What is it about?
The hydrogen dimer, composed of two interacting H2 molecules, presents an interesting challenge for computational methods due to the interplay of interactions: strong intramolecular bonding within each H2 molecule and weaker intermolecular bonding between the two H2 units. To achieve accuracy, computational methods must effectively capture this delicate balance of interactions. For this study, we considered several methods from the frameworks of Density Functional Theory (DFT), the Random Phase Approximation (RPA), and Quantum Monte Carlo (QMC). We evaluated their ability to describe the potential energy surface of the (H2)2 system, focusing on variations in the intermolecular bond distance and relative orientations of the H2 units. Our results reveal that while some methods perform well near equilibrium geometry, many struggle under short range intermolecular interactions. Only methods with an advanced treatment of the electronic exchange-correlation effects provide a reliable description across different regions of the potential energy surface of the (H2)2 system.
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Why is it important?
The hydrogen dimer is a simple yet challenging system for computational methods. Many approaches struggle to evaluate with accuracy the intermolecular interaction between two H2 molecules, especially when the bonding distance is reduced to around 2 Angstrom, a value commonly encountered in the molecular solid phases of hydrogen under megabar pressures. This study emphasizes the importance of using numerical methods that present an advanced description of the electronic exchange-correlation effects. Such approaches are particularly relevant for future research focusing on high-pressure molecular solid hydrogen phases, which display a similar three-dimensional arrangement of H2 molecules.
Perspectives
The phase diagram of solid hydrogen under megabar pressures remains unclear, with several candidate structures proposed to match the available spectroscopic experimental data. The energy differences between these structures are so small that only an accurate treatment of all relevant interactions can help identify the correct phases. I hope the conclusions of this study highlight that, even in the (H2)2 dimer, which can be considered as a simple model for hydrogen solid phases, inaccuracies in the treatment of electronic exchange-correlation effects can significantly alter the calculation results.
Mr. Damian Contant
Universite Paris-Sorbonne
Read the Original
This page is a summary of: Assessing many-body methods on the potential energy surface of the (H2)2 hydrogen dimer, The Journal of Chemical Physics, November 2024, American Institute of Physics,
DOI: 10.1063/5.0235728.
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