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and clusters. In the computed EXAFS spectrum a systematic error with respect to
the experimental spectrum was obtained. In a subsequent study from 2008 the
different dependencies of the calculated EXAFS spectra were studied for liquid
water and hexagonal ice within the supercell approach [
81
]. Several configurations
of AIMD simulations were produced and asymmetrically coordinated water
molecules were observed. For example, such water molecules with only one hydro-
gen bond showed well defined spectral lines which significantly differed from the ice
signals.
For a review of calculations of the X-ray adsorption spectra (XAS) which
especially focused on the transition potential approach and its application to
water, see the recent work of Leetmaa et al. [
82
].
4 Applications in Chemical Engineering
In this section we discuss several studies in which AIMD was applied to special
chemical problems, reactions, and industrial processes.
4.1 Wavefunction Analysis
Many schemes were adapted to analyze the wavefunction (electronic structure) in
AIMD simulations. The most important ones are the Wannier analysis based on
maximally localized Wannier functions (MLWF) [
83
], the electron localization
function (ELF)[
84
], the Fukui function [
85
], and the nucleus-independent chemical
shift maps [
74
].
The usefulness of Wannier functions was demonstrated by Silvestrelli et al. [
86
]
in a study of amorphous silicon. The authors were able to describe the bond
structure using the Wannier functions. The investigation of defect configurations
was possible with a novel degree of accuracy through the Wannier approach [
86
].
Another application of Wannier functions was published by Fitzhenry et al.
investigating silicon-carbon alloys [
87
]. In this study the bond structure was
resolved by the application of Wannier functions and Fitzhenry et al. were able to
identify, classify, and quantify the types of bonding present in the alloy. They were
able to observe three-center bonding and a temperature dependent flipping of bonds
during the simulation [
87
]. In 2005, B
uhl et al. investigated the ionic liquid (see also
Sect.
4.2.2
) 1,3-dimethylimidazolium chloride ([Mmim][Cl]) at 438 K using
CPMD [
88
]. Population analyses showed noticeable charge transfer from anions
to cations and Wannier functions demonstrated this specifically for the CH ··· Cl
hydrogen bonds. Another important tool of the Wannier analysis is the derivation of
local dipole moments. The applications of dipole moment calculations is discussed
in Sect.
4.2.1
.
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