The phase diagram of high pressure hydrogen is of great interest.
Laboratory experiments are difficult and expensive and ab-initio theory is crucial in developing the field. The accuracy of Density Functional Theory (DFT) calculations is limited and often non-predictive. We present a methodology based on Quantum Monte Carlo to study hydrogen at extreme conditions: the Coupled Electron-Ion Monte Carlo (CEIMC).
The first application of CEIMC is in computing the principal Hugoniot of deuterium. We find that the maximum compression along the Hugoniot is ~5% higher than with DFT and ~15% higher than most accurate experimental data.
A second application is to trace the liquid-liquid transition line. A first-order phase transition in the fluid phase between a molecular insulating fluid and a monoatomic metallic fluid has been predicted[4-6]. The existence and precise location of the transition line is relevant for planetary models. Recent experiments reported contrasting results about the location of the transition[7-9]. Theoretical results based on DFT are also very scattered. We report highly accurate CEIMC calculations of this transition finding results that lie between the two experimental predictions, close to that measured in diamond anvil cell experiments but at 25-30 GPa higher pressure. The transition along an isotherm is signaled by a discontinuity in the specific volume, a sudden dissociation of the molecules, a jump in electrical conductivity and loss of electron localization. We discuss the difference observed with respect to the predictions of a different Quantum Monte Carlo method .
Finally a third application of CEIMC is to study the stability of the various crystalline molecular phases of hydrogen. We have performed calculations along the T=200K isotherm in the phase III and along the T=414K isotherm in the phase IV. We report a preliminary comparison between CEIMC results and DFT based results and discuss the electronic character of the various phases.
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