We examine the influence of the main approximations employed in density functional theory descriptions of the solid phase of molecular hydrogen near dissociation. We consider the importance of nuclear quantum effects on equilibrium properties and find that they strongly influence intramolecular properties, such as bond fluctuations and stability. We demonstrate that the combination of both thermal and quantum effects make a drastic change to the predicted optical properties of the molecular solid, suggesting a limited value to dynamical, e.g., finitetemperature predictions based on classical ions and static crystals. We also consider the influence of the chosen exchangecorrelation density functional on the predicted properties of hydrogen, in particular, the pressure dependence of the band gap and the zeropoint energy. Finally, we use our simulations to make an assessment of the accuracy of typically employed approximations to the calculation of the Gibbs free energy of the solid, namely the quasiharmonic approximation for solids. We find that, while the approximation is capable of producing free energies with an accuracy of ≈10 meV, this is not enough to make reliable predictions of the phase diagram of hydrogen from first principles due to the small free energy differences seen between several potential structures for the solid; direct free energy calculations for quantum protons are required in order to make definite predictions.
We examine the influence of the main approximations employed in density functional theory descriptions of the solid phase of molecular hydrogen near dissociation. We consider the importance of nuclear quantum effects on equilibrium properties and find that they strongly influence intramolecular properties, such as bond fluctuations and stability. We demonstrate that the combination of both thermal and quantum effects make a drastic change to the predicted optical properties of the molecular solid, suggesting a limited value to dynamical, e.g., finitetemperature predictions based on classical ions and static crystals. We also consider the influence of the chosen exchangecorrelation density functional on the predicted properties of hydrogen, in particular, the pressure dependence of the band gap and the zeropoint energy. Finally, we use our simulations to make an assessment of the accuracy of typically employed approximations to the calculation of the Gibbs free energy of the solid, namely the quasiharmonic approximation for solids. We find that, while the approximation is capable of producing free energies with an accuracy of ≈10 meV, this is not enough to make reliable predictions of the phase diagram of hydrogen from first principles due to the small free energy differences seen between several potential structures for the solid; direct free energy calculations for quantum protons are required in order to make definite predictions. © 2013 American Physical Society.
Toward a Predictive FirstPrinciples Description of Solid Molecular Hydrogen with DensityFunctional Theory
PIERLEONI, CARLO;
2013
Abstract
We examine the influence of the main approximations employed in density functional theory descriptions of the solid phase of molecular hydrogen near dissociation. We consider the importance of nuclear quantum effects on equilibrium properties and find that they strongly influence intramolecular properties, such as bond fluctuations and stability. We demonstrate that the combination of both thermal and quantum effects make a drastic change to the predicted optical properties of the molecular solid, suggesting a limited value to dynamical, e.g., finitetemperature predictions based on classical ions and static crystals. We also consider the influence of the chosen exchangecorrelation density functional on the predicted properties of hydrogen, in particular, the pressure dependence of the band gap and the zeropoint energy. Finally, we use our simulations to make an assessment of the accuracy of typically employed approximations to the calculation of the Gibbs free energy of the solid, namely the quasiharmonic approximation for solids. We find that, while the approximation is capable of producing free energies with an accuracy of ≈10 meV, this is not enough to make reliable predictions of the phase diagram of hydrogen from first principles due to the small free energy differences seen between several potential structures for the solid; direct free energy calculations for quantum protons are required in order to make definite predictions.File  Dimensione  Formato  

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