Strong Correlation meets Materials Modelling: DMFT and GW in CASTEP

许多现代技术的功能是基于利用材料的特殊物理性能。这可能是半导体的电流控制，数据存储的磁性，固态冰箱的Peltier效应，或MRI扫描仪中使用的超高功率磁体的超导性。支撑这种“功能材料”的未来改进以及新材料和设备的开发的是“材料设计”的思想。计算机模拟方法仅根据特定晶体结构中电子的量子力学行为来预测活性性质，这是任何为未来器件设计“设计材料”的尝试的重要组成部分。该资助的提议者是负责编写CASTEP密度泛函理论（DFT）建模代码的人员之一，这是前一代HECToR国家高性能计算服务中使用的少数几个建模代码之一。CASTEP被授权给近100个英国研究小组，2013年被用于产生118篇研究出版物，全球总数超过4000篇。密度泛函理论是现代材料科学、凝聚态物理和固态化学的大部分选择的模拟方法。它可以处理大量材料，包括块状金属、氧化物、半导体、石墨烯等层状材料、表面等等。它被广泛应用于预测结构和能量性质，如相变、电子输运性质、光谱性质，包括NMR化学位移、非弹性中子和X射线、拉曼和红外、XANES和其他近边电子光谱，但它的成功并不普遍。交换和相关性的基本近似（即LDA，GGA和混合物）对许多包含开放d-和f-壳层的材料具有较差的预测能力，通常被描述为“强相关”。例如，许多过渡金属氧化物被LDA和GGA错误地预测为金属，Jahn-Teller结构畸变是不存在的，涉及电子离域的相变也没有再现。用于治疗强关联系统的方法包括动态平均场理论（DMFT）和所谓的GW近似，它弥补了半局域DFT的不足。然而，这些是目前不容易获得的材料建模社区，部分是因为计算费用，部分是因为缺乏强大的，易于部署modellingcodes. In这里，我们建议实现LDA+DMFT和GW内CASTEP，英国首屈一指的电子结构为基础的材料建模代码。我们的战略目标是大幅扩大英国的强相关建模社区，目前与法国或德国相比，英国的规模还很小。计划交付的成果将提供具有最终用户能力的CASTEP的扩展，而且还将提供进一步开发该方法的软件平台。我们的DMFT实施的一个关键特征将是部队的可用性和结构优化的能力。这将使一个现实的处理复杂的材料与低网站对称性。我们希望这将刺激在英国采用这些方法的增长，从开发人员群体到更广泛的建模社区，就像DFT一样。当与实验研究一起使用时，模拟的力量最有效。联合王国在钻石光源的RIXS和ARPES光束线以及ISIS目标站2中子设施的MERLIN仪器中对针对“强相关”材料的仪器进行了大量投资。这项建议的一个主要特点是与两个设施的科学家合作，提供模拟科学用户界为实验研究选择的一些实际材料的能力。我们相信这将最大限度地发挥工作的影响。

The function of much modern technology is based on exploiting special physical properties of materials. This might be control of electrical current in the case of semiconductors, magnetism for data storage, the Peltier effect for solid-state refrigerators, or superconductivity for ultra high power magnets used in MRI scanners. Underpinning future refinements of such "functional materials" and development of new materials and devices lies the idea of "materials design". Computer simulation methods with the power to predict the active properties based only on the quantum-mechanical behaviour of the electrons in a particular crystal structure are a vital part of any attempt to engineer "designer materials" for future devices.The proposers of this grant are among those responsible for writing the CASTEP density-functional theory (DFT) modelling code, which is among the top few modelling codes used on the previous generation HECToR national high-performance computing service. CASTEP is licensed to nearly 100 UK research groups, was used to generate 118 research publications in 2013 and a worldwide total exceeding 4000.Density-functional theory is the simulation method of choice for a large part of modern materials science, condensed matter physics and solid-state chemistry. It can treat a huge range of materials, including bulk metals, oxides, semiconductors, layered materials such as graphene, surfaces and much more. It is widely used to predict structural and energetic properties such as phase transitions, electronic transport properties, spectroscopic properties including NMR chemical shifts, inelastic neutron and X-Ray, Raman and infrared, XANES and other near-edge electronic spectra.However, its success is not universal. The essential approximations to exchange and correlation (ie the LDA, GGAs and hybrids) have poor predictive power for many materials containing open d- and f-shells, frequently described as "strongly-correlated". For example many transition-metal oxides are erroneously predicted by LDA and GGA to be metallic, Jahn-Teller structural distortions are absent and phase transitions involving electron delocalisation are not reproduced.Methods for treating strongly-correlated systems which remedy the deficiencies of semi-local DFT include dynamical mean-field theory (DMFT) and the so-called GW approximation. However these are currently not readily accessible to the materials modelling community, partly because of computational expense and partly because of the lack of robust, easily deployable modelling codes.Herein we propose to implement both LDA+DMFT and GW within CASTEP, the UK's premier electronic structure-based materials modelling code. Our strategic target is to substantially broaden the strong-correlation modelling community in the UK, which is currently small compared to either France or Germany. The planned deliverables will offer both an extension to CASTEP with end-user capability, but also a software platform for further development of the methodology. A key feature of our DMFT implementation will be the availability of forces, and capability of structure optimization. This will enable a realistic treatment of complex materials with low site symmetry. We hope this will stimulate an growth in the adoption of these methods in the UK from the developer groups to a wider modelling community much as happened with DFT.The power of simulation works best when used hand in hand with experimental studies. The UK has invested heavily in instruments targeted at "strongly-correlated" materials in the RIXS and ARPES beamlines at Diamond Light Source and the MERLIN instruments at ISIS target station 2 neutron facility. A key feature of this proposal is to work with scientists at both facilities to deliver the capability of modelling some of the actual materials selected for experimental study by the scientific user community. We believe this will maximise the impact of the work.