32 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
Ab initio quality neural-network potential for sodium
An interatomic potential for high-pressure high-temperature (HPHT)
crystalline and liquid phases of sodium is created using a neural-network (NN)
representation of the ab initio potential energy surface. It is demonstrated
that the NN potential provides an ab initio quality description of multiple
properties of liquid sodium and bcc, fcc, cI16 crystal phases in the P-T region
up to 120 GPa and 1200 K. The unique combination of computational efficiency of
the NN potential and its ability to reproduce quantitatively experimental
properties of sodium in the wide P-T range enables molecular dynamics
simulations of physicochemical processes in HPHT sodium of unprecedented
quality.Comment: 8 pages, 11 figures, 2 table
Microscopic origins of the anomalous melting behaviour of high-pressure sodium
Recent experiments have shown that sodium, a prototype simple metal at
ambient conditions, exhibits unexpected complexity under high pressure. One of
the most puzzling phenomena in the behaviour of dense sodium is the
pressure-induced drop in its melting temperature, which extends from 1000 K at
~30GPa to as low as room temperature at ~120GPa. Despite significant
theoretical effort to understand the anomalous melting its origins have
remained unclear. In this work, we reconstruct the sodium phase diagram using
an ab-initio-quality neural-network potential. We demonstrate that the
reentrant behaviour results from the screening of interionic interactions by
conduction electrons, which at high pressure induces a softening in the
short-range repulsion. It is expected that such an effect plays an important
role in governing the behaviour of a wide range of metals and alloys.Comment: 5 pages, 4 figures, 30 references, supplementary informatio
Nucleation mechanism for the direct graphite-to-diamond phase transition
Graphite and diamond have comparable free energies, yet forming diamond from
graphite is far from easy. In the absence of a catalyst, pressures that are
significantly higher than the equilibrium coexistence pressures are required to
induce the graphite-to-diamond transition. Furthermore, the formation of the
metastable hexagonal polymorph of diamond instead of the more stable cubic
diamond is favored at lower temperatures. The concerted mechanism suggested in
previous theoretical studies cannot explain these phenomena. Using an ab initio
quality neural-network potential we performed a large-scale study of the
graphite-to-diamond transition assuming that it occurs via nucleation. The
nucleation mechanism accounts for the observed phenomenology and reveals its
microscopic origins. We demonstrated that the large lattice distortions that
accompany the formation of the diamond nuclei inhibit the phase transition at
low pressure and direct it towards the hexagonal diamond phase at higher
pressure. The nucleation mechanism proposed in this work is an important step
towards a better understanding of structural transformations in a wide range of
complex systems such as amorphous carbon and carbon nanomaterials
CP2K: An electronic structure and molecular dynamics software package - Quickstep: Efficient and accurate electronic structure calculations
CP2K is an open source electronic structure and molecular dynamics software package to perform atomistic simulations of solid-state, liquid, molecular, and biological systems. It is especially aimed at massively parallel and linear-scaling electronic structure methods and state-of-the-art ab initio molecular dynamics simulations. Excellent performance for electronic structure calculations is achieved using novel algorithms implemented for modern high-performance computing systems. This review revisits the main capabilities of CP2K to perform efficient and accurate electronic structure simulations. The emphasis is put on density functional theory and multiple post–Hartree–Fock methods using the Gaussian and plane wave approach and its augmented all-electron extension