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== Ecut ==


Used for kinetic energy cutoffwhich controls numberof planewaves at given k point by: (1/2)[(2 Pi)*(k+Gmax)]2=ecut for Gmax. All planewaves inside this "basis sphere" centeredat k are included in the basis (except if dilatmxis defined). Can be specified in Ha (the default), Ry, eV or Kelvin, sinceecut has the'ENERGY' characteristics.(1 Ha=27.2113845 eV) This is the single parameter which can have an enormouseffect on the quality of a calculation; basically the largerecut is, the better converged the calculation is. For fixedgeometry, the total energy MUST always decrease as ecut israised because of the variational nature of the problem.
Usually one runs at least several calculations at various ecutto investigate the convergence needed for reliable results.
For k-points whose coordinates are build from 0 or 1/2,the implementation of time-reversal symmetry that linkscoefficients of the wavefunctions in reciprocal spacehas been realized. See the input variable istwfk.If activated (which corresponds to the Default mode),this input variable istwfk will allow todivide the number of plane wave (npw) treated explicitlyby a factor of two. Still, the final result should be identical withthe 'full' set of plane waves.
See the input variable ecutsm, for thesmoothing of the kinetic energy, needed to optimize unit cell parameters.