type: Entry
title: Periodic Hamiltonian 
name: Periodic
keyword: PERIODIC
is_a: Hamiltonian
description: >
  A bulk material simulated with periodic or twisted boundary conditions.
related: []
required:
  - keyword: NSPIN
    type: section
    description: Two integers, the first the number of spin up electrons and the second the number of spin down.
  - keyword: ATOM
    type: section
    description: >
       Add an atom to the system.  The format of 
       the section is label  charge COOR x y z .  For example, a Lithium atom 
       with atomic charge 3 at (.5, 0, -.5) would be ATOM \{ Li 3 COOR .5 0 -.5 \}.
  - keyword: LATTICEVEC
    type: section
    description: >
      Enforce the 
      periodic boundary conditions as a parallelepiped, with three vectors specifying
      the edges.  For example, a cubic cell with sides 1.0 a.u. is: <br>
      LATTICEVEC { 
       1.0 0.0 0.0 
       0.0 1.0 0.0 
       0.0 0.0 1.0 
      }
optional: 
  - keyword: KPOINT
    type: section
    default: { 0 0 0 } 
    description: >
     Specify the k-point (i.e.,
     boundary conditions) at which the trial wave function is
     constructed. For instance, 0 selects periodic and 1
     selects anti-periodic boundary condition along the
     corresponding lattice vector.
  - keyword: CUTOFF_DIVIDER
    type: Float
    default: 1.0
    description: > 
          Control how many unit cells to generate ghost 
          centers around the simulation cell.  1 is one unit cell, 2 is one 
          half a unit cell, and .5 is 2 unit cells. This will often be set correctly by the 
          conversion program.
  - keyword: EWALD_GMAX
    type: integer
    default: 200
    description: > 
        How far to search to generate the k-mesh for the Ewald summation. Only the vectors with significant weights are kept. If you have a cell with a lattice vector larger than around 300 Bohr, this may need to be increased.


