Improved convergence of self-consistence procedures in the MC SCF theory

To obtain optimized orbitals within the MC SCF theory, the energy surface near a chosen point is approximated by a quadratic function of independent matrix elements of a small orthogonal orbital transformation. The method of a second-order one-electron Hamiltonian (OEH) is developed on the basis of this approximation. A procedure is proposed to define step coordinates, insuring a rapid descent along an average-energy surface also in the cases when the matrix of second energy derivatives has eigenvalues negative or close to zero. The results obtained in applying the OEH method for the calculation of ground and triplet states of uracile in the π-electron approximation are discussed. When a complete matrix of the second energy derivatives is used, the self-consistence procedure is quadratically convergent. An exponential, yet rapid enough convergence is provided by a simplified computation scheme neglecting cross derivatives.     

A fully ab initio potential energy surface for C1H_2 reactive system

An ab initio analytical potential energy surface called BW3 for the CIH2 reactive system is presented. The fit of this surface is based on about 1 200 ab initio energy points, computed with multi-reference configuration interaction(MRCI) and scaling external correlation (SEC) method and a very large basis set. The precision in the fit is very high. The BW3 surface could reproduce correctly the dissociation energy of H2 and HCI, and the endothermicity of the Cl H2 abstraction reaction. For the Cl H2 abstraction reaction, the saddle point of BW3 lies in collinear geometries, and the barrier height is 32.84 kJ/mol; for the H CIH exchange reaction, the barrier of BW3 is also linear, with a height of 77.40 kJ/mol.     

A fully ab initio potential energy surface for ClH2 reactive system

An ab initio analytical potential energy surface called BW3 for the ClH2 reactive system is presented. The fit of this surface is based on about 1 200 ab initio energy points, computed with multi-reference configuration interaction(MRCI) and scaling external correlation (SEC) method and a very large basis set. The precision in the fit is very high. The BW3 surface could reproduce correctly the dissociation energy of H2 and HCl, and the endothermicity of the Cl + H2 abstraction reaction. For the Cl + H2 abstraction reaction, the saddle point of BW3 lies in collinear geometries, and the barrier height is 32.84 kJ/mol; for the H + ClH exchange reaction, the barrier of BW3 is also linear, with a height of 77.40 kJ/mol.     

Wave packet dynamics of H2(v1=8-14)+H2(v2=0-2): the role of the potential energy surface on different reactive and dissociative processes

A time-dependent wave packet method has been used to study different competing products of H(2)+H(2) collisions: four center reaction, collision induced dissociation, reactive dissociation, and three-body complex formation. A three-degree-of-freedom reduced dimensionality model has been used for five different geometries of the colliding complex (parallel H, crossed X, collinear L, and two T-shaped geometries T(I) and T(II)), with reactants in selected vibrational states with one diatom vibrationally "hot" and the other one vibrationally "cold." Product probabilities have been calculated using two potential energy surfaces [J. Chem. Phys. 101, 4004 (1994); J. Chem. Phys. 116, 666 (2002)] in order to compare their performance in the dynamics. The regions of the potential energy surfaces responsible of the threshold behavior of the probabilities have been identified. Overall, we have found that the most recent potential energy surface is less anisotropic, provides a smaller propensity for insertion-type processes, and gives lower energy thresholds.     

A fullyab initio potential energy surface for ClH2 reactive system

Anab initio analytical potential energy surface called BW3 for the ClH₂ reactive system is presented. The fit of this surface is based on about 1 200ab initio energy points, computed with multi-reference configuration interaction(MRCI) and scaling external correlation (SEC) method and a very large basis set. The precision in the fit is very high. The BW3 surface could reproduce correctly the dissociation energy of H₂ and HCl, and the endothermicity of the Cl + H₂ abstraction reaction. For the Cl + H₂ abstraction reaction, the saddle point of BW3 lies in collinear geometries, and the barrier height is 32.84 kJ/mol; for the H + ClH exchange reaction, the barrier of BW3 is also linear, with a height of 77.40 kJ/mol.     

Optimizing conical intersections of solvated molecules: the combined spin-flip density functional theory/effective fragment potential method

Solvent effects on a potential energy surface crossing are investigated by optimizing a conical intersection (CI) in solution. To this end, the analytic energy gradient has been derived and implemented for the collinear spin-flip density functional theory (SFDFT) combined with the effective fragment potential (EFP) solvent model. The new method is applied to the azomethane-water cluster and the chromophore of green fluorescent protein in aqueous solution. These applications illustrate not only dramatic changes in the CI geometries but also strong stabilization of the CI in a polar solvent. Furthermore, the CI geometries obtained by the hybrid SFDFT/EFP scheme reproduce those by the full SFDFT, indicating that the SFDFT/EFP method is an efficient and promising approach for understanding nonadiabatic processes in solution.     

A fully ab initio potential energy surface for ClH2 reactive system

Anab initio analytical potential energy surface called BW3 for the ClH₂ reactive system is presented. The fit of this surface is based on about 1 200ab initio energy points, computed with multi-reference configuration interaction(MRCI) and scaling external correlation (SEC) method and a very large basis set. The precision in the fit is very high. The BW3 surface could reproduce correctly the dissociation energy of H₂ and HCl, and the endothermicity of the Cl + H₂ abstraction reaction. For the Cl + H₂ abstraction reaction, the saddle point of BW3 lies in collinear geometries, and the barrier height is 32.84 kJ/mol; for the H + ClH exchange reaction, the barrier of BW3 is also linear, with a height of 77.40 kJ/mol.     

Cis-->trans, trans-->cis isomerizations and N-O bond dissociation of nitrous acid (HONO) on an ab initio potential surface obtained by novelty sampling and feed-forward neural network fitting

The isomerization and dissociation dynamics of HONO are investigated on an ab initio potential surface obtained by fitting the results of electronic structure calculations at 21 584 configurations by using previously described novelty sampling and feed-forward neural network (NN) methods. The electronic structure calculations are executed by using GAUSSIAN 98 with a 6-311G(d) basis set at the MP4(SDQ) level of accuracy. The average absolute error of the NN fits varies from 0.012 eV (1.22 kJ mol(-1)) to 0.017 eV (1.64 kJ mol(-1)). The average computation time for a HONO trajectory using a single NN surface is approximately 4.8 s. These computation times compare very favorably with those required by other methods primarily because the NN fitting needs to be executed only one time rather than at every integration point. If the average result obtained from a committee of NNs is employed at each point rather than a single NN, increased fitting accuracy can be achieved at the expense of increased computational requirements. In the present investigation, we find that a committee comprising five NN potentials reduces the average absolute interpolation error to 0.0111 eV (1.07 kJ mol(-1)). Cis-trans isomerization rates with total energy of 1.70 eV (including zero point energy) have been computed for a variety of different initial distributions of the internal energy. In contrast to results previously reported by using an empirical potential, where cis-->trans to trans-->cis rate coefficient ratios at 1.70 eV total energy were found to lie in the range of 2.0-12.9 depending on the vibration mode excited, these ratios on the ab initio NN potential lie in the range of 0.63-1.94. It is suggested that this result is a reflection of much larger intramode coupling terms present in the ab initio potential surface. A direct consequence of this increased coupling is a significant decrease in the mode specific rate enhancement when compared to results obtained by using empirical surfaces. All isomerizations are found to be first order in accordance with the results reported by using empirical potentials. The dissociation rate to NO+OH has been investigated at internal HONO energies of 3.10 and 3.30 eV for different distributions of this energy among the six vibrational modes of HONO. These dissociations are also found to be first order. The computed dissociation rate coefficients exhibit only modest mode specific rate enhancement that is significantly smaller than that obtained on an empirical surface because of the much larger mode couplings present on the ab initio surface.     

A theoretical study on the intermolecular interaction of energetic system-Nitromethane dimer

Three optimized geometries of nitromethane dimer have been obtained at the HF/6-31G level.Dimer binding energies have been corrected for the basis set superposition error (BSSE) and the zero point energy.Computed results indicate that the cyclic structure of (CH3NO2)2 is the most stable of three optimized geometries,whose corrected binding energyis 17.29 kJ mol-1 at the MP4SDTQ/6-31G//HF/6-31G level.In the optimized structures of nitromethane dimer,the inter-molecular hydrogen bond has not been found; and the charge-transfer interaction between CH3NO2 subsystems is weak; and the correlation interaction energy makes a little contribution to the intermolecular interaction energy of the dimer.     

A quantum reaction dynamics study of the translational, vibrational, and rotational motion effects on the HD + H3+ reaction

Time-dependent, quantum reaction dynamics wavepacket approach is employed to investigate the impacts of the translational, vibrational, and rotational motion on the HD+H(3)(+) → H(2)D(+) + H(2) reaction using the Xie-Braams-Bowman potential energy surface [Z. Xie, B. J. Braams, and J. M. Bowman, J. Chem. Phys. 122, 224307 (2005)]. We treat this five atom reaction with a seven-degree-of-freedom model by fixing one Jacobi and one torsion angle related to H(3) (+) at the lowest saddle point geometry of the potential energy surface. The initial state selected reaction probabilities show that the rotational excitations of H(+)-H(2) greatly enhance the reactivity with the reaction probabilities increased double at high rotational states compared to the ground state. However, the vibrational excitations of H(3) (+) hinder the reactivity. The ground state reaction probability shows no reaction threshold for this exoergic reaction, and as the translational energy increases, the reaction probability decreases. Furthermore, reactive resonances and zero point energy play very important roles on the reaction dynamics. The obtained integral cross section has the character of an exoergic reaction without a threshold: it decreases with the translational energy increasing. The calculated thermal rate constants using this seven-degree-of-freedom model are in agreement with a later experiment measurement.     

N2O在焦炭表面异相生成和分解机理的密度泛函理论研究

选用合理简化的焦炭模型,对煤焦燃烧过程中N₂O的异相生成和分解机理进行了分子水平上的研究。采用UB3LYP/6-31G(d)密度泛函理论方法优化得到了反应路径上反应物、产物、中间体和过渡态的几何构型和各中间反应的活化能和反应焓变。NO与其预先吸附在焦炭表面解离生成的表面氮组分反应生成N₂O的路径有两个,需要克服的势垒分别为69.3kJ/mol和200.0kJ/mol;NO亦可直接与焦炭中的吡啶氮结合释放出N₂O,该反应路径所需克服的最大势垒为418.0kJ/mol。N₂O可在焦炭表面分解释放出N₂,异相分解反应为一步反应,计算所得活化能为100.8kJ/mol。N₂O的异相生成和异相分解反应均为放热反应。采用经典过渡态理论计算得到了各路径中速率控制步骤的反应速率常数。低温条件下,N₂O的异相分解反应速率略低于其异相生成速率,随着温度的升高,两者逐渐接近,说明高温条件有利于N₂O的异相分解。     

OH-与CH2ClF反应的阴离子产物通道

理论研究了羟基负离子(OH^-)与氟氯代甲烷(CH₂ClF)反应的阴离子产物通道. 分别在B3LYP/6-31+G(d,p)和B3LYP/6-311++G(2d,p)水平上得到反应势能面上各关键物种的优化构型, 进而计算得到谐振频率和零点能. 基于CCSD(T)/6-311+G(3df,3dp)水平的相对能量, 描述了由质子转移和双分子亲核取代(S_N2)过程生成各阴离子产物的途径. 各阴离子产物途径势垒的计算结果表明质子转移过程是实验中的主要产物通道, 与以往实验测量的结论相符. 此外, 计算还显示双分子亲核取代过程得到了非典型的阴离子产物, 其中动力学效应可能会导致F^-的生成.     

Potential-dependent reorientation of thiocyanate on Au electrodes

Thiocyanate (SCN) adsorption on an Au electrode is examined using surface-enhanced Raman scattering (SERS) measurements, along with detailed density functional theory (DFT) calculations. Both the calculation and the spectroscopic measurements show that three different geometries are adopted by SCN adsorption in the potential region studied (0.0 V 相似文献   

Microsolvation of uracil and its conjugate bases: a DFT study of the role of solvation on acidity

The effect of microsolvation on the deprotonation energies of uracil was examined using DFT. The structures of uracil and its N(1) and N(3) conjugate bases were optimized with zero to six associated water molecules. Multiple configurations (upward of 93) of these hydrated clusters were located at PBE1PBE/6-311+G(d,p). Trends in these geometries are discussed, with the waters generally forming chains with small numbers of waters (one-three), rings (three-five waters), or cages (five-six waters). The difference in energy between the N1 and N3 conjugate bases is 13 kcal mol(-1) in the gas phase, and it decreases with each added water up to four. At this point the energy difference has been halved, but addition of a fifth or sixth water has little effect on the energy difference. This is understood in terms of the water structures and their ability to stabilize the negatively charged atoms in the conjugate bases.     

Isomerization study of C5H5NO molecules

The complex potential energy surface (PES) for the isomerization of C₅H₅NO species, including 18 isomers and 23 interconversion transition states, is probed theoretically at the B3LYP/6‐311++G(d,p) and MP2//B3LYP/6‐311++G(d,p) levels of theory. The geometries and relative energies for various stationary points were determined. The zero‐point vibrational energy (ZPVE) corrections have been made to calculate the reliable energy. We predicted a six‐membered ring structure as a global minima isomer I, which is 118.49 and 131.48 kcal · mol^?1 more stable than the least stable, four‐ and three‐membered ring isomer VIII at B3LYP and MP2//B3LYP levels of theory, respectively. The isomers and interconversion transition states have verified by frequency calculation. The intrinsic reaction coordinates (IRC) calculations have been performed to confirm that each transition state is linked by the desired reactants and products. The isomer stability has been studied using relative energies, chemical hardness, and chemical potential. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Finite temperature effects in Na3 and Na3: A path integral Monte Carlo study

Path integral Monte Carlo techniques are used to study Na₃⁺ and Na₃ at finite temperatures. In accord with previous classical trajectory calculations, we find that vibrational motion significantly distorts the clusters from the previously predicted zero temperature geometries, due to the flat Born-Oppenheimer potential energy surface. More importantly, these distorted clusters show significant localized electronic bonding, in contrast to the delocalized bonding found in previous studies of the zero temperature structures.     

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.     

B2Cn+(n=1～9)团簇的结构及其稳定性

采用密度泛函理论(DFT)的B3LYP泛函, 在6-311G*水平上对B2Cn+(n=1～9)团簇的几何构型和电子结构进行了优化和振动频率计算. 结果表明, 在B2Cn+(n=1～9)团簇的基态构型中, B2C2+、B2C3+为具有D∞h对称性的线形结构, B2C7+为具有Cs对称性的立体环状结构, 其余均为平面构型; 其成键顺序为C—C成键优于B—C 成键, B—C成键优于B—B成键. 进一步得到了B2Cn+(n=1～9)团簇的总能量(ET)、零点能(EZ)、摩尔热容(Cp)、标准熵(S0)以及原子化能(ΔEn+). 其结果显示, 随着n的递增, ET、EZ、Cp、S0和ΔEn+数值均呈现增大趋势, 其中EZ数值呈现近似等梯度的增加趋势. 通过对B2Cn+(n=1～9)团簇基态结构的垂直电子亲合势的研究发现, n为奇数的B2Cn+团簇比n为偶数的稳定.     

CH3NO2+X(X=H,OH, CH3, CH2[^3B1], O[^3P])→CH2NO2+XH吸氢反应过渡态 结 构和反应位垒的DFT计算研究

采用DFT(B3LYP)方法,分别在6-311g(d,p),6-311++g(d,p)和自洽相关基组cc-pVIZ水平上优化了基态硝基甲烷和自由基H,OH,CH3,CH2[^3B1]以及O[^3P]等发生吸氢反应时的过渡态结构,并计算了反应的位垒。研究表明,对同一反应,不同基组下优化得到的过渡态几何结构基本一致;反应位垒数值的大小也基本接近,经校正,硝基甲烷同自由基反应位垒的理论计算值同实验结果基本吻合。     

DFT Studies on the Isomerization of Butene Double Bond Catalyzed by 1-Butyl-3-methyl-imidazolium in Ionic Liquid

1 INTRODUCTION Butene and its isomers are important petroleum raw materials. Isomerization reaction of butene plays a key role in the course of C4 alkylation and its reaction mechanism has captured the attention of chemists all along[1, 2]. As a green so…