从单分子到凝聚相水体系的理论描述：势能面和分子动力学最新进展

本文回顾了近十年来水体系的势能面与分子动力学理论研究的最新进展,包括水分子参与的气相反应,固体表面上的吸附与解离动力学,以及从团簇到凝聚相水的结构、振动光谱与统计力学模拟. 近年来再次发展起来的机器学习技术,例如结合置换不变多项式的神经网络,或结合基本不变量的神经网络,已被成熟应用于气相与固体表面体系的高精度势能面构造中. 对于团簇甚至凝聚相水体系,原子中心神经网络方法或基于核的高斯过程方法应用更为广泛. 此外,在多体展开框架下,在气相体系中发展起来的的方法也组成了高维度体系势函数构造的高精度方案. 当前凝聚相水体系面临的主要问题是高精度从头算数据集的积累,兼顾计算精度与效率的双杂化密度泛函是一种可能的解决方案. 在动力学理论方面,无论是化学反应截面计算还是振动光谱模拟,往往需要合理描述水分子中氢原子的量子效应,才能得到较为可靠的理论计算结果. 量子波包动力学方法已经在气相反应机理研究方面有深入的应用,也在包含数个水分子的团簇振动分析中有初步应用. 基于路径积分的分子动力学方法正在较大水团簇以及凝聚相水的结构与谱学模拟方面发挥重要作用.     

固体氧化物燃料电池体系掺杂氧化锆的理论研究：从结构到导电性

固体氧化物燃料电池是一种将化学能(如H₂和O₂)转化为电能的清洁能源系统,它具有高效、低碳以及燃料适应性广的特点. 作为燃料电池的“心脏”,电解质决定了整个电池的性能,其中掺杂氧化锆是最为典型的燃料电池电解质材料. 氧化钇稳定氧化锆在高温下具有优良的离子电导率,广泛应用在固体燃料电池中. 电解质材料的组成和使用温度对电导率的影响在实验和理论上已得到了充分研究. 复合氧化物的原子结构的表征是阐明其导电行为的关键,本文综述了氧化钇稳定氧化锆电解质的结构和导电性研究的最新理论进展,比较了研究该材料所采用的不同的理论方法及其相应结果,并总结了各种方法的优缺点. 重点介绍了利用随机表面行走-神经网络方法取得的最新成果,这些成果和实验结果相吻合. 结果表明,采用机器学习进行原子模拟为理解固体电解质中遇到的复杂物质现象提供了一种经济、高效和准确的方法.     

对浸入水或甲醇中的石墨氧化物的理论研究 (cited: 3)

采用密度泛函理论优化了含水和不含水的石墨氧化物的多种结构. 当层间没有水分子时,优化的层间距在6 ?左右,小于6.5～7 ?的实验值. 反之,水石墨氧化物的层间距和实验值符合很好. 基于优化的石墨氧化物结构,用分子动力学方法模拟了水或甲醇中的石墨氧化物. 对于不含水的石墨氧化物,水和甲醇分子不进入其层间. 而对于含水的石墨氧化物,液体分子进入层间,增大了层间距,半定量地重复了实验现象.     

Al与α-Fe2O3纳米界面铝热反应的从头计算分子动力学研究

在正则系综统下,温度为2000 K时,利用基于密度泛函理论的第一性原理分子动力学计算方法对Al与赤铁矿α-Fe₂O₃的铝热反应进行了研究. 模拟得出Fe–O键的数量随着时间的增加而减小,Al–O键和Fe–Fe键的数量随时间的增加而增大;同时Fe离子总的电荷量随时间的增加而减小,而Al离子总的电荷量随时间的增加而增大. 模拟结果表明,在Al/Fe₂O₃铝热反应中发生了氧化还原反应,Al被氧化,Fe离子被还原;在界面处生成Al–O键,Fe–O键发生断裂;氧化还原反应完成需约3 ps. 关键词： 铝热反应 密度泛函理论 分子动力学 正则系综     

CO分子在线性极化飞秒激光场中的TDDFT研究

本文运用将含时密度泛函理论和分子动力学非绝热耦合的方法,研究了CO分子在不同强度、不同极化方向的激光场中的电离和动力学行为. 研究发现,激光强度越强,CO分子吸收的能量越多,电离越早且电离越强,CO分子键长变长且伸缩振动越剧烈. 此外,CO分子偶极矩的变化及峰值也随着激光强度的增强而增大. 对激光极化方向的研究发现,激光极化方向沿着CO分子轴向时,分子的电离最强且伸缩振动最剧烈. 当激光极化角增大时,CO分子的电离逐渐被抑制且电子的偶极振动对激光极化方向表现出较强的依赖性. 此外研究还表明,CO分子碳原子和氧原子周围电子的弥散方式不同且与激光极化方向有关. 关键词： 含时密度泛函理论 分子动力学 分子电离     

硼掺杂富勒烯储氢-从物理吸附到化学吸附的转变

本文采用密度泛函理论(DFT)中的局域密度近似(LDA)方法对硼(B)掺杂富勒烯(C_(35)B)储氢问题在前人的基础上做了进一步研究,结果表明被C_(35)B吸附的氢分子很容易解离,经历从物理吸附到化学吸附的转变,并且发现解离产物C_(34)BHCH有分子内氢转移反应发生,这时B原子仍能与氢分子有很强的相互作用,最终导致B位置以及与B最邻近的三个C原子上都有氢原子吸附.并利用过渡态理论从热力学上分析了这种反应的发生趋势.     

Rh在CeO_2(111)表面吸附的第一性原理研究

采用基于广义梯度近似的投影缀加平面波(Projector augmented wave)赝势和具有三维周期性边界条件的超晶胞模型,用第一原理计算方法,计算并分析了Rh在CeO_2(111)表面吸附所形成的Rh/CeO_2(111)界面体系的吸附能,价键结构和局域电子结构.考虑了Rh在不同吸附位置的吸附情况.结果表明:1)Rh在CeO_2(111)面有较大的吸附能,表明Rh与CeO_2(111)面有较强的相互作用,而且Rh在位于表面O的三度位,且位于次层氧的顶位的吸附最强;2)Rh的4d态同衬底O的2p态有明显的杂化作用,这是Rh同CeO_2较强作用的主要原因;3)Rh被CeO_2(主要是Rh的次近邻的Ce)氧化为Rh~(3+),并伴随着Ce~(4+)→Ce~(3+)反应的发生.     

Pd与CeO2(111)面的相互作用的第一性原理研究

采用基于广义梯度近似的投影缀加平面波(projector augmented wave) 赝势和具有三维周期性边界条件的超晶胞模型,用第一性原理计算方法,计算并分析了Pd在CeO2(111)面上不同覆盖度时的吸附能,价键结构和局域电子结构. 考虑了单层Pd和1/4单层Pd两种覆盖度吸附的情况. 结果表明:1)在单层吸附时,Pd的最佳吸附位置是O的顶位偏向Ce的桥位;在1/4单层吸附时,Pd最易在O的桥位偏向次层O的顶位吸附.2) 单层覆盖度吸附时,吸附原子Pd之间的作用较强;1/4单层覆盖度吸附时,同层吸附的Pd之间的几乎没有相互作用. 同时在低覆盖度吸附时,Pd和CeO2衬底的相互作用较高覆盖度时强.3) Pd的吸附只对其近邻的衬底原子结构有明显影响.4) Pd在CeO2衬底上吸附后,Pd和CeO2衬底的活性增强,并可能在Pd与CeO2界面处形成活性中心. 以上结果有助于人们对三元催化剂中Pd与CeO2的协同作用机理的理解.     

N、P掺杂硼烯/石墨烯异质结的密度泛函理论研究

采用基于密度泛函理论的第一性原理计算方法系统研究了氮、磷掺杂对硼烯/石墨烯异质结的几何结构和电子性质的影响.结果表明,相较完整硼烯/石墨烯异质结的金属特性,氮、磷掺杂的硼烯/石墨烯异质结均表现为半导体特性.室温下的分子动力学模拟进一步论证了相关体系的动力学稳定性.研究结果能够为硼烯/石墨烯异质结在新型二维半导体材料中的应用提供参考价值.     

采用密度泛函理论与分子动力学对聚甲基丙烯酸甲酯双折射性的理论计算

双折射性是各种光学材料的重要性能之一,具有高双折射率的光学材料在诸多研究及工业领域的应用越来越广泛.然而,作为常用的光学薄膜及光波导材料之一的聚合物材料的双折射性通常却很弱,只能通过实验对其双折射率进行大致的表征,缺乏对其双折射率的系统性理论计算,从而限制了提高聚合物双折射性的研究.本文建立了从聚合物的单体分子结构到多分子链的系统性的双折射率理论计算方法,并借助此方法研究了导致聚合物弱双折射性的限制因素.以聚甲基丙烯酸甲酯(PMMA)为研究对象,运用密度泛函理论研究了其本征双折射率,这里的本征双折射率是指分子链完全取向时其单体单元的双折射率.计算结果表明其本征双折射率高达0.0738左右,并且通过计算给出了PMMA单体单元的平均双折射率色散曲线.采用分子动力学方法研究了该聚合物(包含20个分子链)的材料双折射率.理论计算结果表明,尽管该聚合物具有较大的本征双折射率,但是由于聚合物中分子链取向度极低,聚合物材料最终表现出来的双折射率只有0.00052.本文建立的研究方法及研究结果为研究增强聚合物材料双折射性提供了理论依据.     

Elucidating the role of non-radiative processes in charge transfer of core–shell Si–SiO2 nanoparticles

Crystalline silicon is the most commonly used material in photovoltaics but has limitations due to its high cost and non-tunable band gap. A new approach of using inexpensive, non-toxic materials with layers that have different band gaps which absorb a wide range of the solar spectrum has the potential to dramatically increase the efficiencies and lower the costs. Core–shell Si–SiO₂ nanoparticles are ideally suited for the photovoltaic application and have been synthesised by different groups in an array of sizes allowing for absorption in a wide spectral range. A theoretical investigation of fundamental charge transfer processes in these systems can potentially lead to improved devices. Calculations on a model core–shell interface with the formula Si₂₆₄O₁₆₀ which features a silicon layer sandwiched between two SiO₂ layers were performed using the Vienna ab initio software package. The Perdew–Burke–Ernzerhof functional in the basis of plane waves was used along with pseudopotentials to simulate electronic structure. The nuclear motion was considered using ab initio molecular dynamics. The density of states, absorption spectrum, partial charge densities, and radiative recombination lifetimes have been calculated. This interface shows quantum confinement behaviour similar to a particle in a box. The role of non-radiative recombination was also determined by relaxation dynamics.     

Hydroxide degradation pathways for guanidimidazolium cation: A density functional theory study

The chemical stability of anion exchange membrane especially in strong alkaline medium under high temperature is one of the crucial challenges for the commercialization of alkaline anion exchange membrane fuel cells. Herein, the degradation mechanism of guanidimidazolium ionic group used in anion exchange membrane was calculated by density functional theory method. The whole transition states of every elemental reaction were systematically searched, and the changes of crucial bonds length were further analyzed for understanding of the reaction mechanism. It was found that the reaction pathway of OH^? attacking guanidinium group of guanidimidazolium was the dominant way for the degradation of guanidimidazolium. Through comparing the energy barriers of guanidimidazolium, imidazolium, and guanidinium, it can be concluded that the electron‐inducing effect had an intimate connection with the alkali stability of ionic groups. The chemical stability of the conjugated structure groups decreased when linking with electron‐drawing groups, while it increased when linking with electron‐donating groups. This work provides theoretical basis for the design of decent alkali‐resistance and high‐performance anion exchange membrane.     

质子与羟基碰撞的含时密度泛函理论研究

采用将含时密度泛函理论和分子动力学非绝热耦合的方法,研究了不同入射速度下质子与羟基碰撞的反应动力学.计算了碰撞前后质子动能和羟基动能的变化及羟基电子和质子的运动.计算结果表明,质子沿垂直羟基分子轴方向入射时,质子与羟基碰撞后,质子被反弹且动能损失并俘获了羟基中氧的一部分电子,而丢失部分电子的羟基则获得动能以伸缩振动的形式向计算边界平动.随着入射质子的初动能增加,质子从羟基中俘获的电子增多,碰撞后羟基的键长变长,羟基振动变强而伸缩振动频率降低.此外,还发现质子的入射方向对碰撞过程的激发动力学有很大的影响.质子从不同的方向入射时,质子的入射初动能越大,其损失的动能越多且损失的动能与入射初动能呈线性关系,而入射方向对质子动能损失的影响很小.在质子入射初动能较低(小于25 eV)的情况下,羟基获得的动能与质子入射初动能呈线性关系且与入射方向无关;在质子入射初动能较高(大于25 eV)时,当质子沿羟基分子轴方向入射时,羟基动能的增量远大于质子沿垂直于羟基分子轴方向入射时羟基动能的增量.     

Vibrational analysis of L-serine using the density functional theory

In this paper, we present a computational study of L-serine using ab initio molecular dynamics simulation based on density functional theory (DFT) within the ultrasoft pseudopotentials and generalized-gradient approximation. Taking into account the intermolecular interactions, we can indeed simulate the features of the experimental results very well for L-serine zwitterions in its solid state. The vibrational spectrum of L-serine performed by DFT was in excellent agreement with our previous inelastic incoherent neutron scattering spectra measured at 20K for L-serine in the 10--200meV region on HET spectrometers at ISIS, Rutherford Appleton Laboratory.     

Bucky-corn: van der Waals composite of carbon nanotube coated by fullerenes

ABSTRACT

Can a C₆₀ layer cover a surface of single-wall carbon nanotube (SWCNT) forming an exohedral pure-carbon hybrid with only van der Waals interactions? The aim of the present paper is to address this question and to demonstrate that the fullerene shell layer in such a bucky-corn structure can be stable. Theoretical study of the structure, stability and electronic properties of bucky-corn hybrids is reported for the shell of C₆₀ and C₇₀ molecules on an individual SWCNT, C₆₀ dimers on an individual SWCNT as well as C₆₀ molecules on SWNT bundles. The geometry and total energies of the bucky-corn hybrids were calculated by the molecular dynamics method, while the density functional theory method was used to simulate the electronic band structures.     

Progress in Developments of Inorganic Nanocatalysts for Application in Direct Methanol Fuel Cells

Significant progress has been made in the last few years toward synthesizing highly dispersible inorganic catalysts for application in the electrodes of direct methanol fuel cells. In addition, research toward achieving an efficient catalyst supporting matrix has also attracted much attention in recent years. Carbon black- (Vulcan XC-72) supported Platinum and Platinum-Ruthenium catalysts have for long served as the conventional choice as the cathode and the anode catalyst materials, respectively. Oxygen reduction reaction at the cathode and methanol oxidation reaction at the anode occur simultaneously during the operation of a direct methanol fuel cell. However, inefficiencies in these reactions result in a generation of mixed potential. This, in turn, gives rise to reduced cell voltage, increased oxygen stoichiometric ratio, and generation of additional water that is responsible for water flooding in the cathode chamber. In addition, the lack of long-term stability of Pt-Ru anode catalyst, coupled with the tendency of Ru to cross through the polymer electrolyte membrane and eventually get deposited on the cathode, is also a serious drawback. Another source of potential concern is the fact that the natural resource of Pt and the rare earth metal Ru is very limited, and has been predicted to become exhausted very soon. To overcome these problems, new catalyst systems with high methanol tolerance and higher catalytic activity than Pt need to be developed. In addition, the catalyst-supporting matrix is also witnessing a change from traditionally used carbon powder to transition metal carbides and other high-performance materials. This article surveys the recent literature based on the advancements made in the field of highly dispersible inorganic catalysts for application in direct methanol fuel cells, as well as the progress made in the area of catalyst-supporting matrices.     

First principles determination of structural,electronic and lattice dynamical properties of a model dipeptide molecular crystal

We present the results of ab initio calculations of the structural, electronic and lattice dynamical properties of the solid-state crystal of the glycyl-l-alanine dipeptide. Intramolecular bond lengths are found to be in good agreement with experimental values; lattice constants are in reasonable agreement, although it is found that discrepancies do exist. A hierarchy of hydrogen bond strengths is found, with those between (oppositely-charged) amine and carboxy functional groups being strongest. The crystal is found to be an indirect-bandgap material, with indirect bandgaps ≈4.95 eV, compared to a direct bandgap of 5.00 eV. Analysis of the electronic structure reveals that the electronic states in the near vicinity of the energy gap arise from carboxylate and amide oxygen atoms. The arrangement of both molecules and hydrogen bonds in the unit cell is found to manifest itself in increased bandwidth along specific reciprocal space directions, reflecting coupling brought about by hydrogen bonds. Determination of the zone-centre lattice dynamical behaviour permits the IR absorption spectrum to be explained. Intermolecular hydrogen bonds are found to couple intramolecular motions in adjacent moelcules, revealing the importance of an accurate treatment of intermolecular interactions, even for high-frequency vibronic modes.