Title:	Studies in heterogeneous catalysis and x-ray spectroscopy and properties of water
DNr:	NAISS 2024/6-324
Project Type:	NAISS Medium Storage
Principal Investigator:	Lars G.M. Pettersson <lgm@fysik.su.se>
Affiliation:	Stockholms universitet
Duration:	2025-01-01 – 2026-01-01
Classification:	10304 10407 10302
Homepage:	http://xsolasgroup.fysik.su.se/
Keywords:

Abstract

Our research focuses on structural fluctuations in ambient liquid water and the detailed character of these. A hypothesis is that the low-density (LDL) component assumes clathrate-like local structures while the high-density (HDL) forms chain-like close-packed structures. Investigating this hypothesis, and possible consequences for chemistry and aquatic life of such a picture, is the goal of the ERC AdG GAS-WAT held by the PI. To investigate hypothetical local structures in the liquid we reverse the problem and investigate the survival time of proposed structures in the simulation box. We have developed a flexible and polarizable force-field for water with machine-learned few-body interactions and a very accurate machine-learned representation of high-order electrostatic multipoles and polarizabilities. Here, we perform extensive MD simulations to determine the resulting properties. We now develop and implement embedding techniques describing a central region using the best force-field possible inside a much larger box with a simpler force-field description. Such techniques are already available in modeling active sites in enzymes where waters enter and leave the relevant volume, but in our case it is a very homogeneous situation which requires special consideration. After extensive simulations of XAS spectra of water using structures obtained from PIMD simulations and the MB-pol force-field we have used our SpecSwap-RMC technique to determine how distributions of various structural descriptors should be modified to agree with requirements from XAS and X-ray diffraction. To determine whether the resulting changes also are compatible with the thermodynamics of the system we have developed a path-integral Monte Carlo code where we target either only energetics or, in addition, the modified distributions from XAS and XRD. This requires running very long simulations at a number of temperatures and pressures. The outcome of this study is expected to have significant impact. The hydrogens/protons in the water molecule are Fermions, which leads to different rotational properties of water molecules dependent on the spin-coupling. To include this, also the protons need to be included in the wave function description. Here we have extended the LOWDIN code to do non-orthogonal CI based on a sampling of rotational states on a grid to build the correct rovibrational wave functions. The aim is to investigate the effect of spin-coupling in different H-bonding situations - in particular possible effects on the dynamics in switching from asymmetrical (HDL) to symmetrical (LDL) situations.. Furthermore, we implement real-time (RT) TDDFT simultaneous propagation of the electronic and nuclear density for molecular systems within the deMon2k code. The code is close to production and will be used in large-scale ab initio MD simulations of XES of water without invoking the Born-Oppenheimer approximation. Finally, we perform VASP simulations of short-time (<15 fs) effects on water H-bond connectivity of valence ionizing a fraction of the molecules surrounding the molecule targeted with XES. Also here, a large number of (albeit short) trajectories will be needed for reliable conclusions.