基于密度泛函理论研究二元排斥Yukawa流体的表面结构性质

基于自由能密度泛函理论(DFT)考察了二元排斥Yukawa (HCRY)流体在不同外场下的密度分布. 基于微扰理论, 体系的Helmholtz自由能泛函采用硬球排斥部分和长程色散部分贡献之和, 其中Kierlik和Rosinberg的加权密度近似(WDA)被用来计算硬球排斥部分, 而色散部分采用平均场理论(MFT)进行描述. 为了验证DFT计算结果的合理性, 研究中采用巨正则Monte Carlo(GCMC)模拟计算了在不同主体相密度、硬核直径和位能参数比的条件下二元HCRY混合流体的密度分布. 结果表明, 该DFT计算结果与GCMC模拟值吻合良好.     

密度泛函与分子模拟计算介孔孔径分布比较

用巨正则系综Monte Carlo模拟(GCMC)方法和密度泛函理论（ DFT）结合统计积分方程（SIE）计算了介孔材料的孔径分布.为比较这两种方法,以77 K氮气在介孔活性碳微球中的吸附数据为依据,求出其孔径分布.在GCMC模拟和DFT计算中,流体分子模型化为单点的Lerrnard-Jones球;流体分子与吸附剂材料之间的作用采用平均场理论中的10-4-3模型.在DFT方法中,自由能采用Tarazona 提出的加权近似密度泛函方法(weighted density approximation,WDA)求解.结果表明,对于孔径大于1.125 nm的介孔材料,GCMC和DFT两种方法都可以用来研究介孔材料的孔径分布;对于小于1.125 nm的介孔材料,不能用DFT方法计算孔径分布（DFT方法本身的近似产生了误差）,只能用分子模拟方法.     

甲烷晶体的晶格能和弹性性质: 不同方法及泛函的评估

通过对甲烷晶体进行结构、晶格能和弹性特性的研究, 评估了不包含和包含色散能量修正的密度泛函理论的性能. 我们分别利用不包含色散能量修正的密度泛函理论(DFT) (包含不同的标准泛函和杂化泛函)和包含色散能量修正的密度泛函理论(DFT-D)计算了甲烷晶体特性, 并与实验作对比. 尽管DFT-D 与传统密度泛函理论及杂化密度泛函理论相比, 修正了甲烷晶体中的范德华(vdW)相互作用, 但是一些修正方案过分修正了这种相互作用. 因此, 人们在使用DFT-D方法时务必谨慎.     

几种密度泛函理论公式用于镧系硫属化合物计算的比较

以镧系元素La、Gd、Lu的硫属化合物为对象,系统考察几种密度泛函理论公式对镧系化合物计算的适用情况,考虑了相对论效应的影响. 计算结果显示,相对论效应引起的键长变化在十2Pm到-3Pm之间,引起的键能减小为0. 4～0. 6eV,与采用的密度泛函公式关系不明显. 不同的密度泛函公式对键长的计算结果影响也不太大,但对键能有显著影响,其中LDA(VWN)＋PW86X公式给出最好结果. 交换能梯度校正明显改善键能计算结果,而相关能梯度校正反而使之变差. 简单的X_α公式给出相当好的键能计算结果. 在考虑相对论效应和梯度校正以后,密度泛函理论方法给出比较可靠的键长数值,键能则仍然偏高,但不超过20％.     

钒硫团簇V2S2+、V3S4+的结构和稳定性

用ab initio分子轨道方法(RHF,UHF)和密度泛函(DFT)方法研究了团簇V2S2+、V3S4+的各种可能的几何构型和电子结构,所得理论计算能较好地解释有关实验结果.     

等电子三原子铀化物OUO2+、NUN和NUO+的结构和谐振频率的CCSD(T)计算研究

用小核相对论有效势和CCSD(T)方法计算了三原子铀化物OUO²⁺, NUN和NUO⁺的平衡键长和谐振频率. 计算结果显示U原子内层5s5p5d 电子相关能对这些化合物性质的影响非常小. 除NUN的弯曲振动频率,旋轨耦合效应对这些化合物的结构和频率的影响并不明显. 本文的计算结果与其他研究组的计算结果以及已有的实验值相比符合较好, 这表明作为单参考态方法, CCSD(T)能够对这些体系的键长和频率给出较精确的计算结果. 与此前密度泛函理论(DFT)的计算结果相比, CCSD(T)方法与PBE0泛函的结果吻合最好. 本文的工作有助于在用密度泛函方法研究这些体系时选择合适的交换相关泛函, 也为今后的实验研究提供了新的理论数据.     

统计方法在提高密度泛函理论准确性的研究进展

介绍了神经网络方法、线性回归分析方法和支持向量机模型的原理及其对密度泛函理论计算结果修正的研究进展.这3种统计方法在改进密度泛函理论计算结果准确性方面均有着很大的作用.最后讨论了3种方法亟待解决的问题并对其发展进行了展望.     

磷酸可待因的振动光谱研究

运用密度泛函理论(DFT)计算了磷酸可待因的拉曼和红外光谱.采用B3LYP混合泛函和6-31G基函数组,同时在实验上测量了磷酸可待因的常规拉曼光谱(NRS)和红外吸收光谱(IRS),结果表明:在振动频率上理论结果与实验结果相当一致,根据理论计算的结果对磷酸可待因的振动光谱进行分析,通过理论与实验数据的比较,对磷酸可待因的所有振动谱带进行了全面地归属.     

α-双环-HMX晶体中二聚作用的理论研究

在四种(分别为HF/6-31G*,MP2/6-31G*,B3LYP/6-31G*和PBE1PBE/6-31G*)水平下,用超分子方法(SM)求得α-双环-HMX(四硝基四氮杂双环辛烷)三种二聚体(I,II和III)中的分子间相互作用能.用对称性匹配微扰理论(SAPT)与密度泛函理论(DFT)相结合的方法[SAPT(DFT)方法]求得该三种二聚体的分子间相互作用能及其分量.在四种SM方法中,以MP2求得的相互作用最强,但均弱于SAPT(DFT)的计算结果.SM和SAPT(DFT)两种方法求得的相互作用,均为IIIIII,归因于三者的分子质心平衡间距(Re)6.58(I)6.95(III)8.60(II).考察SAPT(DFT)方法求得的各作用能分量是IIIIII.密度泛函方法对色散能计算的丢失使其求得的相互作用偏小.     

计算机模拟铌原子簇的稳定构型和能量性质

采用密度泛函(DFT)方法结合全局优化"Basin-Hopping"算法研究了铌原子簇: 对于Nb小簇n＝2～6我们用密度泛函方法计算了它们的稳定几何构型和电子结构, 通过拟合计算结果构造铌原子簇势能函数, 并利用该函数和全局优化"Basin-Hopping"算法得到较大铌原子簇(n＝7～20)能量极小的结构. 计算结果表明与实验及其它计算结果相一致.     

The determination of intrinsic reaction coordinates by density functional theory

The first implementation of the intrinsic reaction coordinate (IRC ) method within the density functional theory (DFT ) framework is presented. The implementation has been applied to four different types of chemical reactions represented by the isomerization process, HCN ? HNC (A); the S_N2 process, H^? + CH₄ ? CH₄ + H^? (B); the exchange process, H˙ + HX ? HX + H˙ (X ? F,Cl) (C); and the elimination process, C₂H₅Cl ? C₂H₄ + HCl (D). The present study presents for each process optimized structures and calculated harmonic vibrational frequencies for the reactant(s), the transition state, and the product(s) along with the IRC path connecting the stationary points. The calculations were carried out within the local density approximation (LDA ) as well as the LDA/NL scheme where the LDA energy expression is augmented by Perdew's and Becke's nonlocal (NL ) corrections. The LDA and LDA/NL results are compared with each other as well as the best available ab initio calculations and experimental data. For reaction (D), ab initio calculations based on MP 2 geometries and MP 4SDTQ energies have been added due to the lack of accurate published post-HF calculations on this process. A detailed discussion is provided on the efficiency of the IRC algorithms, the relative accuracy of the DFT and ab initio schemes, as well as the reaction mechanisms of the four reactions. It is concluded that the LDA/NL scheme affords the same accuracy as does the MP 4 method. The post-HF methods seem to overestimate activation energies, whereas the corresponding LDA/NL estimates are too low. The LDA activation energies are even lower than the LDA/NL counterparts. The incorporation of the IRC method into the DFT framework provides a promising and reliable tool for probing the chemical reaction path on the potential energy surfaces, even for large-size systems. IRC calculations by ab initio methods of an accuracy similar to the LDA/NL scheme, such as the MP 4 scheme, are not feasible. © John Wiley & Sons, Inc.     

Structural, electronic, and optical properties of CaCO3 aragonite

Density functional theory ab initio calculations of the structural parameters, electronic structure, carriers effective masses, and optical absorption of the CaCO₃ aragonite polymorph were performed within the local density and generalized gradient approximations, local density approximation (LDA) and generalized gradient approximation (GGA) respectively. A good agreement between the calculated lattice parameters and experimental results was obtained. Both the LDA and GGA results for CaCO₃ aragonite exhibit very close indirect and direct energy gaps, and the computed effective masses are heavy and anisotropic. Two optical absorption regimes related to distinct electronic transitions are predicted by the calculations.     

TheRate: Program for ab initio direct dynamics calculations of thermal and vibrational-state-selected rate constants

We introduce TheRate (THEoretical RATEs), a complete application program with a graphical user interface (GUI) for calculating rate constants from first principles. It is based on canonical variational transition-state theory (CVT) augmented by multidimensional semiclassical zero and small curvature tunneling approximations. Conventional transition-state theory (TST) with one-dimensional Wigner or Eckart tunneling corrections is also available. Potential energy information needed for the rate calculations are obtained from ab initio molecular orbital and/or density functional electronic structure theory. Vibrational-state-selected rate constants may be calculated using a diabetic model. TheRate also introduces several technical advancements, namely the focusing technique and energy interpolation procedure. The focusing technique minimizes the number of Hessian calculations required by distributing more Hessian grid points in regions that are critical to the CVT and tunneling calculations and fewer Hessian grid points elsewhere. The energy interpolation procedure allows the use of a computationally less demanding electronic structure theory such as DFT to calculate the Hessians and geometries, while the energetics can be improved by performing a small number of single-point energy calculations along the MEP at a more accurate level of theory. The CH₄+H↔CH₃+H₂ reaction is used as a model to demonstrate usage of the program, and the convergence of the rate constants with respect to the number of electronic structure calculations. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1039–1052, 1998     

A direct ab initio dynamics study of the water-assisted tautomerization of formamide

Direct ab initio dynamics calculations based on a canonical variational transition-state theory with several multidimensional semiclassical tunneling approximations were carried out to obtain rate constants for the water-assisted tautomerization of formamide. The accuracy of the density functionals, namely, B-LYP, B3-LYP, and BH&H-LYP, were examined. We found that the BH&H-LYP method yields the most accurate transition-state properties when comparing it to ab initio MP2 and QCISD results, whereas B-LYP and B3-LYP methods predict barrier heights too low. Reaction path information was calculated at both the MP2 and nonlocal hybrid BH&H-LYP levels using the 6–31G(d,p) basis set. At the BH&H-LYP level, we found that the zero-point energy motion lowers the barrier to tautomerization in the formamide-water complex by 3.6 kcal/mol. When tunneling is considered, the activation energy at the BH&H-LYP level at 300 K is 17.1 kcal/mol. This is 3.4 kcal/mol below the zero-point-corrected barrier and 7.0 kcal/mol below the classical barrier. Excellent agreement between BH&H-LYP and MP2 rate constants further supports the use of BH&H-LYP for rate calculations of large systems. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 861–874, 1997     

Benchmark comparison of gradient-dependent and local density calculations for bulk silicon and aluminum

Critical evaluation of calculated properties of crystalline Al and Si shows that considerably less is known about the systematic behavior of gradient-dependent approximations (GDAs) to density functional theory than is commonly thought. GDAs appear to be quite sensitive to the technical details of implementation (e.g., pseudopotentials and relativistic corrections). GDA cohesive energies published so far for the two systems typically are substantially better than are LDA values when compared with experiment. The reverse is true for the lattice constants, while the GDA bulk moduli are no better than are LDA values for Al and are worse for Si. Extremely well-controlled test calculations are needed to ascertain the actual behavior of GDAs. © 1997 John Wiley & Sons, Inc.     

Electronic coupling matrix elements from charge constrained density functional theory calculations using a plane wave basis set

We present a plane wave basis set implementation for the calculation of electronic coupling matrix elements of electron transfer reactions within the framework of constrained density functional theory (CDFT). Following the work of Wu and Van Voorhis [J. Chem. Phys. 125, 164105 (2006)], the diabatic wavefunctions are approximated by the Kohn-Sham determinants obtained from CDFT calculations, and the coupling matrix element calculated by an efficient integration scheme. Our results for intermolecular electron transfer in small systems agree very well with high-level ab initio calculations based on generalized Mulliken-Hush theory, and with previous local basis set CDFT calculations. The effect of thermal fluctuations on the coupling matrix element is demonstrated for intramolecular electron transfer in the tetrathiafulvalene-diquinone (Q-TTF-Q(-)) anion. Sampling the electronic coupling along density functional based molecular dynamics trajectories, we find that thermal fluctuations, in particular the slow bending motion of the molecule, can lead to changes in the instantaneous electron transfer rate by more than an order of magnitude. The thermal average, (<|H(ab)|(2)>)(1/2)=6.7 mH, is significantly higher than the value obtained for the minimum energy structure, |H(ab)|=3.8 mH. While CDFT in combination with generalized gradient approximation (GGA) functionals describes the intermolecular electron transfer in the studied systems well, exact exchange is required for Q-TTF-Q(-) in order to obtain coupling matrix elements in agreement with experiment (3.9 mH). The implementation presented opens up the possibility to compute electronic coupling matrix elements for extended systems where donor, acceptor, and the environment are treated at the quantum mechanical (QM) level.     

On the evaluation of molecular electron affinities by approximate density functional theory

The ability of approximate Density Functional Theory to calculate molecular electron affinities has been probed by a series of calculations on the hydrides CH₃, NH₂, OH, and HC₂ as well as the multibonded species CN, BO, N₃, OCN, and NO₂. The simple Hartree–Fock Slater scheme lacks dynamic correlations and underestimates on the average the adiabatic electron affinities (EA_ad) by 0.7 eV. A considerable improvement is obtained by the Local Density Approximation (LDA) in which dynamic correlation is included. Values from LDA calculation underestimate, on the average, the adiabatic electron affinities by 0.4 eV. The best agreement with experiment is obtained by the LDA/NL scheme in which a nonlocal correction recently proposed by Becke is added to the LDA energy expression. The LDA/NL method underestimates EA_ad by 0.2 eV. It is concluded that the LDA/NL method affords EA_ad's in as good agreement with experiment as ab initio techniques in which electron correlation is taken into account by extensive configuration interaction. A full geometry optimization has been carried out on the nine neutral sample molecules as well as the corresponding anions.     

Mechanistic study and kinetic properties of the CF3CHO + Cl reaction

Theoretical investigations have been carried out on the mechanism and kinetics for the reaction of CF 3 CHO + Cl using duallevel direct dynamics method. The potential energy surface information was obtained at the MCQCISD/3//MP2/cc-pVDZ level and the kinetic calculations were done using variational transition state theory with interpolated single-point energy (VTST-ISPE) approach. The calculated results show that the reaction proceeds primarily via the H-abstraction channel, while the Cl-addition channel is unfavorable due to the higher barriers. The improved canonical variational transition-state theory (ICVT) with the small-curvature tunneling correction (SCT) was used to calculate the rate constants. The theoretical rate constants at room temperature are in general agreement with the experimental values. A three-parameter rate constant expression was fitted over a wide temperature range of 200-2000 K.     

Interaction energies in hydrogen-bonded systems: a testing ground for subsystem formulation of density-functional theory

The formalism based on the total energy bifunctional (E[rhoI,rhoII]) is used to derive interaction energies for several hydrogen-bonded complexes (water dimer, HCN-HF, H2CO-H2O, and MeOH-H2O). Benchmark ab initio data taken from the literature were used as a reference in the assessment of the performance of gradient-free [local density approximation (LDA)] and gradient-dependent [generalized gradient approximation (GGA)] approximations to the exchange-correlation and nonadditive kinetic-energy components of E[rhoI,rhoII]. On average, LDA performs better than GGA. The average absolute error of calculated LDA interaction energies amounts to 1.0 kJ/mol. For H2CO-H2O and H2O-H2O complexes, the potential-energy curves corresponding to the stretching of the intermolecular distance are also calculated. The positions of the minima are in a good agreement (less than 0.2 A) with the reference ab initio data. Both variational and nonvariational calculations are performed to assess the energetic effects associated with complexation-induced deformations of molecular electron densities.