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In this paper we discuss some of the details of the calculation of the macroscopic polarization, via the Berry-phase approach, using a general (numerical) local combination of atomic orbitals (LCAO) as a basis set. Previous implementations with a LCAO basis relied on the use of gaussian expansions, where analytical formulas exist for the crucial matrix elements. However, our approach, which only requires the matrix elements of the position operator, can be more easily implemented in the case of numerical orbitals. This work is a necessary first step towards our main goal of applying the SIESTA code to the study of ferroelectric alloys. This code, which uses a numerical LCAO basis set, has been highly optimized for the treatment of large systems. We are confident that it will allow us to treat larger systems than the standard plane-waves methods, while keeping a reasonable accuracy. Test results for representative ferroelectric perovskites are presented. [ABSTRACT FROM AUTHOR]

We study the H[sub 5]O[sub 2][sup +] molecule as a basic unit of the proton transfer in hydrogen-bonded water networks in density-functional theory (DFT) scheme using the finite-difference pseudopotential method. The comparison of dissociation energies obtained from the local-density approximation (LDA) and the generalized-gradient approximation (GGA) with experimental results and other accurate quantum chemistry calculations shows that (1) the GGA in DFT is a reliable scheme for this system, and (2) we can obtain the effect of including higher correlation by using the GGA over the LDA. We also address the technical issue of implementing the GGA exchange-correlation potential in the real space. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]