Scalable electronic structure theory: the embedded electron-pair approximation

量子力学理论提供了计算分子结构和分子行为的方法。计算是复杂的，部分原因是分子有许多相互作用的成分，部分原因是量子力学本身固有的复杂性。对于最简单的系统可以得到精确的量子力学结果，但对于实际问题，需要近似。产生这些近似，然后将它们转化为可用的软件工具的领域是分子电子结构理论。事实证明，化学领域被引用最多的论文描述的是分子电子结构理论的突破。原因是这些方法可以普遍应用：它们可以告诉我们任何分子的结构和反应性，所以它们被大量的化学家使用。目前，密度泛函理论（DFT）和耦合聚类理论（CC）在该领域占据主导地位。前者非常高效（即在计算机上运行得很快），后者对许多问题都非常准确。在使DFT更精确和CC理论更有效方面已经取得了很大的进展；我们小组参与了其中的一些努力。在这个提议中，我们提出了分子电子结构理论的一个新分支，基于一次处理一对电子的概念，但每对电子都嵌入由分子其余部分提供的模型中。这些方法可能是革命性的，因为它们的成本似乎并不比DFT大多少，但它们的精度可以与CC理论竞争。现在是进行这项研究的合适时机，部分原因是对更好的理论方法的需求；部分原因是最近量子嵌入理论的突破为构建新的、潜在的惊人电子结构方法提供了一个绝佳的机会。

The theory of quantum mechanics provides the means to calculate the structure of molecules, and how molecules will behave. The calculations are complicated, partly because a molecule has many interacting components, and partly because of the intrinsic complications of quantum mechanics itself. Exact quantum mechanical results can be obtained for the simplest of systems, but for real problems, approximations are needed. The field that produces these approximations, then converts them into usable software tools is molecular electronic structure theory.It turns out that the most highly cited papers in chemistry describe breakthroughs in molecular electronic structure theory. The reason is that these methods can be applied universally: they can inform us about the structure and reactivity of any molecule, so they are used by an enormous range of chemists.Currently two approximations dominate the field, density functional theory (DFT) and coupled cluster theory (CC). The first is very efficient (ie runs quickly on computers) and the second is amazingly accurate for many problems. There has been a great deal of progress in making DFT more accurate, and CC theory more efficient; our group has been involved in some of these efforts.In this proposal we set out a new branch of molecular electronic structure theory, based on the concept of treating the electrons one pair at a time, but with each pair embedded in a model provided by the rest of the molecule. These methods could be revolutionary, because their cost appears not much greater than that of DFT, but their accuracy could be competitive with CC theory. Now is the right time for this research partly because of the demand for better theoretical methods; and partly because recent breakthroughs in quantum embedding theory give a remarkable opportunity to build new and potentially amazing electronic structure methods.