grid{}

Specifications of the non-uniform rectangular grid lines.

xgrid{} / ygrid{} / zgrid{}

These grid lines set the total device size, so at least two lines have to be present for each relevant direction.

grid{
   xgrid{ ... }
   ygrid{ ... }  # only 2D/3D
   zgrid{ ... }  # only 3D
grid{
   xgrid{   # x position of line and adjacent grid spacing
      line{ pos = 0       spacing = 0.5 }         # 0.5 nm spacing at position   0 nm
      line{ pos = 50      spacing = 0.1 }         # 0.1 nm spacing at position  50 nm
      line{ pos = 100     spacing = 0.5 }         # 0.5 nm spacing at position 100 nm
   }
pos
value

any float

position of the line [nm]
spacing
value

any float >= 0.0

grid spacing adjacent to this line [nm]
Minimum spacing is 10^-3 nm, i.e. 1 pm.
 grid{
    xgrid{ ... }

    ygrid{   # y position of line and adjacent grid spacing (2D or 3D only)
       line{ pos = 0       spacing = 0.5 }         # 0.5 nm spacing at position   0 nm
       line{ pos = 50      spacing = 0.1 }         # 0.1 nm spacing at position  50 nm

             # Here, two grid spacings are defined for y=75 nm, i.e. for y<75 nm a grid spacing of 0.1 nm
             # - and for y>75 nm a grid spacing of 0.5 nm is used.
       line{ pos = 75      spacing = 0.1 }  # <==  # 0.1 nm spacing at position  75 nm
       line{ pos = 75      spacing = 0.5 }  # <==  # 0.5 nm spacing at position  75 nm

       line{ pos = 100     spacing = 0.5 }         # 0.5 nm spacing at position 100 nm
}

Note: This example where at 75 nm an abrupt change in grid spacing is enforced is a bit dangerous. This can have impacts on numerical stability and numerical accurateness as discretized equations are only approximations to the continuous equations being solved. For nonuniform grids, higher order terms for the first or second derivates might get too large compared to slightly nonuniform grids. Abrupt changes in grid spacing rather should be avoided if possible. A corresponding warning message is written to the log file for duplicate grid points and jumps in grid points.

zgrid{   #  (3D only)
   line{ pos = 0       spacing = 0.5 }   # 0.5 nm spacing at position   0 nm
   line{ pos = 50      spacing = 0.1 }   # 0.1 nm spacing at position  50 nm
   line{ pos = 100     spacing = 0.5 }   # 0.5 nm spacing at position 100 nm
}

Note: The program generates a grid that has the specified spacing adjacent to the specified grid lines. If the grid is not equidistant, an exponential grid is generated. In this case, the calculated spacing corresponds only approximately to the specified spacing. The grid spacing used might have influence on the numerical convergence of the equations.

The actually generated grids are also written into files grid_x.dat, grid_y.dat, and grid_z.dat. Especially for more complicate grid definitions or grid definitions employing variables, it is recommended to review these files to make sure that the results match your expectations. You can also use nextnanomat \(\rightarrow\) Output \(\rightarrow\) Show grid to visualize the grid lines.

Repeating grid lines In the same way as regions (==> region{}) can be repeated, also grid lines can be repeated which might be useful for multi-quantum wells, superlattices, Quantum Cascade Lasers, Bragg reflectors, etc. Also, it is possible to make the number of repetitions a variable in a corresponding template.

array{} copies the line object located at pos a few times, namely min times into the negative direction and max times into the positive direction, i.e. the loop runs from -shift*min to shift*max.

xgrid{
  line{ pos = 11.0  spacing = 0.5  # 0.5 nm spacing at position 11.0 nm
    array{ # (optional)
               shift = 10.0    # repeat grid line in x direction by shifting it 10.0 nm (in units of [nm])
               max   = 3       # repeat grid line in x direction by applying the shift 3 (=max) times (Here, 4 lines will be set: The original one specified at pos, and 3 shifted ones.)
                               # max = 0 does not set a repeated (=shifted) grid line; negative values are not allowed.
               min   = 2    }  # (optional, default is 0) repeat grid line in negative x direction 2 times
                               # min = 0 does not set a repeated (=shifted) grid line; negative values are not allowed.
                               # In this example, the grid line at 11.0 nm is repeated 2 (=min) times into the negative direction and 3 (=max) times into the positive direction,
                               # i.e. additional grid lines at 1.0 nm, -9.0 nm and 21.0 nm, 31.0 nm, 41.0 nm will be set.
    array2{ *same as array{}* }  # (optional)

For meaning of array, see array_x{} in structure{}.

For meaning of array2, see array2_x{} in structure{}.

The grid line definitions line{} are processed in the order of their occurrence in order to obtain a list of default grid lines. Here, within each line{} definition, duplicate grid lines created by array{} and array2{} are ignored. After all line{} definitions have been processed, the list of default grid lines is ordered in ascending position (but the order of doubled lines remains unchanged), and intermediate grid lines are added as suggested by the respective grid spacings. Note that array{} and array2{} may accidentally cause doubled grid lines between different line{} definitions. In this case, resulting jumps in grid spacings may be nonobvious and should be checked in the output.

xgrid{
  min_pos = 100.0       # (optional) nm
  max_pos = 200.0       # (optional) nm
  line{ ... }
}

Using the optional variables min_pos and max_pos (defaults are -Infinity and +Infinity), a (half-)range can be defined for each coordinate direction within which all grid lines to be used are located. Grid lines located outside (e.g. due to a position defined by a formula or by a repetition) will be ignored. Please note that specifying a (half-)range in itself does not specify a grid line for the respective range limit(s). Similarly, there is no automatic insertion of grid lines as controlled by the respective spacings between a grid line defined inside of a (half-)range and an adjacent grid line defined outside of the (half-)range.

Per default, all defined grid lines are used for the computation, with the boundaries of the simulation region being defined by the smallest and largest coordinate positions of all grid lines defined for a respective coordinate direction. However, there are circumstances where structural features are partially or completely outside of the simulation region, and thus no grid lines are allowed for these structural features. Here, in order to facilitate reuse of coordinate calculations between region definitions and grid definitions, it is helpful to set min_pos and max_pos to the desired extent of the simulation region in order to prevent unwanted grid lines from being generated. All grid line definitions from line{} which fall outside of the interval [min_pos, max_pos] are ignored. Thus, if the such defined interval is [100, 200], an individual line with pos = 250 would be ignored. Similarly, when a line is repeated using array{} and/or array2{} multiple times, the line repetitions falling outside of the interval would be ignored.

Jumps in grid spacing

By default, jumps in grid spacings are not allowed. Optionally, they can be switched on.

allow_spacing_jumps
options

yes or no

default

no

xgrid{
  ...
  allow_spacing_jumps = yes
  line{ pos = 0       spacing = 0.5 }   # 0.5 nm spacing at position   0 nm
  line{ pos = 50      spacing = 0.5 }   # 0.5 nm spacing at position  50 nm
  line{ pos = 50      spacing = 0.1 }   # 0.1 nm spacing at position  50 nm
  line{ pos = 100     spacing = 0.1 }   # 0.5 nm spacing at position 100 nm
}

Here, left of the point at x=50 nm, a grid spacing of 0.5 nm is used, and right of the point at x=50 nm, a grid spacing of 0.1 nm is used which is a jump in grid spacing. allow_spacings_jumps = yes is not recommended due to a loss in numerical accuracy and numerical stability. The only purpose of this feature is to generate input files that are compatible to input files of the nextnano³ software which supports jumps by default.