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arXiv 1602.07461physics.chem-ph
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小分子非简谐振动计算中局域模的作用

On the benefits of localized modes in anharmonic vibrational calculations for small molecules

Pawel T. Panek, Christoph R. Jacob

摘要:

对于相对较小的分子,非简谐振动计算已经是计算上的要求。振动组态相互作用(VCI)计算的主要瓶颈在于势能面的构造和激发空间的大小。为了解决这些变化,我们使用局域模坐标来构造势能面,并进行振动自洽场(L-VSCF)和L-VCI计算[P. T. Panek,Ch. R. Jacob,ChemPhysChem 15,3365(2014)],用于两个原型测试情况(乙烯和呋喃分子)的所有振动模式。我们发现,模式之间的互耦合减少时,从正常模式坐标切换到局域模式坐标。当使用这样的本地化模式坐标,我们观察到一个更快的收敛的$n$模式展开的势能面。这使得可以忽略势能面的$n$模式展开中的高阶贡献或近似混合势能面中的高阶贡献,这显著减少了构造非谐势能面的计算工作量。此外,我们发现,当使用本地化模式坐标,相对于VCI激发空间的收敛进行得更顺利,并在低阶的错误显着减少。这使得有可能设计出低成本的模型,用于获得非谐波校正的第一近似值。这表明,局域模式坐标的使用可以是有益的已经在小分子的非谐振动计算,并提供了一个可能的途径,使这种精确的计算也为较大的分子。

英文摘要:

Anharmonic vibrational calculations can already be computationally demanding for relatively small molecules. The main bottlenecks lie in the construction of the potential energy surface and in the size of the excitation space in the vibrational configuration interaction (VCI) calculations. To address these challanges, we use localized-mode coordinates to construct potential energy surfaces and perform vibrational self-consistent field (L-VSCF) and L-VCI calculations [P. T. Panek, Ch. R. Jacob, ChemPhysChem 15, 3365 (2014)] for all vibrational modes of two prototypical test cases, the ethene and furan molecules. We find that the mutual coupling between modes is reduced when switching from normal-mode coordinates to localized-mode coordinates. When using such localized-mode coordinates, we observe a faster convergence of the $n$-mode expansion of the potential energy surface. This makes it possible to neglect higher-order contributions in the $n$-mode expansion of the potential energy surface or to approximate higher-oder contributions in hybrid potential energy surfaces, which reduced the computational effort for the construction of the anharmonic potential energy surface significantly. Moreover, we find that when using localized-mode coordinates, the convergence with respect to the VCI excitation space proceeds more smoothly and that the error at low orders is reduced significantly. This makes it possible to devise low-cost models for obtaining a first approximation of anharmonic corrections. This demonstrates that the use of localized-mode coordinates can be beneficial already in anharmonic vibrational calculations of small molecules, and provides a possible avenue for enabling such accurate calculations also for larger molecules.

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