固定节面量子MonteCarlo的一个新算法

提出了一个固定节面量子MOnTECArlO的新算法,与前人的算法相比,其“指导函数”的优化不是在扩散前,而是在扩散过程中同步进行;这不仅在机时上是节省的,更重要的是优化与扩散两者按相同的取样方式进行,达到互相改善的目的;这一优化方案是超线性收敛的,它能加快粒子的扩散;在扩散过程中,“指导函数”的节面不断发生改变,这有利于减小“固定节面误差”.这一新算法已被运用到CH_2的X--3b_1态和A--1A_1态,以及nH_2的π-X--2b_1态和σ-A--2A_1态总能量的计算,由此算出了CH_2单一三重态的“劈开”能△E_(S-T)=(45.542±1.840)kJ/MOl和nH_2的σ-π“劈开”能△E_(σ-π)=(141.644±1.589)kJ/MOl.计算结果表明这一新算法在精度、统计误差和计算量方面比一般固定节面量子MOnTECArlO方法都要优越得多

Structure, Energetics and Vibrational Frequency Shifts of Water Molecules Confined Inside Single-walled Carbon Nanotubes:A DFT Study

利用密度泛函理论中的杂化泛函M06-2X研究了受限于不同管径单壁碳纳米管(SWCnT)内水分子团簇(H2O)n=3~6的结构、能量以及振动频率.结果显示由于SWCnT的限域效应,水分子团簇的几何构型与在真空相比发生了巨大变化,如受限(H2O)6能形成单链锯齿型水分子构型.随着管直径的增大,纳米管与水之间的相互作用逐渐减弱,但水分子之间的氢键相互作用能变化不大.对比受限和真空下水分子O—H振动频率发现,绝大部分O—H的振动频率由于碳纳米管与水的相互作用而发生了红移.AIM理论分析显示O—H振动频率的红移应归因于其电子密度的减小.这也表明碳纳米管绝非简单的几何限制效应,而是与水分子之间存在弱电子相互作用,主要包括H…π氢键作用和O…π轨道作用,从而导致水分子的小部分电荷转移到了SWCnT上.The study of confined water molecules has attracted much attention because of their fascinating properties.In the present work, small water clusters(H2O)n=3~6 encapsulated in different diameter of single-walled carbon nanotubes(SWCNT) were investigated using density functional theory(DFT).The DFT-based M06-2X method along with mixed basis sets(6-311+G(d, p)//6-31+G(d)) was employed in all calculations for structure, interaction energy, charge transfer and vibrational frequency.The results indicate that as compared to water molecules in vacuum, the geometries of confined water molecules changed significantly due to the confinement effect of carbon nanotubes, for example, the(H2O)6 cluster can form chain-like configuration via hydrogen bond.The tube-water interaction energy decreased with the increase of the nanotubes diameter, while the hydrogen bond energy between water molecules changes slightly.Vibrational analysis reveals the red shift of the majority of the O—H stretching modes inside SWCNT compared to vacuum vibrational frequencies due to the tube-water interaction.The Bader's theory of atoms in molecule(AIM theory) has been used to characterize the tube-water interactions.Electron density ρ that reflects the strength of a bond has been used to explain the phenomenon of the red shift.The AIM analysis suggests that the red shift of O—H vibrational frequencies for confined water molecules should be attributed to the decrease of the electron density ρ.This fact demonstrates that the tube-water interaction is not a simple effect of geometry confinement, but weak electronic interaction between the molecular orbital of the confined water molecules and the delocalized π electrons from carbon nanotube.It mainly includes the H…π hydrogen bond interaction and O…π orbital interaction, resulting in a slight charge transfer from water molecules to SWCNT.Therefore, when constructing carbon nanotube devices, one should not ignore the existence of water molecules.We hope that our results can provide a guidance to the understanding of the behavior of water molecules encapsulated inside nanoscale environment.国家重点基础研究发展计划(No.2012CB214901); 国家自然科学基金(No.51274197)资助~

Hartree-Fock量子MonteCarlo方法

量子MOnTECArlO(QMC)方法已成功地应用于量子化学领域,对QMC的评述请参考

Theoretical Study of Substituent Effects on Bond Dissociation Enthalpies in Lignite Model Compounds

为了探究褐煤热解过程中氧桥键C—O均裂这一重要反应,选取α-O-4和β-O-4类结构单元作为褐煤模型化合物,运用不同密度泛函计算了部分模型化合物中C—O的离解焓,并以CbS-Qb3作为理论基准值进行比较,最后选取M05-2X进行离解焓计算.结果显示,对于选定的α-O-4和β-O-4类模型化合物,其平均离解焓分别为51.0 kCAl/MOl和66.1 kCAl/MOl.周围取代环境能显著影响C—O离解焓,芳环上存在给电子基团(OH,OCH3和CH3)能降低C—O离解焓,而吸电子基团COOH则能增加其离解焓.然后深层次分析了取代基效应对C—O离解焓的影响.此外,分子内氢键的形成对离解焓也有很大的影响.C—O的离解焓与其键长没有特定的相关性,不能简单的通过C—O键长来预测其离解焓.Lignite is an abundant natural resource that is a potential source of clean fuel and value-added chemicals.The mechanisms by which thermal and catalytic treatments deconstruct lignite remain elusive,which is where quantum mechani-cal calculations can offer fundamental insights.In order to investigate the cleavage of C—O bridge bond,which is the critical step in the thermal decomposition of lignite,the α-O-4 and β-O-4 types of structural units are selected as lignite model com-pounds to calculate the C—O bond dissociation enthalpies using several kinds of density functional theory methods(B3PW91,B3P86,PBE1PBE,BMK,M06-2X and M05-2X) at 6-31+G(d,p) level.By the comparison between the results and the theoretical benchmark values provided by CBS-QB3 method,M05-2X functional was applied for the calculations on C—O bond dissociation enthalpies.The present results indicate that the C—O average bond dissociation enthalpies are 51.0 kcal/mol and 66.1 kcal/mol for the α-O-4 and β-O-4 types of model compounds,respectively.Local substituents have a great withdrawing groups such as carboxyl group.Then the substituent effects are deeply analyzed on the basis of the ground-state effect on the C—O bond dissociation enthalpies,the C—O bond dissociation enthalpies will decrease when the adjacent arene rings are substituted by electron donating groups(OH,OCH3 and CH3),while the results are opposite for the electron effect and radical effect.An electron donating group can stabilize the phenoxy radicals(radical effect),however,an electron withdrawing group has the opposite effect.In most cases,the radical effect is more important than the ground-state effect.Furthermore,there is a negligible correlation between the C—O bond distances and strengths,and the C—O bond dissocia-tion enthalpies cannot be predicted so easily.Interestingly,the C—O bond dissociation enthalpies can be significantly influ-enced by the intramolecular hydrogen bond,if the intramolecular hydrogen bond still exists after the cleavage of the C—O bond,the bond dissociation enthalpies will be lower.国家重点基础研究发展计划(No.2012CB214901); 国家自然科学基金(No.51274197)资助~

Numerical simulation of the generation and development process of typhoon "Fung-Wong"(浅谈台风“凤凰”生成及发展过程的数值模拟)

应用NCEP再分析资料，分析了2008年8号台风“凤凰”生成和发展过程中大尺度稳定凝结潜热、积云对流潜热和海面感热3种热力因子的作用.结果表明：对流凝结潜热释放是台风“凤凰”维持和发展的主要热力因子，积云对流潜热释放不但使台风中心增暖并使台风中间层上升运动增强，从而促使台风加强和发展；海面感热通过海气相互作用使海面凤及对流层涡度增强，可能是台风初始低压的形成机制；大尺度稳定凝结潜热在台风发展过程中虽然值较小，但其通过影响台风眼区的垂直下沉气流，可促使台风暖心低压进一步发展