arXiv subjects

Lars G. M. Pettersson

Publications and source records attributed to Lars G. M. Pettersson.

7 recordsLinked to original sources

arXiv2019

A Two-State Picture of Water and the Funnel of Life

Here I show that experimental and simulation data on liquid water using vibrational (infrared and Raman) and X-ray (absorption and emission) spectroscopies, as well as recent data from X-ray scattering, are fully consistent with a two-state picture of water. At ambient conditions there are fluctuations between a dominating high-density liquid (HDL) and a low-density form (LDL). These are related to the two forms of amorphous ice at very low temperature, high-density amorphous (HDA) and low-density amorphous (LDA), which interconvert in a first-order-like transition. This transition line is assumed to continue into the so-called No-mans land as a liquid-liquid transition and terminate in a critical point with very large fluctuations between the two liquid forms. These fluctuations extend in a funnel-like region up to ambient temperatures and pressures and give water its unusual properties which are fundamental to life. With this picture we find simple, intuitive explanations of the anomalous properties of water, such as the density maximum at 4 C, why ice floats, and why the compressibility and heat capacity grow as the liquid is cooled. We summarize by noting that in this picture, water is not a complicated liquid, but two normal liquids with a complicated relationship.

cond-mat.soft↗

arXiv2017

X-ray Emission Spectrum of Liquid Ethanol : Origin of Split Peaks

The X-ray emission spectrum of liquid ethanol was calculated using density functional theory and a semi-classical approximation to the Kramers-Heisenberg formula including core-hole-induced dynamics. Our spectrum agrees well with the experimental spectrum. We found that the intensity ratio between the two peaks at 526 and 527 eV assigned as 10a' and 3a" depends not only on the hydrogen bonding network around the target molecule, but also on the intramolecular conformation. This effect is absent in liquid methanol and demonstrates the high sensitivity of X-ray emission to molecular structure. The dependence of spectral features on hydrogen-bonding as well as on dynamical effects following core-excitation are also discussed.

physics.chem-ph↗

arXiv2017

How square ice helps lubrication

In the context of friction we use atomistic molecular-dynamics simulations to investigate water confined between graphene sheets over a wide range of pressures. We find that thermal equilibration of the confined water is hindered at high pressures. We demonstrate that, under the right conditions, square ice can form in an asperity, and that it is similar to cubic ice VII and ice X. We simulate sliding of atomically flat graphite on the square ice and find extremely low friction due to structural superlubricity. The conditions needed for square ice to form correspond to low sliding speeds, and we suggest that the ice observed in experiments of friction on wet graphite is of this type.

cond-mat.mtrl-sci↗

arXiv2013

The Boson peak in supercooled water

We perform extensive molecular dynamics simulations of the TIP4P/2005 model of water to investigate the origin of the Boson peak reported in experiments on supercooled water in nanoconfined pores, and in hydration water around proteins. We find that the onset of the Boson peak in supercooled bulk water coincides with the crossover to a predominantly low-density-like liquid below the Widom line $T_W$. The frequency and onset temperature of the Boson peak in our simulations of bulk water agree well with the results from experiments on nanoconfined water. Our results suggest that the Boson peak in water is not an exclusive effect of confinement. We further find that, similar to other glass-forming liquids, the vibrational modes corresponding to the Boson peak are spatially extended and are related to transverse phonons found in the parent crystal, here ice Ih.

cond-mat.soft↗

arXiv2011

Ab initio van der Waals interactions in simulations of water alter structure from mainly tetrahedral to high-density-like

The structure of liquid water at ambient conditions is studied in ab initio molecular dynamics simulations using van der Waals (vdW) density-functional theory, i.e. using the new exchange-correlation functionals optPBE-vdW and vdW-DF2. Inclusion of the more isotropic vdW interactions counteracts highly directional hydrogen-bonds, which are enhanced by standard functionals. This brings about a softening of the microscopic structure of water, as seen from the broadening of angular distribution functions and, in particular, from the much lower and broader first peak in the oxygen-oxygen pair-correlation function (PCF), indicating loss of structure in the outer solvation shells. In combination with softer non-local correlation terms, as in the new parameterization of vdW-DF, inclusion of vdW interactions is shown to shift the balance of resulting structures from open tetrahedral to more close-packed. The resulting O-O PCF shows some resemblance with experiment for high-density water (A. K. Soper and M. A. Ricci, Phys. Rev. Lett., 84:2881, 2000), but not directly with experiment for ambient water. However, an O-O PCF consisting of a linear combination of 70% from vdW-DF2 and 30% from experiment on low-density liquid water reproduces near-quantitatively the experimental O-O PCF for ambient water, indicating consistency with a two-liquid model with fluctuations between high- and low-density regions.

cond-mat.mtrl-sci↗

arXiv2009

SpecSwap RMC: A novel reverse Monte Carlo approach using a discrete configuration space and pre-computed properties

We present a novel approach to reverse Monte Carlo (RMC) modeling, SpecSwap-RMC, which makes use of pre-computed property data from a discrete configuration space replacing atomistic moves with swap moves of contributions to a sample-set representing the average, or summed property. The approach is particularly suitable for disordered systems and properties which require significant computer time to compute. We demonstrate the approach by fitting jointly and separately the EXAFS signal and x-ray absorption spectrum (XAS) of ice Ih using as SpecSwap sample-set 80 configurations from a space of 1382 local structures with associated pre-computed spectra. As an additional demonstration we compare SpecSwap and FEFFIT fits of EXAFS data on crystalline copper finding excellent agreement.

physics.data-an↗

arXiv2007

Half or full core-hole in density functional theory X-ray absorption spectrum calculations of water?

We analyze the performance of two different core-hole potentials in the theoretical modeling of XAS of ice, liquid and gas phase water; the use of a full core-hole (FCH) in the calculations, as suggested by Hetenyi et al. [J. Chem. Phys. 120 (18), 8632 (2004)], gives poor agreement with experiment in terms of intensity distribution as well as transition energies, while the half core hole (HCH) potential, in the case of water, provides a better compromise between initial and final state effects, leading to good agreement with the experimental data.

cond-mat.soft↗