193 research outputs found
Recent achievements in ab initio modelling of liquid water
The application of newly developed first-principle modeling techniques to
liquid water deepens our understanding of the microscopic origins of its
unusual macroscopic properties and behaviour. Here, we review two novel ab
initio computational methods: second-generation Car-Parrinello molecular
dynamics and decomposition analysis based on absolutely localized molecular
orbitals. We show that these two methods in combination not only enable ab
initio molecular dynamics simulations on previously inaccessible time and
length scales, but also provide unprecedented insights into the nature of
hydrogen bonding between water molecules. We discuss recent applications of
these methods to water clusters and bulk water.Comment: 23 pages, 17 figure
Electronic signature of the instantaneous asymmetry in the first coordination shell of liquid water
Interpretation of the X-ray spectra of water as evidence for its asymmetric
structure has challenged the conventional symmetric nearly-tetrahedral model
and initiated an intense debate about the order and symmetry of the hydrogen
bond network in water. Here, we present new insights into the nature of local
interactions in water obtained using a novel energy decomposition method. Our
simulations reveal that while a water molecule forms, on average, two strong
donor and two strong acceptor bonds, there is a significant asymmetry in the
energy of these contacts. We demonstrate that this asymmetry is a result of
small instantaneous distortions of hydrogen bonds, which appear as fluctuations
on a timescale of hundreds of femtoseconds around the average symmetric
structure. Furthermore, we show that the distinct features of the X-ray
absorption spectra originate from molecules with high instantaneous asymmetry.
Our findings have important implications as they help reconcile the symmetric
and asymmetric views on the structure of water.Comment: Accepted by Nature Commu
A hybrid approach to Fermi operator expansion
In a recent paper we have suggested that the finite temperature density
matrix can be computed efficiently by a combination of polynomial expansion and
iterative inversion techniques. We present here significant improvements over
this scheme. The original complex-valued formalism is turned into a purely real
one. In addition, we use Chebyshev polynomials expansion and fast summation
techniques. This drastically reduces the scaling of the algorithm with the
width of the Hamiltonian spectrum, which is now of the order of the cubic root
of such parameter. This makes our method very competitive for applications to
ab-initio simulations, when high energy resolution is required.Comment: preprint of ICCMSE08 proceeding
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