HARTREE-FOCK PERTURBED-CLUSTER TREATMENT OF LOCAL DEFECTS IN CRYSTALS .2. THE ENERGY-DEPENDENT COUPLING MATRICES

This paper is the second of a series to illustrate the characteristics of EMBED, a computer program for the ab-initio Hartree-Fock study of defects in crystals. The subjects are the energy-dependent coupling matrices M(e), which are a key quantity of the perturbed-cluster theory, since they are used for coupling the cluster pseudo-eigen-function to the host-crystal solution. A finite-temperature formulation is introduced for these matrices: they are defined as an integral over the energy axis (lambda) of the projected density of states (PDOS), rho(lambda), times a ''thermal function'', [f(lambda) - f(e)]/(e - lambda). Different approximations for the latter function are proposed, in order to allow the integral to be performed analytically, yet accurately enough. The influence of the computational parameters (''temperature'', degree of the polynomial expansion of the PDOS, position of the Fermi energy) are discussed with reference to the problem of interstitial atomic hydrogen in bulk lithium.