精确固定节面量子Monte：Carlo差值法

本文提出了精确固定节面量子Monte Carlo差值法，这个新算法能够在精确固定节面量子Monte Carlo方法的基础上直接计算两个体系之间的能量差，且使计算结果的统计误差达到10-2 kJ/mol 数量级，获得电子相关能90%以上。我们把这个新算法应用于分子势能面的研究中，使用一个“刚性移动”模型，利用Jacobi变换使分子两个几何构型的能量计算具有很好的正相关性，因而能得到准确的能量差值，于是精确的分子势能面就可以得到。这个新算法已经被使用到BH分子基态势能曲线和H₃分子势能面的研究。这个算法还可应用于分子光谱、化学反应能量变化值等领域的研究。     

精确固定节面量子Monte Carlo差值法

提出了精确固定节面量子Monte Carlo差值法, 这个新算法能够在精确固定节面量子Monte Carlo方法的基础上直接计算两个体系之间的能量差, 且使计算结果的统计误差达到10-5 hartree 数量级, 获得电子相关能90%以上. 我们把这个新算法应用于分子势能面的研究中, 使用一个“刚性移动”模型, 利用Jacobi变换使分子两个几何构型的能量计算具有很好的正相关性, 因而能得到准确的能量差值, 由此就可以得到精确的分子势能面.     

Ne-CO2的从头算势能面及微波光谱

采用三重激发校正的耦合簇[CCSD(T)]方法和大基组计算了范德华复合物Ne-CO2的分子间势能面. 分子间相互作用能的计算采用考虑了基组重叠误差修正的超分子方法. 计算结果表明, 该势能面有两个极小值点, 分别对应T形构型和线性Ne-OCO构型. 采用离散变量表象(DVR)方法及Lanczos算法计算了Ne-CO2的振转能级. 计算结果表明, 体系势能面支持22个振动束缚态. 计算得到的微波光谱的跃迁频率与实验值吻合得很好.     

N_2H体系的势能面及振动激发态的理论研究

本文采用含Davidson校正的多参考组态相互作用MRCI+Q方法以及aug-cc-pVQZ基组计算了N2H体系的基态与第一激发态近24000个从头算能量点,使用三次样条插值法构建了高精度的全域绝热势能面.基态势能面具有一个极小点以及高于极小点0.44 eV的鞍点.基于该势能面上发生的N(~4S)+NH(X~3∑~-)→H(~2S)+N_2(X~1∑_g~g)反应是一个无能垒的放热反应,放热量为6.172 eV,比过去的势能面更接近实验值(5.956 eV).基态与第一激发态势能面间存在明显的锥形交叉,说明两个态之间存在非绝热耦合.此外,采用Lanczos算法预测了基态N_2H分子的振动能级.     

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.     

OCS电子基态势能面与振动光谱的理论研究

本文采用键长-键角内标系下的自洽场-组态相互作用方法精确计算了OCS分子的振动高激发态能级,并结合实验观测到的振动能级利用非线性最小二乘法优化电子基态势能函数中的势能参数。由优化所得的势能面计算出的振动激发态能级与50个实验观测到的振动能级比较,标准偏差为0.08cm^-^1。此外,还用该势能面计算了OCS同位素分子的振动能级,计算结果与实验值也十分吻合。     

确定三原子分子势能面的SCF-CI方法

建议根据振动高激发态的实验能级确定三原子分子势能面的SCF-CI方法.此法中使用键长-键角内坐标系下的SCF-CI方法来精确地计算振动高激发态的能级及其对势能参数的一阶微分,并使用LMF算法来优化势能参数.为验证此方法,优化了水分子的势能面,计算出水分子的振动高激发态能级与70个观测到的振动能级相比较,标准偏差为1.15cm~(-1).     

复合物Kr-CS2的一个新的四维的势能面及其红外光谱的预测

用高精度的量子化学从头算方法构建了Kr-CS₂体系的精确的四维势能面.对该势能面不仅考虑了分子间的振动方式,而且考虑了单体CS₂分子内的ν₁对称伸缩和ν₃反对称伸缩.采用了量子化学超分子方法在[CCSD（T）]-F12水平上构建了从头算势能面.将得到的四维势能面作积分,便得到CS₂处于振动基态和激发态的复合物平均势能面,这两个态的势能面均有一个T型的全局最小值和两个相等的线性极小值.还通过径向部分采用离散变量表象法（DVR）和角度部分采用有限基组表象法（FBR）结合Lanczos循环算法对Kr-CS₂复合物的振转能级进行了计算,对光谱常数进行了预测.另外,也预测了Kr-CS₂复合物的带心位移（-1.2357 cm^-1）.     

He-N2O的从头算势能面及振转能级

采用超分子MP4方法和较大的基组计算得到了He-N₂O体系的分子间势能面,发现该势能面有3个极小值点,分别对应T形构型及两个线性He-ONN和He-NNO构型.同时采用离散变量表象方法预测了体系的振转能级,计算结果表明,MP4势能面支持5个振动束缚态.     

Xe-N2O复合物的分子间势能面和振转光谱的理论研究

采用CCSD(T)方法研究了范德华分子体系Xe—N2O复合物的势能面和振转光谱性质,研究表明,该势能面有两个极小点,分别对应T构型和线性Xe—ONN构型,采用离散变量表象和Lanczos算法计算了体系的振转能级,计算结果表明,CCSD(T)势能面支持97个振动束缚态,并对能级进行了指认,计算得到的Xe—N2O转动跃迁频率与实验值吻合得很好。     

Differential diffusion quantum Monte Carlo method: determination of potential energy surfaces of molecules

A differential approach for self-optimizing diffusion Monte Carlo calculation was proposed in this paper, which is a new algorithm combining three techniques such as optimizing, diffusion and correlation sampling. This method can be used to directly compute the energy differential between two systems in the diffusion process, making the statistical error of calculation be reduced to order of 10-5 hartree, and recover about more than 80% of the correlation energy. We employed this approach to set up a potential energy surface of a molecule, used a "rigid move" model, and utilized Jacobi transformation to make energy calculation for two configurations of a molecule having good positive correlation. So, an accurate energy differential could be obtained, and the potential energy surface with good quality can be depicted. In calculation, a technique called "post-equilibrium remaining sample was set up firstly, which can save about 50% of computation expense. This novel algorithm was used to study the potenti     

Statistical mechanics of energy transfer in gas–surface scattering: A dynamical Lie algebraic approach

The dynamical Lie algebraic (DLA) method is used to describe statistical mechanics of energy transfer in rotationally inelastic molecule–surface scattering. Statistical average values of an observable for the scattering system are calculated in terms of density operator formalism in statistical mechanics. Employing a cubic expansion procedure of molecule–surface interaction potential leads to generation of a dynamical Lie algebra. Thus these statistical average values as a function of the group parameters can be obtained analytically in this formulation. The group parameters can be found from solving a set of coupled nonlinear differential equations. The DLA method, which has no need for determination of transition probabilities in advance as made routinely in the calculation, offers an efficient alternative to the method for computing the statistical average values. This method is much less computationally intensive because most of calculations can be analytically carried out. The average final rotational energies and their dependence on the main dynamic variables and the average interaction potential are presented for the rotationally inelastic scattering of NO molecules from a flat, static Ag(111) surface. Direct comparison is made between the predictions of this model calculation and experiment. The model reproduces well the degree of rotational excitation and correlation between the average final translational and the average rotational energies. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Method of local increments for the calculation of adsorption energies of atoms and small molecules on solid surfaces. 2. CO/MgO(001)

The method of local increments is used in connection with an embedded cluster approach and wave function based quantum chemical ab initio methods to describe the adsorption of a single CO molecule on the MgO(001) surface. The first step in this approach is a conventional Hartree-Fock calculation. The occupied orbitals are then localized by means of the Foster-Boys localization procedure, and the full system is decomposed into several "subunits" that consist of the orbitals localized at the CO molecule and at the Mg and O atoms of the MgO cluster. The correlation energy is expanded into a series of local n-body increments that are evaluated separately and independently. In this way, big savings in computer time can be achieved because (a) the treatment of a large system is replaced with a series of much faster calculations for small subsystems and (b) the big basis sets necessary for describing dispersion effects are only needed for the atoms in the respective subsystem while all other atoms can be treated by medium size Hartree-Fock type basis sets. The coupled electron pair approach, CEPA, an approximate coupled cluster method, is used to calculate the correlation energies of the various subsystems. For the vertical adsorption of CO on top a Mg atom of the MgO(001) surface with the C atom toward Mg, the individual one- and two-body increments are calculated as functions of the CO-MgO separation and a full potential energy curve is constructed from them. A very shallow minimum with an adsorption energy of 0.016 eV at a Mg-C distance of 3.04 ? is found at the Hartree-Fock level, while inclusion of correlation (dispersion) effects shortens the Mg-C distance to 2.59 ? and yields a much larger adsorption energy of 0.124 eV. This is in very good agreement with the best experimental value of 0.14 eV. The basis set superposition error, BSSE, was fully corrected for by the counterpoise method and the bonding mechanism was analyzed at the Hartree-Fock level by means of the constrained space orbital variation, CSOV, analysis.     

On the classical simulation of unimolecular reaction processes

The numerical simulation of the internal motions of a molecule undergoing a unimolecular reaction on an assumed potential energy surface requires the step-by-step solution of a set of simultaneous differential equations. After several thousand time steps, due to differences in the handling of rounding errors in different computing systems, the situation often arises that no two computing machines will give the same result for a given trajectory, even when running the identical algorithm.     

氮原子在Ni平坦和缺陷表面上的吸附和振动

 构造了氮－镍相互作用的5－参数Morse势，研究了氮原子在Ni（\r\n100），Ni（110）和Ni（111）平坦表面的吸附和振动，获得了氮原子\r\n在三个低指数表面的吸附位、吸附构型、结合能和本征振动等数据，计\r\n算结果与实验结果非常吻合．同时，与Ni（100）表面对比，系统研究\r\n了氮原子在Ni（510）台阶面的吸附和扩散．计算结果表明，氮原子在\r\n台阶下部形成最稳定的吸附态，台阶对下台面上扩散的氮原子形成捕获\r\n势，对上台面上扩散的氮原子形成反射势．     

Dynamic and spectroscopic studies of single molecules physisorbed on graphite substrates. 2. Application to the ammonia molecule

In the present paper, the theoretical approach developed in paper 1 is applied to an NH(3) molecule adsorbed on a graphite substrate. The potential energy surfaces (PESs) for the interaction between the molecule and the graphite crystal are described in detail. The molecule exhibits two quasi-equivalent angular position minima of energy ("up" and "down") along the perpendicular axis to the surface. The PES calculations also indicate that the NH(3) molecule has a rotational motion that is moderately hindered, with an energy barrier value of about 14 meV and also a quasi-free lateral translational motion above the surface, indicating a weak corrugation of the graphite (0001) surface. The isosteric heat of adsorption is calculated and is in agreement with the experimental one. Finally, the infrared absorption spectra for the vibrational mode frequency regions are obtained.     

Comparison of kinetic Monte Carlo and molecular dynamics simulations of diffusion in a model glass former

We present results from kinetic Monte Carlo (KMC) simulations of diffusion in a model glass former. We find that the diffusion constants obtained from KMC simulations have Arrhenius temperature dependence, while the correct behavior, obtained from molecular dynamics simulations, can be super-Arrhenius. We conclude that the discrepancy is due to undersampling of higher-lying local minima in the KMC runs. We suggest that the relevant connectivity of minima on the potential energy surface is proportional to the energy density of the local minima, which determines the "inherent structure entropy." The changing connectivity with potential energy may produce a correlation between dynamics and thermodynamics.