Laboratório de Espectroscopia Molecular Instituto de Química, Universidade de São Paulo CP 26077, CEP 05513-970, São Paulo, SP, Brazil.
J Chem Phys. 2012 Sep 14;137(10):104510. doi: 10.1063/1.4751548.
Time correlation functions of current fluctuations were calculated by molecular dynamics (MD) simulations in order to investigate sound waves of high wavevectors in the glass-forming liquid Ca(NO(3))(2)·4H(2)O. Dispersion curves, ω(k), were obtained for longitudinal (LA) and transverse acoustic (TA) modes, and also for longitudinal optic (LO) modes. Spectra of LA modes calculated by MD simulations were modeled by a viscoelastic model within the memory function framework. The viscoelastic model is used to rationalize the change of slope taking place at k ~ 0.3 Å(-1) in the ω(k) curve of acoustic modes. For still larger wavevectors, mixing of acoustic and optic modes is observed. Partial time correlation functions of longitudinal mass currents were calculated separately for the ions and the water molecules. The wavevector dependence of excitation energies of the corresponding partial LA modes indicates the coexistence of a relatively stiff subsystem made of cations and anions, and a softer subsystem made of water molecules.
为了研究玻璃形成液体 Ca(NO(3))(2)·4H(2)O 中的高波矢声波，通过分子动力学 (MD) 模拟计算了电流涨落的时间相关函数。得到了纵向 (LA) 和横向声学 (TA) 模式以及纵向光学 (LO) 模式的色散曲线 ω(k)。通过 MD 模拟计算的 LA 模式谱通过记忆函数框架内的粘弹性模型进行建模。粘弹性模型用于合理化在声子模式的 ω(k) 曲线中在 k~0.3 Å(-1) 处发生的斜率变化。对于更大的波矢，观察到声子和光子模式的混合。分别为离子和水分子计算了纵向质量流的部分时间相关函数。相应部分 LA 模式的激发能的波矢依赖性表明，由阳离子和阴离子组成的相对刚性子系统与由水分子组成的较软子系统共存。
