Ab initio x-ray scattering of liquid water

Abstract

A direct calculation of the coherent x-ray scattering spectrum of liquid water under ambient conditions is presented using ab initio density functional theory. The experimental data are compared with the calculated x-ray scattering spectra retrieved from the trajectories of two Car–Parrinello molecular dynamics runs of about 10 ps with 32 and 64 water molecules in the simulation cell, respectively. Furthermore, the x-ray spectra obtained from molecular dynamics runs of 20 ps for 128 water molecules using empirical water models are presented. All calculations were performed with the QUICKSTEP code, which includes an all-electron implementation of the Gaussian augmented plane wave method. It is shown that by this approach one can assess how well different models describe the structural properties of water.

X-ray scattering intensities of water at extreme pressure and temperature

“…The formalism for determination of I(Q) and g(R OO ) has been reviewed elsewhere [4,6,7,14] and is briefly included herein for the sake of completeness. The x-ray scattering of an electron density generated form a condensed phase simulation trajectory can be calculated via the following equation:…”

“…The effect is likely to be even more pronounced at high pressure-temperature conditions, where molecular bonding changes dramatically, causing the formation of new classes of materials [16]. With recent advances in ab initio codes [6,17], it has become possible but costly to directly calculate the total x-ray scattering intensity of a condensed phase simulation via Fourier Transform of the one-electron density. We are thus in a unique position to assess the efficacy of standard atomic form factors and to determine more accurate values for high pressures and temperatures.…”

“…As stated in [6], plane wave pseudopotentials are not well suited to describing the electron density near the nuclei. Thus the allelectron version of the code was used.…”

“…All of our calculations used the BLYP exchange correlation functional, as well. The smooth part of the electron charge density was was expanded to an energy cutoff of 130 Ry, as conducted previously [6].…”

“…In water, the x-ray scattering is dominated by oxygen-oxygen correlations, because most of the electron density of the molecules is centered around the oxygen atoms. X-ray scattering has the advantage over neutron scattering in that the data does not need to be massively corrected before conversion to real space [4][5][6][7]. In addition, due to recent advances in ab initio codes, x-ray scattering spectra can be readily calculated using a combination of all electron methods and density functional theory [6,7].…”