水二聚体解离能的量子Monte Carlo计算
Dissociation energy of the water dimer from Quantum Monte Carlo calculations
摘要：
本文报道了用量子蒙特卡罗（QMC）方法研究水二聚体的电子离解能。采用全电子和赝势方法对水分子的电子基态进行了变分量子蒙特卡罗（VMC）和扩散量子蒙特卡罗（DMC）计算。我们已经使用了Slater-Jastrow试验波函数与B3 LYP类单粒子轨道，我们已经纳入回流相关。当引入回流关联时，水单体的总能量降低约4-5 mHa，得到DMC能量为-76.42830（5）Ha，其仅比实验值高10 mHa。在我们的赝势DMC计算中，我们比较了使用定域近似获得的水单体和二聚体的总能量与Casula最近提出的变分方案[Phys.Rev.B74，161102（R）（2006）]。Casula格式的时间步长误差较大，能量外推到零时间步长的结果总是高于局域近似的结果。然而，当采取能量差时，误差抵消，产生彼此误差条内的电子解离能。在我们的各种全电子和赝势计算中得到的离解能范围在5.03（7）和5.47（9）kcal/mol之间，与实验符合得很好。我们的计算给出了单体的偶极矩在1.897（2）和1.909（4）德拜之间，二聚体的偶极矩在2.628（6）和2.672（5）德拜之间。
英文摘要：
We report a study of the electronic dissociation energy of the water dimer using quantum Monte Carlo (QMC) techniques. We have performed variational quantum Monte Carlo (VMC) and diffusion quantum Monte Carlo (DMC) calculations of the electronic ground state of the water monomer and dimer using all-electron and pseudopotential approaches. We have used Slater-Jastrow trial wave functions with B3LYP-like single-particle orbitals, into which we have incorporated backflow correlations. When backflow correlations are introduced, the total energy of the water monomer decreases by about 4-5 mHa, yielding a DMC energy of -76.42830(5) Ha, which is only 10 mHa above the experimental value. In our pseudopotential DMC calculations, we have compared the total energies of the water monomer and dimer obtained using the locality approximation with those from the variational scheme recently proposed by Casula [Phys. Rev. B 74, 161102(R) (2006)]. The time step errors in the Casula scheme are larger and the extrapolation of the energy to zero time step always lies above the result obtained with the locality approximation. However, the errors cancel when energy differences are taken, yielding electronic dissociation energies within error bars of each other. The dissociation energies obtained in our various all-electron and pseudopotential calculations range between 5.03(7) and 5.47(9) kcal/mol and are in good agreement with experiment. Our calculations give monomer dipole moments which range between 1.897(2) and 1.909(4) Debye and dimer dipole moments which range between 2.628(6) and 2.672(5) Debye.
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