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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Pruning the Hamiltonian Matrix in MULTIMODE: Test for C2H4 and Application to CH3NO2 Using a New Ab Initio Potential Energy Surface
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Pruning the Hamiltonian Matrix in MULTIMODE: Test for C2H4 and Application to CH3NO2 Using a New Ab Initio Potential Energy Surface

机译：在多模式下修剪哈密顿矩阵：使用新的从头算势能面测试C2H4并将其应用于CH3NO2

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摘要

We report vibrational self-consistent field/virtual state configuration interaction energies of nitromethane using the code MULTIMODE and a new full-dimensional potential energy surface (PES). The PES is a precise, permutationally invariant linear least-squares fit to 17 049 electronic energies, using the CCSD(T)-F12b method with HaDZ basis (cc-pVDZ basis for H atoms, and aug-cc-pVDZ basis for C, O, N atoms). Nitromethane has 15 vibrational degrees of freedom, including one that is a nearly free internal methyl torsion, which is accurately described by the PES. This torsional mode makes vibrational calculations very challenging and here we present the results of calculations without it. Nevertheless, 14-mode calculations are still challenging and can lead to very large Hamiltonian matrices. To address this issue, we apply a pruning scheme, suggested previously by Handy and Carter, that reduces the size of matrix without sacrificing accuracy in the eigenvalues. The method is briefly described here in the context of partitioning theory. A new and more efficient implementation of it, coded in the latest version of MULTIMODE program, is described. The accuracy and efficiency are demonstrated for 12-mode C2H4 and then applied to CH3NO2. Agreement with available experimental values of the CH3NO2 14 fundamentals is very good. Diffusion Monte Carlo calculations in full dimensionality are done for the zero-point energy and wavefuction. These indicate that the torisonal motion is nearly a free-rotor in this state.
机译：我们使用代码MULTIMODE和新的全尺寸势能面（PES）报告了硝基甲烷的振动自洽场/虚拟状态配置相互作用能。 PES是精确的，排列不变的线性最小二乘法，它使用具有HaDZ的C​​CSD（T）-F12b方法（H原子为CC-pVDZ，C的aug-cc-pVDZ为基础，适合于17049电子能量）， O，N原子）。硝基甲烷具有15个振动自由度，其中一个几乎是自由的内部甲基扭转，PES对此进行了准确描述。这种扭转模式使振动计算非常具有挑战性，在这里我们介绍了没有它的计算结果。尽管如此，14模计算仍然具有挑战性，并且可能导致非常大的哈密顿矩阵。为了解决这个问题，我们采用了Handy和Carter先前建议的修剪方案，该方案可以在不牺牲特征值精度的情况下减小矩阵的大小。这里在分区理论的背景下简要描述该方法。描述了它的新的和更有效的实现，该实现以MULTIMODE程序的最新版本编码。演示了12模态C2H4的准确性和效率，然后将其应用于CH3NO2。与CH3NO2 14基本原理的可用实验值一致。对零点能量和波动进行了全维扩散Monte Carlo计算。这些表明在这种状态下，姿态运动几乎是自由转子。

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