of sub-keys to specify various details of the chemical system under evaluation


Specifies the geometry of the molecular system as a list of rows, one row per atom.

The name of an atom type. It must be the standard one- or two-characters symbol for the chemical element: H, He, Li, and so on.
This specifies the coordinates of the atom. The x, y, z values of the Cartesian coordinates are by default interpreted in Ångstrom.

The net charge of the molecule can be controlled with this optional sub-key. If this sub-key is omitted the net total charge of the molecule is by default zero.

The net total charge of the molecule.

Information about the periodicity of the system is given through this sub-key. Its presence is optional, and it implies a periodic system. The subsequent computation will therefore evaluate the system accordingly. A list of up to three vectors (one per row) for the cell must be specified.

Three floating point values defining the periodicity vector along a given direction. One, two, or three vectors can be specified (each on a different row) to express linear, planar or bulk periodicity, respectively. If one vector is specified, periodicity must develop along the x axis. If two vectors are specified, periodicity must develop along the xy plane. The unit is the same as in the Atoms section.

Allows the application of a strain tensor to the lattice. The values of eps1 to eps6 represent the unique elements of the strain tensor, as follows:

eps1  eps6  eps5
eps6  eps2  eps4
eps5  eps4  eps3
This optional keyword modifies how the Atoms coordinates are interpreted. When the keyword is present, coordinates will be interpreted as fractions of the periodicity lattice vectors, instead of absolute geometric positions in 3D space. Necessarily, the presence of this sub-key requires Lattice to be specified.

The default atomic masses used in DFTB are taken from Nuclear Physics A 565, 1-65 (1993). The atomic masses of selected atom types can be redefined via the AtomMasses block key. The units of the atomic mass is unified atomic mass unit.

Example: Three different hydrogen isotopes:

   N         0.000000    0.000000    0.010272
   H        -0.471582   -0.816803    0.407861
   H.D       0.943163    0.000000    0.407861
   H.T      -0.471582    0.816803    0.407861
   H.D 2.01410
   H.T 3.01605


The Units key is optional. It allows to specify different units for Length, Angle, and Time, in place of the default ones.

  {length    angstrom|bohr}
  {angle     degree|radian}
  {time      femtosec|au}

The default values are angstrom, degree, and femtosec.