耗散粒子动力学及其应用的新进展

耗散粒子动力学(dissipative particle dynamics,DPD),是1992年提出的一种针对于复杂流体的介观模拟方法,凭着相对于其它模拟方法的高计算效率以及能够模拟流体水力学行为的优势,目前已经成为研究复杂流体动力学行为的主要模拟方法之一,同时也是联系微观结构与宏观实验的重要桥梁,在辅助研究复杂流体性质方面起到了重要的作用,本文对这一模拟方法在近几年的发展及其应用做了全面的总结。首先介绍了近年来方法本身的改进和完善,接着详细总结了目前的应用研究热点。作为一种计算机实验工具,这种方法近年来在对复杂流体的研究上进展迅速,既能用于预测平衡过程的结构和性质,又能用于研究非平衡过程的动力学因素。     

二维三角格子上高分子动态 Monte Carlo模拟的协同运动算法

在动态Monte Carlo模拟的协同运动算法中,几个相邻的链节可以同时运动,这可以理解为高分子链中张力的作用引起的协同运动。将这一算法用于二维三角格子模型上RW链和SAW链的模拟。结果表明RW链的动力学行为符合Rouse理论,说明说明该算法可以用于高分子动力学研究,其优点是不需要使用键长涨落模型。     

Triton X-100/甲苯/水三元体系界面张力的耗散颗粒动力学模拟

采用耗散颗粒动力学方法在介观层次上模拟了非离子表面活性剂Triton X-100 在油/水界面上的分布行为, 并把用于油/水二元体系界面张力的计算方法拓展到含表面活性剂的三元体系. 利用该方法可以得到与实验数值吻合的界面张力数据. 另外, 模拟结果直观展示了表面活性剂界面张力与界面密度的关系, 为表面活性剂复配增效理论提供了依据. 该模拟方法给出的微观信息可以为驱油体系配方筛选和表面活性剂有效应用提供指导.     

非等温硼酸酯化反应动力学研究

通过对酯化反应特点的分析,建立了只需跟踪反应出水量、反应时间和相应的体系温度,便可对非等温条件下酯化反应动力学进行定量分析的实验方法,并导出了用于计算动力学参数的理论公式.在此基础上研究了二甘醇独丁醚硼酸酯的合成动力学,经计算得知该反应为二级反应,表现活化能为98.33kj/mol,频率因子为1.048×10^7L·mol^-1·S^-1.文中通过一组不同升温速率条件下的实验数据,对反应动力学参数及所导计算公式进行了验证.     

电解质溶液自扩散系数的布朗动力学模拟

采用布朗动力学方法对电解质溶液进行了模拟,在传统布朗动力学的基础上综合考虑了流体力学的影响,并且引入SmartMonteCarlo方法的接受概率,避免了离子不现实的移动和位型重叠,这样不仅可以将模拟过程中的时间步长大幅度提高,而且还可使溶质在相空间的演化过程更接近实际.模拟过程以电解质溶液的原始模型为基础,将溶剂看作连续介质,溶质分子之间的相互作用采用软核加静电的势能函数模型,长程静电力采用Ewald加和的处理方法.模拟得到KCl和NaCl溶液的径向分布函数g+-(r),g++(r)和g--(r),并与文献中HNC计算以及模拟的结果进行比较,使用推广的Green-Kubo公式模拟计算溶液中各种离子的自扩散性质,计算结果与实验数据吻合良好.     

连锁聚合反应的动态Monte Carlo模拟及其浓度效应

用动态Monte Carlo方法和键长涨落模型相结合分别对自由基聚合、离子聚合动力学过程进行了计算机模拟, 在三维空间中统计了转化率、聚合度、分子量的数量和重量分布, 并将模拟结果和理论预测进行了对比分析, 同时得到不同浓度下均方回转半径与平均链长的标度关系, 说明三维空间中标度指数与浓度的相关性. 还初步考察了扩散对反应体系分子量的影响. 说明动态Monte Carlo方法用于研究连锁聚合反应动力学过程是有效的, 而且能够同时得到经典的聚合反应Monte Carlo模拟方法所难以得到的链构象、分子扩散等空间相关的信息.     

电解质水溶液传递性质的布朗动力学模拟研究

在传统布朗动力学的基础上, 考虑流体力学的影响, 并且引入Smart Monte Carlo方法的接受概率, 对电解质溶液进行布朗动力学模拟, 得到不同浓度和温度下KCl溶液中离子间的径向分布函数, 并且与超网链积分方程理论计算结果进行了比较, 同时, 模拟了KCl和NaCl溶液的摩尔电导率. 模拟过程基于电解质溶液的原始模型, 溶剂被看作连续介质, 溶质分子之间的相互作用采用软核加静电的势能函数模型, 长程静电力的处理采用Ewald加和方法. 结果显示, 流体力学的作用对于电解质溶液的结构性质没有明显的影响, 但是对于传递性质的影响显著; 考虑流体力学作用的布朗动力学模拟结果与实验数据吻合良好.     

流场中单链高分子过孔输运的动力学行为

采用布朗动力学方法,通过模拟不同温度和流速下,不同链长的单链高分子穿过纳米孔洞的动力学行为,证实了临界流速的存在,并验证了理论预测的结果,即临界流量随体系温度线性增大,且与链长无关.还对高分子链过孔输运时间以及输运过程中链构象的变化进行了研究.模拟结果为明晰在剪切流场中高分子穿过纳米孔洞的动力学行为提供了有益参考.     

基于Monte Carlo模拟的化学反应动力学参数估算

提出并采用基于MonteCarlo模拟与动力学实验相结合的化学反应动力学参数估算方法,由基元反应确定MonteCarlo模拟具体做法,将MonteCarlo模拟结果与动力学实验结果相比较,根据比较结果自动调整和优化动力学参数,从而无需事先确定动力学方程即可有效估算各种化学反应的动力学参数值.采用该方法估算了丙烯氨氧化反应动力学参数,并对估算结果进行了分析与讨论.     

辛算法在准经典轨迹计算中的应用: H+H2体系

采用辛算法模拟了H + H₂在BKMP势能面上的长时间动力学行为, 并和Runge-Kutta方法计算的结果进行了比较.结果表明,采用Runge-Kutta方法模拟长时动力学行为时体系中出现动力学耗散行为,导致无法真实的模拟体系的物理实际;而采用辛算法模拟精确度更高,四阶辛算法误差只有Runge-Kutta法的约百分之一,六阶辛算法的误差则更小,体系中也未见动力学耗散行为.这些结果说明,在进行长时动力学行为模拟的时候,在保持体系能量守恒性上,辛算法应当是一种更好的积分方法.     

Are azobenzenophanes rotation-restricted?

We simulated the photoisomerization dynamics of an azobenzenophane with a semiclassical surface hopping approach and a semiempirical reparametrized quantum mechanics/molecular mechanics Hamiltonian. Only one of the two azobenzene chromophores in the molecule is taken into account quantum mechanically: the other one is treated by molecular mechanics. Both n-->pi* and pi-->pi* excitations are considered. Our results show that the photoisomerization reaction mainly involves the rotation around the N=N double bond. The excited state relaxation features are in qualitative agreement with experimental time-resolved fluorescence results.     

Optimal control of ultrafast cis-trans photoisomerization of retinal in rhodopsin via a conical intersection

Optimal control simulation is applied to the cis-trans photoisomerization of retinal in rhodopsin within a two-dimensional, two-electronic-state model with a conical intersection [S. Hahn and G. Stock, J. Phys. Chem. B 104, 1146 (2000)]. For this case study, we investigate coherent control mechanisms, in which laser pulses work cooperatively with a conical intersection that acts as a "wave-packet cannon." Optimally designed pulses largely consist of shaping subpulses that prepare a wave packet, which is localized along a reaction coordinate and has little energy in the coupling mode, through multiple electronic transitions. This shaping process is shown to be essential for achieving a high target yield although the envelopes of the calculated pulses depend on the local topography of the potential-energy surfaces around the conical intersection and the choice of target. The control mechanisms are analyzed by considering the motion of reduced wave packets in a nuclear configuration space as well as by snapshots of probability current-density maps.     

Functional and Basis Set Dependence for Time-Dependent Density Functional Theory Trajectory Surface Hopping Molecular Dynamics: Cis-Azobenzene Photoisomerization

Within three functionals (TD-B3LYP, TD-BHandHLYP, and TD-CAM-B3LYP) in combination with four basis sets (3-21g, 6-31g, 6-31g(d), and cc-pvdz), global switching (GS) trajectory surface hopping molecular dynamics has been performed for cis-to-trans azobenzene photoisomerization up to the S₁(nπ*) excitation. Although all the combinations show artificial double-cone structure of conical intersection between ground and first excited states, simulated quantum yields and lifetimes are in good agreement with one another; 0.6 (±5%) and 40.5 fs (±10%) by TD-B3LYP, 0.5 (±10%) and 35.5 fs (±4%) by TD-BHandHLYP, and 0.44 (±9%) and 35.2 fs (±10%) by TD-CAM-B3LYP. By analyzing distributions of excited-state population decays, hopping spots, and typical trajectories with performance of 12 functional/basis set combinations, it has been concluded that functional dependence for given basis set is slightly more sensitive than basis set dependence for given functional. The present GS on-the-fly time-dependent density functional theory (TDDFT) trajectory surface hopping simulation can provide practical benchmark guidelines for conical intersection driven excited-state molecular dynamics simulation involving in large complex system within ordinary TDDFT framework. © 2019 Wiley Periodicals, Inc.     

Superior Z→E and E→Z photoswitching dynamics of dihydrodibenzodiazocine, a bridged azobenzene, by S1(nπ*) excitation at λ = 387 and 490 nm

The ultrafast Z→E and E→Z photoisomerisation dynamics of 5,6-dihydrodibenzo[c,g][1,2]diazocine (1), the parent compound of a class of bridged azobenzene-based photochromic molecular switches with a severely constrained eight-membered heterocyclic ring as central unit, have been studied by femtosecond time-resolved spectroscopy in n-hexane as solvent and by quantum chemical calculations. The diazocine contrasts with azobenzene (AB) in that its Z rather than E isomer is the energetically more stable form. Moreover, it stands out compared to AB for the spectrally well separated S(1)(nπ*) absorption bands of its two isomers. The Z isomer absorbs at around λ = 404 nm, the E form has its absorption maximum around λ = 490 nm. The observed transient spectra following S(1)(nπ*) photoexcitation show ultrafast excited-state decays with time constants τ(1) = 70 fs for the Z and <50 fs for the E isomer reflecting very fast departures of the excited wave packets from the S(1) Franck-Condon regions and τ(2) = 270 fs (320 fs) related to the Z→E (resp. E→Z) isomerisations. Slower transient absorption changes on the time scale of τ(3) = 5 ps are due to vibrational cooling of the reaction products. The results show that the unique steric constraints in the diazocine do not hinder, but accelerate the molecular isomerisation dynamics and increase the photoswitching efficiencies, contrary to chemical intuition. The observed isomerisation times and quantum yields are rationalised on the basis of CASPT2//CASSCF calculations by a S(1)/S(0) conical intersection seam at a CNNC dihedral angle of ≈96° involving twisting and torsion of the central CNNC moiety. With improved photochromism, high quantum yields, short reaction times and good photostability, diazocine 1 and its derivatives constitute outstanding candidates for photoswitchable molecular tweezers and other applications.     

An apparent angle dependence for the nonradiative deactivation of excited triplet states of sterically constrained, binuclear ruthenium(II) bis(2,2':6',2' '-terpyridine) complexes

The photophysical properties are reported for a series of binuclear ruthenium(II) bis(2,2':6',2"-terpyridine) complexes built around a geometrically constrained, biphenyl-based bridge. The luminescence quantum yield and lifetime increase progressively with decreasing temperature, but the derived rate constant for nonradiative decay of the lowest-energy triplet state depends on the length of a tethering strap attached at the 2,2'-positions of the biphenyl unit. Since the length of the strap determines the dihedral angle for the central C-C bond, the rate of nonradiative decay shows a pronounced dependence on angle. The minimum rate of nonradiative decay occurs when the dihedral angle is 90 degrees, but there is a maximum in the rate when the dihedral angle is about 45 degrees. This effect does not appear to be related to the extent of electron delocalization at the triplet level but can be explained in terms of variable coupling with a low-frequency vibrational mode associated with the strapped biphenyl unit.     

The photoisomerization mechanism of azobenzene: a semiclassical simulation of nonadiabatic dynamics

We have simulated the photoisomerization dynamics of azobenzene, taking into account internal conversion and geometrical relaxation processes, by means of a semiclassical surface hopping approach. Both n-->pi* and pi-->pi* excitations and both cis-->trans and trans-->cis conversions have been considered. We show that in all cases the torsion around the N==N double bond is the preferred mechanism. The quantum yields measured are correctly reproduced and the observed differences are explained as a result of the competition between the inertia of the torsional motion and the premature deactivation of the excited state. Recent time-resolved spectroscopic experiments are interpreted in the light of the simulated dynamics.     

Quantum dynamics study of fulvene double bond photoisomerization: the role of intramolecular vibrational energy redistribution and excitation energy

The double bond photoisomerization of fulvene has been studied with quantum dynamics calculations using the multi-configuration time-dependent Hartree method. Fulvene is a test case to develop optical control strategies based on the knowledge of the excited state decay mechanism. The decay takes place on a time scale of several hundred femtoseconds, and the potential energy surface is centered around a conical intersection seam between the ground and excited state. The competition between unreactive decay and photoisomerization depends on the region of the seam accessed during the decay. The dynamics are carried out on a four-dimensional model surface, parametrized from complete active space self-consistent field calculations, that captures the main features of the seam (energy and locus of the seam and associated branching space vectors). Wave packet propagations initiated by single laser pulses of 5-25 fs duration and 1.85-4 eV excitation energy show the principal characteristics of the first 150 fs of the photodynamics. Initially, the excitation energy is transferred to a bond stretching mode that leads the wave packet to the seam, inducing the regeneration of the reactant. The photoisomerization starts after the vibrational energy has flowed from the bond stretching to the torsional mode. In our propagations, intramolecular energy redistribution (IVR) is accelerated for higher excess energies along the bond stretch mode. Thus, the competition between unreactive decay and isomerization depends on the rate of IVR between the bond stretch and torsion coordinates, which in turn depends on the excitation energy. These results set the ground for the development of future optical control strategies.     

H-vinyl conical intersections for dienes: a mechanism for the photochemical hula twists

The potential role of "H-vinyl" conical intersections in a photochemical process known as the hula twist has been evaluated. The H-vinyl conical intersections of butadiene are explored with complete active space self-consistent field calculations, and proposals about the geometries involved in the hula-twist mechanism are discussed and contrasted with the conventional one-bond flip mechanism of cis-trans isomerization.     

Ab initio nonadiabatic quantum dynamics of cyclohexadiene/hexatriene ultrafast photoisomerization

Reaction mechanisms of the ultrafast photoisomerization between cyclohexadiene and hexatriene have been elucidated by the quantum dynamics on the ab initio potential energy surfaces calculated by multireference configuration interaction method. In addition to the quantum wave-packet dynamics along the two-dimensional reaction coordinates, the semiclassical analyses have also been carried out to correctly estimate the nonadiabatic transition probabilities around conical intersections in the full-dimensional space. The reaction time durations of radiationless decays in the wave-packet dynamics are found to be generally consistent with the femtosecond time-resolution experimental observations. The nonadiabatic transition probabilities among the ground (S0), first (S1), and second (S2) excited states have been estimated by using the semiclassical Zhu-Nakamura formula considering the full-dimensional wave-packet density distributions in the vicinity of conical intersections under the harmonic normal mode approximation. The cyclohexadiene (CHD) ring-opening process proceeds descending on the S1(1 1B) potential after the photoexcitation. The major part of the wave-packet decays from S1(1 1B) to S1(2 1A) by the first seam line crossing along the C2-symmetry-breaking directions. The experimentally observed ultrafast S1-S0 decay can be explained by the dynamics through the S1-S0 conical intersection along the direction toward the five-membered ring. The CHD: hexatriene (HT) branching ratio is estimated to be approximately 5:5, which is in accordance with the experiment in solution. This branching ratio is found to be mainly governed by the location of the five-membered ring S1-S0 conical intersection along the ground state potential ridge between CHD and HT.