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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Comparison of ab Initio and DFT Electronic Structure Methods for Peptides Containing an Aromatic Ring: Effect of Dispersion and BSSE
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Comparison of ab Initio and DFT Electronic Structure Methods for Peptides Containing an Aromatic Ring: Effect of Dispersion and BSSE

机译：包含芳香环的肽的从头算和DFT电子结构方法的比较：分散和BSSE的影响

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摘要

We establish that routine B3LYP and MP2 methods give qualitatively wrong conformations for flexible organic systems containing pi systems and that recently developed methods can overcome the known inadequacies of these methods. This is illustrated for a molecule (a conformer of the Tyr-Gly dipeptide) for which B3LYP/ 6-31+G(d) and MP2/6-31+G(d) geometry optimizations yield strikingly different structures [Mol. Phys. 2006, 104, 559-570]: MP2 predicts a folded "closed-book" conformer with the glycine residue located above the tyrosine ring, whereas B3LYP predicts a more open conformation. By employing different levels of theory, including the local electron correlation methods LMP2 (local MP2) and LCCSD(TO) (local coupled cluster with single, double, and noniterative local triple excitations) and larg basis sets (aug-cc-pVnZ, n = D, T, Q), it is shown that the folded MP2 minimum is an artifact caused by large intramolecular BSSE (basis set superposition error) effects in the MP2/6-31+G(d) calculations. The B3LYP functional gives the correct minimum, but the potential energy apparently rises too steeply when the glycine and tyrosine residues approach each other, presumably due to missing dispersion effects in the B3LYP calculations. The PWB6K and M05-2X functionals, designed to give good results for weak interactions, remedy this to some extent. The reduced BSSE in the LMP2 calculations leads to faster convergence with increasing basis set quality, and accurate results can be obtained with smaller basis sets as compared to canonical MP2. We propose LMP2 as a suitable method to study interactions with pi-electron clouds.
机译：我们建立了常规的B3LYP和MP2方法对于包含pi系统的柔性有机系统给出的定性错误构象，并且最近开发的方法可以克服这些方法的已知不足之处。 B3LYP / 6-31 + G（d）和MP2 / 6-31 + G（d）几何优化产生明显不同的结构的分子（Tyr-Gly二肽的构象异构体）对此进行了说明。物理2006，104，559-570]：MP2预测折叠的“封闭书”构象异构体，其甘氨酸残基位于酪氨酸环上方，而B3LYP预测更开放的构象。通过采用不同层次的理论，包括局部电子相关方法LMP2（局部MP2）和LCCSD（TO）（具有单，双和非迭代局部三重激发的局部耦合簇）和较大的基集（aug-cc-pVnZ，n = D，T，Q），表明折叠的MP2最小值是由MP2 / 6-31 + G（d）计算中的大分子内BSSE（基集叠加误差）效应引起的伪影。 B3LYP官能团给出了正确的最小值，但是当甘氨酸和酪氨酸残基彼此接近时，势能显然会急剧上升，这可能是由于B3LYP计算中缺少分散效应所致。 PWB6K和M05-2X功能旨在为弱交互提供良好的结果，在某种程度上可以对此进行补救。与标准MP2相比，LMP2计算中降低的BSSE导致收敛速度加快，基础集质量提高，并且使用较小的基础集即可获得准确的结果。我们提出LMP2作为研究与π电子云相互作用的合适方法。

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