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

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(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.