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Current Research Topics: HEDP Theory Group

First-Principles Equation of State (FPEOS)

For ICF and HED applications, we have been using ab initio quantum molecular-dynamics (QMD) and path-integral Monte Carlo (PIMC) methods to study the equation-of-state of materials under high pressures. Comparing the generated FPEOS tables with traditional EOS models, we can identify what important physics is determining the EOS property of materials, in particular in the warm-dense-matter (WDM) regime. These FPEOS studies have covered DT [1,2], CH [3,4], Be [5], and Si [6,7] for the past several years. Implementing these FPEOS tables into our ICF/HED hydrocodes helps to redefine reliable 1-D target designs. We are currently improving the QMD method by implementing the temperature dependent xc-functionals for more-accurate and mid-/high-Z materials. Some examples of our recent results are given:

[1] S. X. Hu et al., Phys. Rev. Lett. 104, 235003 (2010).

[2] S. X. Hu et al., Phys. Rev. B 84, 224109 (2011).

[3] S. X. Hu, T. R. Boehly, and L. A. Collins, Phys. Rev. E 89, 063104 (2014).

[4] S. X. Hu et al., Phys. Rev. E 92, 043104 (2015).

[5] Y. H. Ding and S. X. Hu, Phys. Plasmas 24, 062702 (2017).

[6] S. X. Hu et al., Phys. Rev. B 94, 094109 (2016).

[7] S. X. Hu et al., Phys. Rev. E 95, 043210 (2016).

First-Principles Opacity Table (FPOT) of Warm Dense Matter

Opacity/emissivity determines how much x-ray radiation is absorbed/emitted in systems. Once materials are highly compressed and heated, atoms and ions in such HED systems can no longer be viewed as individual entities. The surrounding plasma environment will significantly alter the opacity and emissivity in such HED systems. For ICF and HED applications, we have been using the ab initio quantum-molecular-dynamics (QMD) method, based on density-functional theory, to study the first-principles optical properties of materials under high pressures. These FPOT studies have covered DT [8], C [9], and CH [10] so far. Implementing FPOT in our ICF/HED hydrocodes also redefines reliable 1-D target designs. We are currently developing the QMD method, by writing a real-space discrete-variable-representation, all-electron, TD-DFT code for studying the optical properties of mid-/high-Z materials. Some examples of our recent results show that traditional continuum-lowering models can be wrong for strongly coupled and degenerate plasmas:

[8] S. X. Hu et al., Phys. Rev. E 90, 033111 (2014).

[9] S. X. Hu, Phys. Rev. Lett. 119, 065001 (2017).

[10] S. X. Hu et al., Phys. Rev. B 96, 144203 (2017).

Ab initio Studies of Transport Properties in HED Plasmas

Transport properties, including thermal/electrical conductivity, diffusivity, viscosity, and stopping-power, are important quantities to know for ICF and HED experiment simulations. These properties essentially determine both energy and mass transport in such systems. For the past few years, we have been using quantum molecular-dynamics (QMD) method, based on density-functional theory, to study the first-principles transport properties of materials under high pressures. These studies have covered the thermal conductivity and ionization of DT [11] and CH [12] and their effects on ICF simulations [13]. Currently, we are developing a TD-DFT code for extending our calculations of the transport properties of HED plasmas to high-temperature regimes. Some examples of our recent results show that traditional thermal-conduction models can be wrong for strongly coupled and degenerate plasmas:

Figures showing temperature and thermal conductivity

[11] S. X. Hu et al., Phys. Rev. E 89, 043105 (2014).

[12] S. X. Hu et al., Phys. Plasmas 23, 042704 (2016).

[13] S. X. Hu et al., Phys. Plasmas 22, 056304 (2015).

Developing Accurate Density-Functional-Theory (DFT) Methods

The accuracy and efficiency of the modern DFT method really depends on the advancement in finding the best exchange-correlation functionals. To that end, we have put effort on developing an accurate presentation for the free-energy functional over a wide range of state conditions [14]. In addition, improving the orbital-free DFT simulations for high-temperature plasmas is one of our current focuses, through introducing temperature-dependent xc-functional [15]. We wish to continue this works to accurately simulate warm dense matter.

Deuterium electronic pressure as a function of T for the finite-T GGA (“KDT16”) and ground-state
PBE XC functionals, as well as PIMC reference results. Ab initio MD simulations, Γ-point only, for 128 atoms
(8500 steps, T ≤ 40 kK) or for 64 atoms (4500 steps T ≤ 62.5 kK)

[14] V. V. Karasiev, J. W. Dufty, and S. B. Trickey, Phys. Rev. Lett. 120, 076401 (2018).

[15] V. V. Karasiev et al., Phys. Rev. Lett. 112, 076403 (2014).

Combing QMD-CMD for Simulations of HED dynamics

In collaboration with Profs. Chuang Ren and Niaz Abdolrahim from ME at the University of Rochester, we are planning to combine the QMD method with classical molecular-dynamics (CMD) simulations to tackle dynamical problems in HED sciences. Basically, QMD can give the ion–ion interaction potentials in an HED system, while the CMD method can use such first-principles potentials to simulate the dynamics of large systems consisting of thousands or even up to millions of atoms. The goals are to answer many fundamental questions, such as:

How quick phase-transition happens under dynamic loading?
What are the transient states behind shocks?
How fast thermal equilibrium among species occurs in a shocked mixture?