Further development of internally contracted multi-reference coupled-cluster theory

精确的第一性原理方法是现代量子化学的重要组成部分。它们可用于可靠地预测瞬时物种或危险化合物的能量和性质，并为更接近的方法提供内部标准。申请人发展的内收缩多参考耦合团簇(ICMRCC)理论承诺将标准耦合团簇理论的高精度转移到具有复杂电子结构的系统中，如多自由基和具有开放d壳层的过渡金属化合物。在这个项目中，我们希望进一步改进icMRCC理论的方法论基础，并提供一种新的有效实施，使其能够充分利用该方法的潜力。具体的发展是对成熟的icMRCC理论的实用近似的调查，以及允许处理任意激发态和准简并情况的多态扩展。通过使用嵌入方法，该方法也将在本项目期间进一步开发，我们将能够将新开发的方法应用于大分子，例如，用于计算催化剂反应中心的能量学或分子磁铁的磁耦合的实际模型。

Accurate first-principles methods are an important building block of modern quantum chemistry. They may serve for the reliable prediction of energies and properties of transient species or hazardous compounds and provide an internal standard for more approximate methods. The internally-contracted multireference coupled-cluster (icMRCC) theory developed by the applicant promises to transfer the high accuracy of standard coupled-cluster theory to systems with complex electronic structure, like polyradicals and transition metal compounds with open d-shells. In this project, we want to further improve the methodological foundations of the icMRCC theory and provide a new and efficient implementation that allows to take full advantage of the potential of the method. Specific developments are the investigation of pragmatic approximations to the full-blown icMRCC theory and a multistate extension, allowing to treat arbitrary excited states and quasi-degenerate cases. By using an embedding method that will also be further developed during this project, we will be able to apply the newly developed methods to large molecules, for instance, to realistic models for computing the energetics at reaction centers of catalysts or magnetic couplings of molecular magnets.