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利用神经网络力法,基于47783个高精度从头算能量点构建了反应体系H+CH4←→H2+CH3的一个全域势能面.通过大最的准经典轨线以及量子散射计算测试了势能面的收敛性质.这个势能面对于拟合过程以及从头算点的数目都已经完全收敛,拟合误差很小县比Shepard插值的势能面计算速度更快,代表了此标志性多原子反应体系最好的势能面. 相似文献
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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. 相似文献
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把李代数方法得到的SO~2分子的代数Hamiltonian,利用相干态基经典化并找到一个新的变换,将分子的键角引入,而得到SO~2分子的势能面。由该势能面计算的解离能,所给出的势能面的立体图和相应的等高线以及力常数与其他方法给出的相一致。该方法可以推广到多原子分子及反应体系。 相似文献
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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 相似文献
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HNCS与CX(X=H,F,Cl)自由基反应的理论研究 总被引:6,自引:0,他引:6
用量子化学密度泛函理论的UB3LYP方法,在6-31 G^*水平上按BERNY能量梯度解析法全参数优化了HNCS与CX(X=H,F,Cl)反应势能面上各驻点的几何构型,通过同一水平的振动频率分析确认了中问体和过渡态,并得到各驻点的零点能校正(Ezpc).通过内禀反应坐标(IRC)计算确认了反应物、中间体、过渡态和产物的相关性并得到最小能量途径(MEP).为了得到体系势能面的更准确信息,在各驻点的UB3LYP/6-31 G^*构型基础上,又进行了UQCISD(T)/6-311 G^**水平上的单点能计算,得到体系的势能面信息和可能的反应机理.应用变分过渡态理论及最小能量途径半经典绝热基态(MEPSAG)、小曲率半经典绝热基态(SCSAG)隧道效应校正的方法计算了标题反应在250~1500K温度范围内的速率常数.研究结果表明,HNCS与CX自由基反应是通过分子间H原子迁移及N—C键的断裂,生成产物CS NCXH.反应均为放热反应. 相似文献
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Generic semi-analytical energy gradients are derived and implemented for NDDO-type methods, by using numerical integral and
Fock matrix derivatives in the context of an otherwise analytical approach for configuration interaction (CI) and other non-variational
treatments. The correctness, numerical precision, and performance of this hybrid approach are established through comparisons
with fully numerical and fully analytical calculations. The semi-analytical evaluation of the CI gradient is generally much
faster than the fully numerical computation, but somewhat slower than a fully analytical calculation, which however shows
the same scaling behavior. It is the method of choice whenever a fully analytical CI gradient is not available due to the
lack of analytical integral derivatives. The implementation is generic in the sense that it can easily be extended to any
new NDDO-type Hamiltonian. The present development of a semi-analytical CI gradient will facilitate studies of electronically
excited states with recently proposed NDDO methods that include orthogonalization corrections.
Dedicated to Professor Karl Jug on the occasion of his 65th birthday 相似文献
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The use of gradient techniques for the development of energy-optimized basis sets has been investigated. The region where the energy surface is approximately quadratic with a positive definite Hessian is found to be very small for large basis sets. However, scaled Newton-Raphson methods prove quite effective even when the starting point is outside this region. The analytic calculation of the Hessian is found to be most efficient in terms of computing time. 相似文献
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The expressions of analytical energy gradients in density functional theory and their implementation in programs are reported. The evaluation of analytical energy gradients can be carried out in the fully 4-component relativistic, approximate relativistic, and nonrelativistic density functional calculations under local density approximation or general gradient approximation with or without frozen core approximation using different basis sets in our programs. The translational invariance condition and the fact that the one-center terms do not contribute to the energy gradients are utilized to improve the calculation accuracy and to reduce the computational effort. The calculated results of energy gradients and optimized geometry as well as atomization energies of some molecules by the analytical gradient method are in very good agreement with results obtained by the numerical derivative method. 相似文献
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A method is described for molecular mechanics calculations based on a cubic B-spline approximation of the potential energy. This method is useful when parts of the system are allowed to remain fixed in position so that a potential energy grid can be precalculated and used to approximate the interaction energy between parts of a molecule or between molecules. We adapted and modified the conventional B-spline method to provide an approximation of the Empirical Conformational Energy Program for Peptides (ECEPP) potential energy function. The advantage of the B-spline method over simpler approximations is that the resulting B-spline function is C2 continuous, which allows minimization of the potential energy by any local minimization algorithm. The standard B-spline method provides a good approximation of the electrostatic energy; but in order to reproduce the Lennard–Jones and hydrogen-bonding functional forms accurately, it was necessary to modify the standard B-spline method. This modification of the B-spline method can also be used to improve the accuracy of trilinear interpolation for simulations that do not require continuous derivatives. As an example, we apply the B-spline method to rigid-body docking energy calculations using the ECEPP potential energy function. Energies are calculated for the complex of Phe-Pro-Arg with thrombin. For this system, we compare the performance of the B-spline method to that of the standard pairwise summation in terms of speed, accuracy, and overhead costs for a variety of grid spacings. In our rigid-body docking calculations, the B-spline method provided an accurate approximation of the total energy of the system, and it resulted in an 180-fold reduction in the time required for a single energy and gradient calculation for this system. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 71–85, 1998 相似文献
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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 相似文献
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We investigate and test an algorithm suitable for the parallel calculation of the potential energy of a protein, or its spatial gradient, when the protein atoms interact via pair potentials. This algorithm is similar to one previously proposed, but it is more efficient, having half the interprocessor communications costs. For a given protein, we show that there is an optimal number of processors that gives a maximum speedup of the potential energy calculation compared to a sequential machine. (Using more than the optimum number of processors actually increases the computation time). With the optimum number the computation time is proportional to the protein size N. This is a considerable improvement in performance compared to sequential machines, where the computation time is proportional to N2. We also show that the dependence of the maximum speedup on the message latency time is relatively weak. 相似文献
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The use of Hermite Gaussian auxiliary function densities from the variational fitting of the Coulomb potential for the calculation of exchange-correlation potentials is discussed. The basic working equations for the energy and gradient calculation are derived. The accuracy of this approximation for optimized structure parameters and bond energies are analyzed. It is shown that the quality of the approximation can be systematically improved by enlarging the auxiliary function set. Average errors of 0.5 kcal/mol are obtained with auxiliary function sets including f and g functions. The timings for a series of alkenes demonstrate a substantial performance improvement. 相似文献
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We present analytical expressions to calculate the gradient of the water-accessible surface area of proteins with respect to Cartesian coordinates and dihedral angles. A detailed mathematical analysis leads to corrected equations for the gradient calculation used previously in the ANAREA program. To study the hydrophobic effect of solvent-protein interactions, our expressions have been implemented to further improve the program package FANTOM. We used this version of FANTOM to minimize the ECEPP/2 and the hydrophobic energy of tendamistat. © 1993 John Wiley & Sons, Inc. 相似文献
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Josep Maria Anglada Emili Besalú Josep Maria Bofill Ramon Crehuet 《Journal of computational chemistry》2001,22(4):387-406
Knowledge of the location of saddle points is crucial to the study the chemical reactivity. Using a path following method defined in a reduced potential energy surface, and starting at either the reactant or product region, we propose an algorithm that locates the corresponding saddle point. The reduced potential energy surface is defined by the set of molecular geometry parameters, namely bond distances, bond angles, and dihedral angles that undergo the largest change for the reaction under consideration; the rest of the coordinates are forced to have a null gradient. Consequently, the proposed method can be seen as a new formulation of the distinguished coordinate method. The method is based on a quadratic model; consequently, it only requires the calculation of the energy and the gradient. The Hessian matrix is normally updated except in the first step and the steps where the resulting updated Hessian matrix is not adequate. Some examples are presented and analyzed. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 387–406, 2001 相似文献
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A global analytical potential energy surface for the ground state of H(3)(-) has been constructed by fitting an analytic function to the ab initio potential energy values computed using coupled cluster singles and doubles with perturbative triples [CCSD(T)] method and Dunning's augmented correlation consistent polarized valence triple zeta basis set. Using this potential energy surface, time-dependent quantum mechanical wave packet calculations were carried out to calculate the reaction probabilities (P(R)) for the exchange reaction H(-)+H(2)(v, j)-->H(2)+H(-), for different initial vibrational (v) and rotational (j) states of H(2), for total angular momentum equal to zero. With increase in v, the number of oscillations in the P(R)(E) plot increases and the oscillations become more pronounced. While P(R) increases with increase in rotational excitation from j=0 to 1, it decreases with further increase in j to 2 over a wide range of energies. In addition, rotational excitation quenches the oscillations in P(R)(E) plots. 相似文献