基本不变量神经网络解析梯度方法的研究

提升势能面的运行速度对于动力学模拟至关重要. 相对于计算简单、 但耗时更长的数值梯度计算, 直接求解势能面梯度的解析公式能够大幅提高势能面的运行效率. 本文发展了基本不变量神经网络解析梯度的生成方法. 计算解析梯度的代码可以通过程序自动生成. 对大量数据点进行测试后, 证明了该方法可以得到正确的势能面梯度输出结果. 通过测试不同势能面的调用时间, 发现采用解析梯度方法能够带来10倍以上的性能提升. 随着体系的增大, 这种性能提升也会越明显.     

多参考态二级微扰方法的改进

本文提出一个新的扩大模型空间的方案用于改进多参考态二级微扰理论(MRPT2)计算. 新方案保持了原方案中扩大模型空间之前的简单程序结构, 理论上完全可以避免势能面计算中入侵态的出现, 并在一系列比较计算中得到证实. 新MRPT2程序是研究分子激发态和电子光谱的有用工具.     

Huckel—Hubbard参数化法及氮叶立德[2,3]和氢[1,3]σ键…

本文建议一种Ｈｕｃｋｅｌ－Ｈｕｂｂａｒｄ参数化法，并用Ｈｕｃｋｅｌ－Ｈｕｂｂａｒｄ理论首次计算了氮叶立德［２，３］和氮［１，３］σ键迁移反应的基态和低激发态势能面。根据计算得到的势能面，对相应的基态和激发态反应途径进行了讨论，得到有价值的结论。     

势能面特征与过渡态熵及A因子

反应体系的势能面,对了解反应的微观过程起着重要的作用,它的特征决定了化学反应的机理.原则上,由反应体系的Schrodinger方程的解,可得到体系能量随核间距变化的函数,从而获得势能面.除少数简单反应外,几乎无法精确求得复杂反应体系的势能面.因而,除从头算法外,人们先后发展了计算势能面的一些半经验方法.对某一反应,文献中可能记载好几个势能面,因此,在分析反应或计算反应的各物理量时,应当说明所应用的是何种势能面.我们曾指出,过渡态熵的可靠性,有赖于提供过渡态参数的势能面.本文从下述基元反应     

D+HCN反应的SVRT含时波包理论研究

基于Horst的势能面,用SVRT(SemirigidVibratingRotorTarget)方法对D+HCN反应进行了含时波包动力学研究,计算得到了不同初始振转态的总反应几率和积分反应截面,采用UniformJ-shifting方法得到该反应的热速率常数.计算结果与H+HCN反应进行了比和讨论.     

Hückel－Hubbard参数化法及氮叶立德［2，3］和氢［1，3］σ键迁移的研究

本文建议一种Ｈüｃｋｅｌ－Ｈｕｂｂａｒｄ参数化法，并用Ｈüｃｋｅｌ－Ｈｕｂｂａｒｄ理论首次计算了氮叶立德［２，３］和氢［１，３］σ键迁移反应的基态和低激发态势能面，根据计算得到的势能面，对相应的基态和激发态反应途径进行了讨论，得到有价值的结论。     

基于神经网络的分子体系势能面构造

本文介绍了近年来使用神经网络构造分子体系势能面的进展,在以前的研究基础上完善了一套完整的分子构型选择方案,在严格的势能面评价标准下,完全解决了势能面构造过程中分子构型选择的难题.基于此方法,采用一系列拟合技巧,结合大量反应动力学计算,成功构造了一系列重要体系的高精度从头算势能面,并能得到可靠的化学动力学结果.     

一种确定反应中间态几何特征和能量的综合性方法

通过综合使用传统的过渡态优化算法、数学统计工具以及人工神经网络算法(ANN)找到一种不依赖于反应物起始构象而得到化学反应中过渡态结构和能量的方法. 在两个反应物互相接近的过程中, 每一步的几何构象都对应着一个系统能量值. 本研究的目的是尽可能地收集处在反应能量面上的这种能量点值. 通过采用几何参数作为自变量对势能面进行模拟研究, 得到了势能面上对应过渡态结构的一阶鞍点. 采用乙醛负离子和甲醛作为反应物, 对经典的醛醇缩合反应中的亲核进攻步骤进行了研究. 对内禀反应坐标(IRC)路径的计算是从反应物的三组不同起始构象出发, 最终获得了反应势能面上的96个点. 本研究中的势能面采用人工神经网络算法进行模拟研究, 并利用交叉验证方法评估得到的结果, 避免了采用人工神经网络算法时过度拟合情况的发生.     

Ne-HCl势能面和振转光谱的理论研究

利用量子化学计算方法CCSD(T)和大基组aug-cc-pVTZ加键函数3_s3_p2_d对Ne-HCl体系的分子间势能面进行了理论研究.结果表明,势能面上有两个势阱,分别对应于线性Ne-ClH和Ne-HCl构型.通过精确求解核运动方程发现,该从头算势能面分别支持5个(对Ne-HCl)和7个(Ne-DCl)振动束缚态.计算得到的振转跃迁频率与实值值吻合.     

He-N20偶极矩面及振转跃迁强度的理论研究

用超分子MP4方法和大基组(aug-cc-pVTZ)及键函数得到He-N2O体系的分子间从头算势能面及偶极矩面．用离散变量表象方法计算了^4He-N2O及^3He-N2O体系的振转能级，并进一步计算得到其振转跃迁强度，计算结果很好地解释了实验现象。     

Ab initio study of the ground and excited states of HCP and its isomer HPC

The potential energy surface of HCP converting to HPC in its ground electronic state has been investigated with ab initio methods at levels up to MP2/6-311G**//MP4/6-311G** as well as TZV + + ** CASSCF. All geometries are fully optimized and compare favorably to previous theoretical and experimental values. The HCP molecule is predicted to be 85.4 kcal/mol lower in energy than its linear isomer at the-MP2/6-31G*//MP2/6-31G* level. The energy barrier for hydrogen rearrangement is found to be merely 2.3 kcal from the HPC end. CASSCF studies were initiated to clarify the low barrier and lent support to a flat surface as HPC converts to stable, linear HCP at the TZV + + ** level. CASSCF also predicts that HPC is unstable with respect to bending. Harmonic vibrational frequencies for HCP results in 5% accuracy or better. A bent triplet is found to be the lowest excited state using the CASSCF method. © 1993 John Wiley & Sons, Inc.     

SCF and CASSCF studies of geometrical rearrangements in LiBO

The minimum energy path for the rearrangement LiOB OBLi was calculated with the SCF approximation using a double-zeta plus polarization basis set. Stationary points on the potential surface were studied with the help of the CASSCF method using different choices of active space. The results indicate that LiBO may be regarded as a polytopic type molecule. The relative energies of different geometrical configurations changed considerably when the CASSCF method was used, compared to those obtained at the SCF level, demonstrating the importance of correlation effects for this system.     

An ab initio study toward understanding the mechanistic photochemistry of acetamide

The potential energy surfaces for CH(3)CONH(2) dissociation into CH(3) + CONH(2), CH(3)CO + NH(2), CH(3)CN + H(2)O, and CH(3)NH(2) + CO in the ground and lowest triplet states have been mapped with DFT, MP2, and CASSCF methods with the cc-pVDZ and cc-pVTZ basis sets, while the S(1) potential energy surfaces for these reactions were determined by the CASSCF/cc-pVDZ optimizations followed by CASSCF/MRSDCI single-point calculations. The reaction pathways leading to different photoproducts are characterized on the basis of the computed potential energy surfaces and surface crossing points. A comparison of the reactivity among HCONH(2), CH(3)CONH(2), and CH(3)CONHCH(3) has been made, which provides some new insights into the mechanism of the ultraviolet photodissociation of small amides.     

Nonadiabatic decomposition of gas-phase RDX through conical intersections: an ONIOM-CASSCF study

Topographical exploration of nonadiabatically coupled ground- and excited-electronic-state potential energy surfaces (PESs) of the isolated RDX molecule was performed using the ONIOM methodology: Computational results were compared and contrasted with the previous experimental results for the decomposition of this nitramine energetic material following electronic excitation. One of the N-NO(2) moieties of the RDX molecule was considered to be an active site. Electronic excitation of RDX was assumed to be localized in the active site, which was treated with the CASSCF algorithm. The influence of the remainder of the molecule on the chosen active site was calculated by either a UFF MM or RHF QM method. Nitro-nitrite isomerization was predicted to be a major excited-electronic-state decomposition channel for the RDX molecule. This prediction directly corroborates previous experimental results obtained through photofragmentation-fragment detection techniques. Nitro-nitrite isomerization of RDX was found to occur through a series of conical intersections (CIs) and was finally predicted to produce rotationally cold but vibrationally hot distributions of NO products, also in good agreement with the experimental observation of rovibrational distributions of the NO product. The ONIOM (CASSCF:UFF) methodology predicts that the final step in the RDX dissociation occurs on its S(0) ground-electronic-state potential energy surface (PES). Thus, the present work clearly indicates that the ONIOM method, coupled with a suitable CASSCF method for the active site of the molecule, at which electronic excitation is assumed to be localized, can predict hitherto unexplored excited-electronic-state PESs of large energetic molecules such as RDX, HMX, and CL-20. A comparison of the decomposition mechanism for excited-electronic-state dimethylnitramine (DMNA), a simple analogue molecule of nitramine energetic materials, with that for RDX, an energetic material, was also performed. CASSCF pure QM calculations showed that, following electronic excitation of DMNA to its S(2) surface, decomposition of this molecule occurs on its S(1) surface through a nitro-nitrite isomerization producing rotationally hot and vibrationally cold distributions of the NO product.     

Unexpected CASSCF bistability phenomenon

One exhibits a problem in which two strongly nonorthogonal complete active-space self-consistent field (CASSCF) solutions are obtained. The problem concerns a molecular frame which presents two stable geometries at the CASSCF level, quinoidal and diradical forms, in disagreement with experiment which indicates a unique minimum corresponding to an intermediate geometry. Those two stable solutions are obtained in a significant domain of intermediate geometries and are related with the CASSCF wave functions of the two stable structures. Obtaining a reliable potential surface starting from CASSCF solutions (even from larger CAS) appears as a very difficult task. © 1997 John Wiley & Sons, Inc.     

HSO自由基电子基态激发态的CAS计算研究

使用CASSCF方法和ANO-L基组优化了HSO自由基的基态和3个低占据激发态的结构, 并采用包括更多电子动态相关能的CASPT2方法进行了单点能校正. 频率计算结果表明, 优化的4个几何为势能面上的稳定点. 通过电子结构的研究合理地解释了各个激发态相对于电子基态的结构变化.     

Singlet and triplet potential surfaces for the O2+C2H4 reaction

Electronic structure calculations at the CASSCF and UB3LYP levels of theory with the aug-cc-pVDZ basis set were used to characterize structures, vibrational frequencies, and energies for stationary points on the ground state triplet and singlet O(2)+C(2)H(4) potential energy surfaces (PESs). Spin-orbit couplings between the PESs were calculated using state averaged CASSCF wave functions. More accurate energies were obtained for the CASSCF structures with the MRMP2/aug-cc-pVDZ method. An important and necessary aspect of the calculations was the need to use different CASSCF active spaces for the different reaction paths on the investigated PESs. The CASSCF calculations focused on O(2)+C(2)H(4) addition to form the C(2)H(4)O(2) biradical on the triplet and singlet surfaces, and isomerization reaction paths ensuing from this biradical. The triplet and singlet C(2)H(4)O(2) biradicals are very similar in structure, primarily differing in their C-C-O-O dihedral angles. The MRMP2 values for the O(2)+C(2)H(4)→C(2)H(4)O(2) barrier to form the biradical are 33.8 and 6.1 kcal/mol, respectively, for the triplet and singlet surfaces. On the singlet surface, C(2)H(4)O(2) isomerizes to dioxetane and ethane-peroxide with MRMP2 barriers of 7.8 and 21.3 kcal/mol. A more exhaustive search of reaction paths was made for the singlet surface using the UB3LYP/aug-cc-pVDZ theory. The triplet and singlet surfaces cross between the structures for the O(2)+C(2)H(4) addition transition states and the biradical intermediates. Trapping in the triplet biradical intermediate, following (3)O(2)+C(2)H(4) addition, is expected to enhance triplet→singlet intersystem crossing.     

Theoretical investigation of the stereoselective stepwise cope rearrangement of a 3-vinylmethylenecyclobutane

The potential energy surface of the rearrangement of 3-vinylmethylenecyclobutane to 4-methylenecyclohexene has been studied computationally using density functional theory (B3LYP) and complete active space ab initio methods (CASSCF and CASPT2). The parent reaction is nonconcerted and occurs through several parallel diradical pathways. Transition structures and diradical intermediates are highly comparable in energy, with no deep potential energy well on the potential energy surface. In the substituted system, stereoelectronic effects of the trialkylsiloxy group regulate torquoselectivity in the bond-breaking processes and this, combined with low barriers to cyclization, leads to a stepwise Cope rearrangement that is, nevertheless, stereoselective.     

Photostability via a sloped conical intersection: a CASSCF and RASSCF study of pyracylene

An extensive ab initio study of the ground- and excited-state potential energy surfaces of pyracylene is presented in this work. CASSCF calculations show that there is an accessible sloped S0/S1 conical intersection, which leads to ultrafast internal conversion and explains the observed photostability. RASSCF calculations (using a well-defined subset of the CASSCF configurations) are shown to be able to reproduce CASSCF results satisfactorily and will therefore be useful for larger systems where CASSCF is currently too expensive. MRCI and MRPT2 energy corrections are computed to assess the ionic character of the excited states. Finally, MMVB calculations are also benchmarked against CASSCF, to assess the reliability of this parametrized method for excited states of large conjugated polycyclic aromatic hydrocarbons.     

Accurate MRCI and CC study of the most relevant stationary points and other topographical attributes for the ground-state C(2)H(2) potential energy surface

Extensive ab initio calculations have been performed to determine the energy, geometry, vibrational frequencies, and relative energetics of all stationary points of the C(2)H(2) ground-state potential-energy surface. The geometries of acetylene and vinylidene minima as well as all transition states are reported at the CASSCF, MRCI, and CCSD(T) levels with aug-cc-pVXZ basis sets. Other more advanced levels of CC theory have also been utilized where judged adequate, mostly for check purposes. Also reported are theoretical limiting values of the energetics of the reaction, deduced from series of computations using the USTE extrapolation method. The data here reported should be valuable for modeling a single-sheeted global potential energy surface for the title system.