The ferroelectric to paraelectric (PE) phase transition of KH₂PO₄ (KDP) is investigated as a stringent test of the first-principles, normal modes framework proposed for calculating anharmonic quantum nuclear motion. Accurate spatially resolved momentum distribution functions (MDFs) are directly calculated from the nuclear wavefunction, overcoming the limitations of path-integral molecular dynamics methods. They indicate coherent, correlated tunneling of protons across hydrogen bonds in the PE phase in agreement with neutron Compton scattering data and reproduces the key features of the experimental MDF. It further highlights the role of Slater’s lateral configurations in the PE phase. The analysis in terms of normal modes demonstrates the importance of collective, correlated proton motion and underlines the value of the employed framework in interpreting experimental data. This also makes the framework very attractive for application to deuterated KDP to further elucidate the nature of the PE transition and to systems exhibiting strong quantum nuclear effects in general.

中文翻译：

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第一性原理在KDP中空间分辨的质子动量分布

KH ₂ PO ₄的铁电至顺电（PE）相变（KDP）作为对第一性原理，建议用于计算非谐量子核运动的标准模态框架的严格测试而进行了研究。直接从核波函数计算出精确的空间分辨动量分布函数（MDF），克服了路径积分分子动力学方法的局限性。他们表明质子穿过PE相中氢键的连贯关联隧道与中子康普顿散射数据一致，并重现了实验MDF的关键特征。它进一步突出了Slater的横向构型在PE阶段的作用。正常模式下的分析证明了集体相关的质子运动的重要性，并强调了所采用框架在解释实验数据中的价值。