Nuclear quantum effects in water (0803.3635v1)

Abstract: In this work, a path integral Car-Parrinello molecular dynamics simulation of liquid water is performed. It is found that the inclusion of nuclear quantum effects systematically improves the agreement of first principles simulations of liquid water with experiment. In addition, the proton momentum distribution is computed utilizing a recently developed open path integral molecular dynamics methodology. It is shown that these results are in good agreement with neutron Compton scattering data for liquid water and ice.

• The paper uses path integral molecular dynamics simulations to show nuclear quantum effects are crucial for reconciling theoretical models of water with experimental data.
• Including nuclear quantum effects leads to a softer liquid water structure with oxygen-oxygen radial distribution functions that align more closely with experimental results.
• The simulations accurately capture the qualitative features of the proton momentum distribution in water and ice, and highlight differences in hydrogen bonding due to quantum effects.

Insights into Nuclear Quantum Effects in Water

The research paper titled "Nuclear quantum effects in water" by Joseph A. Morrone and Roberto Car explores the significant role of nuclear quantum effects on the properties of liquid water and ice. The paper employs path integral Car-Parrinello molecular dynamics (PI-CPMD) simulations to address discrepancies between first-principles simulations and experimental data, particularly focusing on the proton momentum distribution derived from neutron Compton scattering experiments.

The investigation highlights the substantial influence of nuclear quantum effects on the structural and dynamic properties of water, which have traditionally been overlooked in classical molecular dynamics simulations. Utilizing an advanced open path integral molecular dynamics methodology, this work calculates proton momentum distribution, demonstrating its alignment with experimental observations and identifying discrepancies which shed light on underlying quantum mechanics.

Key Findings and Numerical Results

1. Structural Agreement with Experiment: The inclusion of nuclear quantum effects leads to a softer structure of liquid water compared to classical simulations. This softening is quantitatively evidenced by the radial distribution functions (RDFs). Specifically, the first peak of the oxygen-oxygen RDF in the path integral simulation is measured at 2.84, which aligns more closely with experimental neutron and x-ray scattering results than the result from classical simulations.
2. Proton Momentum Distribution: By implementing an "open" path integral molecular dynamics approach, the computed proton momentum distributions exhibit qualitative agreement with neutron Compton scattering data, capturing phase-specific variations between liquid and solid water. Although the computed momentum distribution in ice is broader than observed experimentally, the essential trend of a shorter tail in ice, indicative of a more intact hydrogen bond network, is correctly reproduced, thus overcoming limitations noted in force field-based studies.
3. Environmental Sensitivity: The distribution of dipole moments and the fraction of broken hydrogen bonds indicate slight but significant differences due to nuclear quantum effects. The simulations show a broader dipole distribution and a concurrent increase in fluidity indicated by an 11% fraction of broken hydrogen bonds in quantum simulations versus 7% in classical ones.

Implications and Future Directions

The successful integration of nuclear quantum effects into molecular dynamics simulations using PI-CPMD offers vital corrections to the perceived overstructuring in classical simulation models based on Density Functional Theory (DFT). However, even with the inclusion of quantum effects, some discrepancies remain due to potential limitations in the exchange-correlation approximations of DFT or the employed plane wave basis sets.

Practically, these findings emphasize the necessity of considering nuclear quantum effects in simulations to achieve more accurate and predictive models, particularly for systems where hydrogen bonding plays a crucial role. Theoretical advancements in the methodology can extend to a broader range of systems, including biological proton wires where proton tunneling is significant. These future explorations may offer deeper insights into the quantum nature of hydrogen bonding and the thermodynamic properties of aqueous and biological systems.

This research represents a significant stride toward reconciling computational predictions with experimental data in water systems, offering a robust framework for future studies aiming to capture subtle quantum mechanical effects in complex molecular environments.