Patkowski Konrad, Cencek Wojciech, Jankowski Piotr, Szalewicz Krzysztof, Mehl James B, Garberoglio Giovanni, Harvey Allan H
Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA.
J Chem Phys. 2008 Sep 7;129(9):094304. doi: 10.1063/1.2975220.
Nonrelativistic clamped-nuclei energies of interaction between two ground-state hydrogen molecules with intramolecular distances fixed at their average value in the lowest rovibrational state have been computed. The calculations applied the supermolecular coupled-cluster method with single, double, and noniterative triple excitations [CCSD(T)] and very large orbital basis sets-up to augmented quintuple zeta size supplemented with bond functions. The same basis sets were used in symmetry-adapted perturbation theory calculations performed mainly for larger separations to provide an independent check of the supermolecular approach. The contributions beyond CCSD(T) were computed using the full configuration interaction method and basis sets up to augmented triple zeta plus midbond size. All the calculations were followed by extrapolations to complete basis set limits. For two representative points, calculations were also performed using basis sets with the cardinal number increased by one or two. For the same two points, we have also solved the Schrodinger equation directly using four-electron explicitly correlated Gaussian (ECG) functions. These additional calculations allowed us to estimate the uncertainty in the interaction energies used to fit the potential to be about 0.15 K or 0.3% at the minimum of the potential well. This accuracy is about an order of magnitude better than that achieved by earlier potentials for this system. For a near-minimum T-shaped configuration with the center-of-mass distance R=6.4 bohrs, the ECG calculations give the interaction energy of -56.91+/-0.06 K, whereas the orbital calculations in the basis set used for all the points give -56.96+/-0.16 K. The computed points were fitted by an analytic four-dimensional potential function. The uncertainties in the fit relative to the ab initio energies are almost always smaller than the estimated uncertainty in the latter energies. The global minimum of the fit is -57.12 K for the T-shaped configuration at R=6.34 bohrs. The fit was applied to compute the second virial coefficient using a path-integral Monte Carlo approach. The achieved agreement with experiment is substantially better than in any previous work.
已计算出两个基态氢分子之间的非相对论性固定核相互作用能，其中分子内距离固定在其最低振转态的平均值。计算采用了具有单、双和非迭代三激发的超分子耦合簇方法[CCSD(T)]以及非常大的轨道基组——高达补充键函数的增强五重ζ尺寸。在主要针对较大间距进行的对称适配微扰理论计算中使用了相同的基组，以对超分子方法进行独立检验。使用全组态相互作用方法和高达增强三重ζ加键中尺寸的基组计算了超出CCSD(T)的贡献。所有计算之后都外推到完全基组极限。对于两个代表性点，还使用基数增加一或二的基组进行了计算。对于相同的两个点，我们还使用四电子显式相关高斯(ECG)函数直接求解了薛定谔方程。这些额外计算使我们能够估计用于拟合势能的相互作用能的不确定性，在势能阱最小值处约为0.15 K或0.3%。该精度比该系统早期势能所达到的精度大约高一个数量级。对于质心距离R = 6.4玻尔的近最小T形构型，ECG计算给出的相互作用能为 -56.91±0.06 K，而用于所有点的基组中的轨道计算给出 -56.96±0.16 K。计算得到的点由一个解析四维势能函数拟合。拟合相对于从头算能量的不确定性几乎总是小于后者能量的估计不确定性。对于R = 6.34玻尔的T形构型，拟合的全局最小值为 -57.12 K。该拟合用于通过路径积分蒙特卡罗方法计算第二维里系数。与实验达成的一致性比以往任何工作都要好得多。
