Anharmonic free energies and phonon dispersions from the stochastic self-consistent harmonic approximation: Application to platinum and palladium hydrides
Anharmonic free energies and phonon dispersions from the stochastic self-consistent harmonic approximation: Application to platinum and palladium hydrides
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DOI：
10.1103/physrevb.89.064302
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发表时间：
2014-02-10
影响因子：
3.7
通讯作者：
Mauri, Francesco
中科院分区：
文献类型：
--
作者：
Errea, Ion;Calandra, Matteo;Mauri, Francesco
基于密度泛函理论的谐波计算是描述金属和绝缘体声子谱的常用方法。然而，包含非调和效应是微妙的，因为它依赖于摄动理论，需要相当多的计算机时间，随着细胞尺寸的增加而快速增加。此外，当谐波解动态不稳定或声子能量的非谐波修正量大于谐波频率本身时，微扰理论失效。本文给出了自洽调和近似的一种随机实现，可以有效地处理非摄动状态下任意温度下的非调和性。该方法是基于自由能相对于一个由任意调和哈密顿量描述的试验密度矩阵的最小化。对试验谐波哈密顿量中的所有自由参数，即平衡位置、声子频率和极化矢量进行最小化。自由能的梯度是按照随机过程计算的。该方法可用于计算电子-声子耦合的热力学性质、动力学性质甚至Eliashberg函数的非调和修正。相对于摄动理论，系统尺寸的标度得到了极大的改善。该方法的有效性在强非谐波钯和铂氢化物中得到了验证。在这两种情况下，我们预测谐波声子谱的强非调和修正，远远超过摄动极限。在钯氢化物中，我们计算了超出准谐波近似的热力学性质，而在PtH中，我们证明了当包括非谐波效应时，先前基于谐波近似计算的高超导临界温度在100 GPa时被强烈抑制。
Harmonic calculations based on density-functional theory are generally the method of choice for the description of phonon spectra of metals and insulators. The inclusion of anharmonic effects is, however, delicate as it relies on perturbation theory requiring a considerable amount of computer time, fast increasing with the cell size. Furthermore, perturbation theory breaks down when the harmonic solution is dynamically unstable or the anharmonic correction of the phonon energies is larger than the harmonic frequencies themselves. We present here a stochastic implementation of the self-consistent harmonic approximation valid to treat anharmonicity at any temperature in the nonperturbative regime. The method is based on the minimization of the free energy with respect to a trial density matrix described by an arbitrary harmonic Hamiltonian. The minimization is performed with respect to all the free parameters in the trial harmonic Hamiltonian, namely, equilibrium positions, phonon frequencies, and polarization vectors. The gradient of the free energy is calculated following a stochastic procedure. The method can be used to calculate thermodynamic properties, dynamical properties, and even anharmonic corrections to the Eliashberg function of the electron-phonon coupling. The scaling with the system size is greatly improved with respect to perturbation theory. The validity of the method is demonstrated in the strongly anharmonic palladium and platinum hydrides. In both cases, we predict a strong anharmonic correction to the harmonic phonon spectra, far beyond the perturbative limit. In palladium hydrides, we calculate thermodynamic properties beyond the quasiharmonic approximation, while in PtH, we demonstrate that the high superconducting critical temperatures at 100 GPa predicted in previous calculations based on the harmonic approximation are strongly suppressed when anharmonic effects are included.