First-Principles Study on Non-Collinear Magnetic States and Magneto-Strain Effects

(1)非共线磁体的第一性原理研究我们对液氧的分子动力学进行了从头算，液氧是一种分子流体，其中单个O_2单元携带一个分子磁矩。除了原子和电子结构外，我们的模拟还描述了非共线磁结构的演化。原子结构显示出强烈的第一邻位分子平行排列的倾向。磁结构表现出很强的短程反铁磁关联，与自旋极化中子衍射数据相一致。在结构和磁性质上观察到的短程关联主要是由O2分子碰撞的适当轨迹造成的。我们的模拟还揭示了几个长寿命的O_4单元的存在，它们的存活时间超过了碰撞期间观察到的平均停留时间的四倍。(2)密度泛函微扰理论的发展和实现：从第一原理出发的晶格动力学我们实现了密度泛函微扰理论(DFPT)的变分公式来研究简谐晶格动力学。在线性响应理论中，通过最小化总能量的二阶导数，计算了电荷密度和波函数对原子位移的响应。该实现是基于平面波基集合和赝势的。对于一阶波函数的优化，首次采用了阻尼型分子动力学(MD)格式和拉格朗日乘子法。我们发现这种方法是非常稳健的，并且给出了波函数的稳定收敛。计算一阶波函数的计算量与基态计算相当，与其他迭代方法类似。该方法简单，适用于基于CAR-Parrinello MD方案的MD码的DFPT实现。

(1) First-principles study on noncollinear magentismWe have carried out an ab initio molecular dynamics of liquid oxygen, a molecular fluid in which the individual O_2 units carry a molecular magnetic moment. In addition to the atomic and electronic structures, our simulation describes the evolution of the noncollinear magnetic structure. The atomic structure shows a strong preference for parallel alignment of first-neighbor molecules. The magnetic structure shows strong short range antiferromagnetic correlations, in agreement with spin-polarized neutron diffraction data. The short range correlations, observed in both structural and magnetic properties, primarily result from appropriate trajectories of colliding O_2 molecules. Our simulation also reveals the occurrence of several long-living O_4 units which survive for time periods longer than four times the average residence time observed during collisions.(2) Development and implementation of density functional perturbation theory : Lattice dynamics from first principlesWe have implemented the variational formulation of density functional perturbation theory (DFPT) to investigate the harmonic lattice dynamics. Response of the charge density and wave functions to atomic displacements within linear response theory, is computed by minimizing the second-order derivative of total energy. The implementation is based on a plane-wave basis set and pseudopotentails. For the optimization of the first-order wave functions, the damped molecular dynamics (MD) scheme and the Lagrange multiplier method is adopted for the first time. We find that this method is very robust and gives a stable convergence of the wave functions. The computational efforts to calculate the first-order wave functions are comparable to the ground state calculations, similar to other iterative methods. Our method is simple and suitable for the implementation of DFPT to the MD code which is based on the original Car-Parrinello MD scheme.