Computational Chemistry of Clusters and Crystals

因此，伊利诺伊大学厄巴纳-香槟分校的Hirata得到了化学系化学理论、模型和计算方法项目、材料研究部门凝聚态物质和材料理论项目以及计算和数据启用科学与工程计划(CDS&amp；E)的支持，以开发用于研究分子晶体的计算方法和软件。分子晶体是一大类重要的固体，由定义明确的分子单元通过弱相互作用结合而成。它们包括自然界中最丰富和最重要的固体，如地球和其他行星的大气物种的冰。合成化学家可以塑造聚集成超结构的分子，如果这些超结构是结晶的，那么它也是分子晶体。一些爆炸物和许多毒品都属于这一类。一些分子晶体显示出光学和电学特性，使其适合用于太阳能电池等光电子器件。这个项目的目标是开发一种通用的计算方法，用于分子晶体和相关的离子晶体以及有机分子超导体。这位首席研究员和他的同事开发了预测有机晶体固体的结构、光学和热学性质以及相行为的软件，具有前所未有的准确性，并在高压化学、地球化学、行星科学和材料科学中得到应用。本次研究活动涉及物理化学创新教育。一系列物理化学课程被录制下来，并与一组匹配的问题一起在线提供，释放出所有面对面的课堂时间用于解决问题、学生对解决方案的解释和讨论。分子晶体的能量被近似为嵌入在晶体自洽确定的静电环境中的其组成碎片的能量之和。碎片能量又通过复杂的分子从头算电子结构方法进行评估。这使得在有限的温度和压力(结构、状态方程、红外、拉曼、非弹性中子散射谱、热容、热焓、吉布斯能量)下，在诸如二阶和更高阶微扰理论或耦合团簇理论的高保真水平下，可以精确地计算固体的各种性质。该项目将该方法应用到健壮且文档齐全的软件中，该软件利用了该方法的自然并行性，并使其可供更广泛的科学界使用。此外，该项目还将这种方法扩展到能带和离子晶体以及有机分子(超导体)。使这些计算成为可能的基本思想是分子轨道线性组合(LCAO)晶体轨道理论，这是LCAO分子轨道概念的粗粒度扩展，自其诞生以来一直主导着计算量子化学。例如，通过将有机分子晶体的波函数展开为其电荷组态的线性组合，这些电荷组态又被上述嵌入碎裂方案处理，该方法描述了这些固体中组成分子单元之间的电荷转移，从而描述了电荷密度波、自旋密度波、金属状态以及可能的超导状态。

So Hirata of the University of Illinois at Urbana-Champaign is supported by an award from the Chemical Theory, Models and Computational Methods program in the Chemistry Division, the Condensed Matter and Materials Theory program in the Division of Material Research and the Computational and Data-enabled Science and Engineering Program (CDS&E) to develop computational approaches and software for the study of molecular crystals. Molecular crystals are a large, important class of solids that consist of well-defined molecular units bound by weak interactions. They include nature's most abundant and important solids such as the ices of the atmospheric species of Earth and other planets. Synthetic chemists can fashion molecules that aggregate into superstructures, which, if crystalline, are also molecular crystals. Some explosives and many drugs fall into this category. Some molecular crystals display optical and electronic properties making them suitable for optoelectronic devices such as solar cells. The goal of this project is to develop a general computational method for molecular crystals and related ionic crystals as well as organic molecular superconductors. The principal investigator and his coworkers develop software to predict the structure, optical and thermal properties, and phase behavior of organic crystalline solids with unprecedented accuracy and applications in high-pressure chemistry, geochemistry, planetary science, and materials science. This research activity involves innovative education in physical chemistry. A series of physical chemistry lectures is recorded and made available online with a matching set of problems, releasing all face-to-face classroom hours for problem solving, student's explanations of solutions, and discussions. The energy of a molecular crystal is approximated as a sum of the energies of its constituent fragments embedded in the self-consistently determined electrostatic environment of the crystal. The fragment energies are, in turn, evaluated by sophisticated molecular ab initio electronic structure methods. This allows an accurate calculation of a variety of properties of solids under finite temperature and pressure (structure, equation of state, infrared, Raman, inelastic neutron scattering spectra, heat capacity, enthalpy, Gibbs energy) at such high levels of fidelity as second- and higher-order perturbation theory or coupled-cluster theory. This project implements this method into robust and well-documented software that exploits the method's natural parallelism and makes it available for the broader scientific community. Furthermore, the project extends this method to energy bands and ionic crystals as well as organic molecular (super) conductors. The underlying idea that enables these calculations is the linear-combination-of-molecular-orbital (LCAO) crystal-orbital theory, a coarse-grained extension of the LCAO molecular-orbital concept, which has dominated computational quantum chemistry since its inception. By expanding the wave function of an organic molecular crystal, for instance, as a linear combination of its charge configurations, which, in turn, are treated by the aforementioned embedded-fragmentation scheme, this method describes charge transfer between constituent molecular units in these solids and thus charge density waves, spin density waves, and metallic as well as possibly superconducting states.