EXTRAPOLATION OF REAL-SPACE QUANTUM-CHEMICAL CALCULATIONS FROM FINITE-SIZE SUPERCELLS TO THE IDEAL INFINITE SYSTEM .3. APPLICATION TO 2-DIMENSIONAL SYSTEMS

A previously presented extrapolation method of the density matrix of a standard finite supercell calculation to an infinite supercell is ext ended to two-dimensional systems. The density matrix of the finite sup ercell is transformed into q space where it is interpolated and extrap olated in such a way that all its fundamental properties are guarantee d by construction. The resulting modified density matrix contains info rmation from a continuum of q points in the first Brillouin zone which allows for a more realistic calculation of properties like the total energy E-tot per atom and the band structure epsilon(i)(q). It is show n that this more realistic calculation takes better care of the crysta l symmetries and is essential in reproducing both important degeneraci es in the band structure and rotationally symmetric results. In the sp ecial case of pi electrons only, an exact analytical solution for the density matrix of the infinite supercell is presented. (C) 1997 John W iley & Sons, Inc.