Li X, Ahuja C, Harrison J F, Hunt K L C
Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, USA
J Chem Phys. 2007 Jun 7;126(21):214302. doi: 10.1063/1.2121548.
Collision-induced light scattering, impulsive stimulated scattering, and subpicosecond-induced birefringence all depend on the transient changes Deltaalpha in molecular polarizabilities that occur when molecules collide. Ab initio results for Deltaalpha are needed to permit comparisons with accurate experimental results for these spectra and for refractive index virial coefficients and dielectric virial coefficients. In this work, we provide results for Deltaalpha for a pair of hydrogen molecules, treated at CCSD(T) level, with an aug-cc-pV5Z (spdf) basis set. Our values replace the best previous ab initio results for the variation of Deltaalpha with intermolecular separation, the self-consistent-field results obtained by Bounds [Mol. Phys. 38, 2099 (1979)] with a relatively small (3s2p) basis set for H2. For the six geometrical configurations studied by Bounds, the inclusion of correlation and improvements in the basis tend to increase both the trace Deltaalpha(0)0 and the anisotropy Deltaalpha2m of the pair polarizability. The change in the anisotropy is relatively small, but our values for the trace differ by factors of 2 or more from Bounds' results. For use in computing experimental line shapes, intensities, and virial coefficients, we have calculated Deltaalpha for 18 different relative orientations of a pair of H2 molecules, with the intermolecular separation R ranging from 2 a.u. (3 a.u. for a linear pair) to 10 a.u. The H2 bond length is fixed at the vibrationally averaged internuclear separation in the ground state r=1.449 a.u. Our results agree well with the CCSD(T) results for Deltaalpha obtained by Maroulis [J. Phys. Chem. A 104, 4772 (2000)] for two pair configurations of H2...H2 (linear and T-shaped) at a fixed internuclear distance of R=6.5 a.u. in a [6s4p1d] basis. As the intermolecular distance increases (for R>or=8 a.u.), the spherical-tensor components of Deltaalpha converge to the results from a long-range model that includes dipole-induced-dipole (DID) interactions, higher-multipole induction, nonuniformity of the local field, hyperpolarization, and van der Waals dispersion. Deviations from the first-order DID model are still evident for R between 8 and 10 a.u. in most orientations of the pair. At shorter range, overlap damping, exchange, and orbital distortion reduce both Deltaalpha0(0) and Deltaalpha(2)0 below their long-range limiting forms.
碰撞诱导光散射、脉冲受激散射和亚皮秒诱导双折射均取决于分子碰撞时分子极化率的瞬态变化Δα。需要从头计算得到的Δα结果，以便与这些光谱、折射率维里系数和介电维里系数的精确实验结果进行比较。在这项工作中，我们给出了一对氢分子在CCSD(T)水平、采用aug-cc-pV5Z (spdf)基组处理时的Δα结果。我们的值取代了之前关于Δα随分子间距离变化的最佳从头计算结果，即Bounds [《分子物理学》38, 2099 (1979)] 使用相对较小的(3s2p)基组对H₂得到的自洽场结果。对于Bounds研究的六种几何构型，包含相关性和基组的改进往往会增加成对极化率的迹Δα(0)0和各向异性Δα2m。各向异性的变化相对较小，但我们的迹值与Bounds的结果相差2倍或更多。为了用于计算实验线形、强度和维里系数，我们计算了一对H₂分子在18种不同相对取向下的Δα，分子间距离R范围从2 a.u.（线性对为3 a.u.）到10 a.u.。H₂键长固定为基态振动平均核间距r = 1.449 a.u.。我们的结果与Maroulis [《物理化学杂志A》104, 4772 (2000)] 在固定核间距R = 6.5 a.u.、采用[6s4p1d]基组对H₂...H₂的两种成对构型（线性和T形）得到的CCSD(T)水平的Δα结果吻合得很好。随着分子间距离增加（对于R≥8 a.u.），Δα的球张量分量收敛到一个长程模型的结果，该模型包括偶极诱导偶极（DID）相互作用、高阶多极诱导、局部场的不均匀性、超极化和范德华色散。在该对分子的大多数取向中，对于8到10 a.u.之间的R，与一阶DID模型的偏差仍然明显。在较短距离时，重叠阻尼、交换和轨道畸变会使Δα0(0)和Δα(2)0都低于它们的长程极限形式。
