This keyword is needed only for for 1-, 2-, and 3-dimensional periodic systems.
The system keyword can assume the following values
- polymer -- system with 1-d translational symmetry (not currently available with NWPW module).
- surface -- system with 2-d translational symmetry (not currently available with NWPW module).
- crystal -- system with 3-d translational symmetry.
- molecule -- no translational symmetry (this is the default)
When the system possess translational symmetry, fractional coordinates are used in the directions where translational symmetry exists. This means that for crystals x, y and z are fractional, for surfaces x and y are fractional, whereas for polymers only z is fractional. For example, in the following H2O layer input (a 2-d periodic system), x and y coordinates are fractional, whereas z is expressed in Angstroms.
geometry units angstrom
O 0.353553 0.353553 2.100000000
H 0.263094 0.353553 2.663590000
H 0.444007 0.353553 2.663590000
Since no space group symmetry is available yet other than P1, input of cell parameters is relative to the primitive cell. For example, this is the input required for the cubic face-centered type structure of bulk MgO.
system crystal
lat_a 2.97692
lat_b 2.97692
lat_c 2.97692
alpha 60.00
beta 60.00
gamma 60.00
end