Introduction

Plane Wave (PW) basis sets offer several advantages over Linear Combination of Atomic Orbital (LCAO) basis sets when used in ab initio electronic structure calculations. These include the availability of FFT algorithms allowing efficient transformation between real- and reciprocal-space representations, systematic convergence with a single parameter - the cut off energy of the PW basis - and efficient calculation of the ionic forces. LCAO basis sets are significantly more efficient in terms of basis size than a PW basis, even when Pseudopotentials are used in conjunction with the later. In addition LCAO basis sets offer a natural way of specifying quantities such as atomic charge, orbital population, charge transfer etc.

This document describes an application which addresses the latter of these considerations by implementing a population analysis scheme for the CASTEP and CETEP PW pseudopotential codes. This analysis is performed using a projection of the PW states onto a localised basis set by the technique described by Sanchez-Portal et al. [1]. A population analysis of these projected states can then be used to determine quantities such as atomic charges and bond populations. The formalism of Mulliken [2] is used in order to perform this analysis. This technique is commonly applied in the analysis of calculations performed using localised basis sets and hence is widely used in Chemistry and Biochemistry. However, it has not been routinely applied to PW calculations. It is widely accepted that the absolute magnitude of the atomic charges have little physical meaning, as they display an extreme sensitivity to the atomic basis set with which they are calculated [3]. However, consideration of their relative values, in contrast to their absolute magnitudes, can yield useful information (see [4,5,6]).

The following section describes the theory underlying the codes. Section 3 describes the use of the applications to analyse output from the CASTEP and CETEP packages. Notes on use, distribution and modification of the code may be found in Section 4.

Please send any comments or reports of bugs in the code or documentation to
mds21@phy.cam.ac.uk.