Development of correlation operator approach to model electron correlation

南卡罗来纳大学的Vitaly Rassolov在化学学部化学理论、模型和计算方法项目的支持下，开发了一种新的方法，将电子相关效应建模为两粒子通用线性算子的平均值。这种方法是对日益流行的密度泛函理论（DFT）的补充，并且在计算复杂性和严密性方面与之相当，可能比DFT具有优势：(i)算子以与基态相同的精度描述激发态；（ii）模拟分子间作用力是一种自然的形式；（iii）它与多参考方法兼容，这是描述化学键断裂、过渡金属中的磁相互作用和激发态等现象所必需的。通过线性算子严格描述相关相互作用涉及复杂的数学，从而限制了方法的小系统。PI通过扩展高斯函数中的算子来克服这个问题，这是电子结构领域的主要内容。展开使用精确的解析表达式执行，使电子结构代码与高斯函数一起工作变得容易。这项研究改进和扩展了计算工具，以便更好地理解和控制化学反应，理解和预测分子间的作用力，以及描述对太阳能转换至关重要的激发态。在已建立的软件包中实现本研究的新想法将扩大它们在许多领域使用量子化学软件的大型科学家社区的可用性。

Vitaly Rassolov of the University of South Carolina is supported by the Chemical Theory, Models and Computational Methods program in the Chemistry Division to develop a novel approach for modeling electron correlation effects as averages over two-particle universal linear operators. This approach, complementary to the increasingly popular Density Functional Theory (DFT) and comparable to it in computational complexity and rigor, may have advantages over DFT: (i) the operators describe excited states with the same level of accuracy as ground states; (ii) it is a natural formalism to model intermolecular forces; (iii) it is compatible with multireference methods, necessary to describe such phenomena as chemical bond-breaking, magnetic interactions in transition metals, and excited states. Rigorous description of correlation interaction via linear operators involves complicated mathematics, thus limiting the approach to small systems. The PI overcomes this by expanding the operators in Gaussian functions, which are a staple in the electronic structure field. The expansions are performed using exact analytic expressions, leaving electronic structure codes with an easy task of working with Gaussians.This research improves and expands computational tools for better understanding and control of chemical reactions, understanding and prediction of intermolecular forces, and description of excited electronic states which are essential for solar energy conversion. Implementation of novel ideas from this research in established software packages will widen their availability to the large community of scientists, in many fields, who utilize quantum chemistry software.