Anharmonic Coupling of Stretching Vibrations in Ice: A Periodic VSCF and VCI Description
Anharmonic Coupling of Stretching Vibrations in Ice: A Periodic VSCF and VCI Description
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冰中拉伸振动的非谐耦合：周期性 VSCF 和 VCI 描述
DOI：
10.1021/acs.jctc.2c00217
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发表时间：
2022
影响因子：
5.5
通讯作者：
Ruggiero, Michael T.
中科院分区：
文献类型：
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作者：
Schireman, Raymond G.;Maul, Jefferson;Erba, Alessandro;Ruggiero, Michael T.
水冰的O-H伸缩振动的非谐性的特征在于使用的振动自洽场（VSCF）和振动组态相互作用（VCI）的方法，其中考虑声子-声子耦合明确地通过数值计算的高阶项的核势的周期性实施。研究了低温质子有序的水冰相（即冰XI）。拉伸模式的耦合非谐处理的净效应不仅仅是各自谐波光谱频率的刚性蓝移，而是它们的相对光谱位置的复杂变化，这不能通过基于谐波计算的简单缩放策略来捕获。所采用的技术允许核势的非谐项的分级处理，这是有效识别主导因素的关键。我们表明，非谐独立模近似-只描述的“内在非谐性”的O-H伸展-是无法捕捉正确的物理，和O-H伸展之间的耦合必须加以描述。谐波法向坐标的检查允许识别O-H拉伸运动的特定特征，其最有可能实现强模式-模式耦合。最后，通过耦合O-H伸展到冰XI的所有其他可能的模式（太赫兹集体振动，分子振动，弯曲），我们确定了显着影响O-H伸展状态的特定类型的运动：特别是，分子振动被发现比分子弯曲更影响伸展状态。
The anharmonicity of O–H stretching vibrations of water ice is characterized by use of a periodic implementation of the vibrational self-consistent field (VSCF) and vibrational configuration interaction (VCI) methods, which take phonon–phonon couplings explicitly into account through numerical evaluation of high-order terms of the nuclear potential. The low-temperature, proton-ordered phase of water ice (namely, ice XI) is investigated. The net effect of a coupled anharmonic treatment of stretching modes is not just a rigid blue-shift of the respective harmonic spectral frequencies but rather a complex change of their relative spectral positions, which cannot be captured by simple scaling strategies based on harmonic calculations. The adopted techniques allow for a hierarchical treatment of anharmonic terms of the nuclear potential, which is key to an effective identification of leading factors. We show that the anharmonic independent-mode approximation─only describing the “intrinsic anharmonicity” of the O−H stretches─is unable to capture the correct physics, and that couplings among O−H stretches must be described. Inspection of harmonic normal coordinates allows identification of specific features of the O–H stretching motions which most likely enable strong mode–mode couplings. Finally, by coupling O–H stretches to all other possible modes of ice XI (THz collective vibrations, molecular librations, bendings), we identify specific types of motion which significantly affect O–H stretching states: in particular, molecular librations are found to affect the stretching states more than molecular bendings.