Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA.
Lawrence Livermore National Laboratory, Livermore, California 94550, USA.
J Chem Phys. 2018 Mar 14;148(10):102318. doi: 10.1063/1.5001208.
Carbon-hydrogen plasmas and hydrocarbon materials are of broad interest to laser shock experimentalists, high energy density physicists, and astrophysicists. Accurate equations of state (EOSs) of hydrocarbons are valuable for various studies from inertial confinement fusion to planetary science. By combining path integral Monte Carlo (PIMC) results at high temperatures and density functional theory molecular dynamics results at lower temperatures, we compute the EOSs for hydrocarbons from simulations performed at 1473 separate (ρ, T)-points distributed over a range of compositions. These methods accurately treat electronic excitation effects with neither adjustable parameter nor experimental input. PIMC is also an accurate simulation method that is capable of treating many-body interaction and nuclear quantum effects at finite temperatures. These methods therefore provide a benchmark-quality EOS that surpasses that of semi-empirical and Thomas-Fermi-based methods in the warm dense matter regime. By comparing our first-principles EOS to the LEOS 5112 model for CH, we validate the specific heat assumptions in this model but suggest that the Grüneisen parameter is too large at low temperatures. Based on our first-principles EOSs, we predict the principal Hugoniot curve of polystyrene to be 2%-5% softer at maximum shock compression than that predicted by orbital-free density functional theory and SESAME 7593. By investigating the atomic structure and chemical bonding of hydrocarbons, we show a drastic decrease in the lifetime of chemical bonds in the pressure interval from 0.4 to 4 megabar. We find the assumption of linear mixing to be valid for describing the EOS and the shock Hugoniot curve of hydrocarbons in the regime of partially ionized atomic liquids. We make predictions of the shock compression of glow-discharge polymers and investigate the effects of oxygen content and C:H ratio on its Hugoniot curve. Our full suite of first-principles simulation results may be used to benchmark future theoretical investigations pertaining to hydrocarbon EOSs and should be helpful in guiding the design of future experiments on hydrocarbons in the gigabar regime.
碳氢等离子体和碳氢化合物材料是激光冲击实验、高能密度物理和天体物理领域广泛关注的对象。碳氢化合物的准确状态方程（EOS）对于惯性约束聚变到行星科学等各种研究都具有重要价值。通过结合高温下的路径积分蒙特卡罗（PIMC）结果和低温下的密度泛函理论分子动力学结果，我们在 1473 个独立的（ρ，T）点的模拟中计算了碳氢化合物的 EOS，这些点分布在一系列组成范围内。这些方法无需可调参数或实验输入即可准确地处理电子激发效应。PIMC 也是一种准确的模拟方法，能够在有限温度下处理多体相互作用和核量子效应。因此，这些方法提供了一种基准质量的 EOS，在暖稠密物质区域超越了半经验和托马斯-费米方法。通过将我们的第一性原理 EOS 与 CH 的 LEOS 5112 模型进行比较，我们验证了该模型中比热假设的合理性，但认为格林艾森参数在低温下过大。基于我们的第一性原理 EOS，我们预测聚苯乙烯的主冲击 Hugoniot 曲线在最大冲击压缩时比无轨道密度泛函理论和 SESAME 7593 预测的曲线软 2%-5%。通过研究碳氢化合物的原子结构和化学键，我们发现化学键在 0.4 到 4 兆巴的压力区间内的寿命急剧下降。我们发现线性混合假设对于描述碳氢化合物的 EOS 和冲击 Hugoniot 曲线在部分离化原子液体的范围内是有效的。我们对辉光放电聚合物的冲击压缩进行了预测，并研究了氧含量和 C:H 比对其 Hugoniot 曲线的影响。我们的整套第一性原理模拟结果可用于基准未来关于碳氢化合物 EOS 的理论研究，并有助于指导在 gigabar 范围内对碳氢化合物进行未来实验的设计。
