We derive a low-scaling $G_0W_0$ algorithm for molecules, using pair atomic density fitting (PADF) and an imaginary time representation of the Green's function and describe its implementation in the Slater type orbital (STO) based Amsterdam density functional (ADF) electronic structure code. We demonstrate the scalability of our algorithm on a series of water clusters with up to 432 atoms and 7776 basis functions and observe asymptotic quadratic scaling with realistic threshold qualities controlling distance effects and basis sets of triple-$ζ$ (TZ) plus double polarization quality. Also owing to a very small prefactor, with these settings a $G_0W_0$ calculation for the largest of these clusters takes only 240 CPU hours. With errors of 0.24 eV for HOMO energies in the GW100 database on the quadruple-$ζ$ level, our implementation is less accurate than canonical all-electron implementations using the larger def2-QZVP GTO-tpye basis set. Apart from basis set errors, this is related to the well-known shortcomings of the GW space-time method using analytical continuation techniques as well as to numerical issues of the PADF-approach of accurately representing diffuse AO-products. We speculate, that these difficulties might be overcome by using optimized auxiliary fit sets with more diffuse functions of higher angular momenta. Despite these shortcomings, for subsets of medium and large molecules from the GW5000 database, the error of our approach using basis sets of TZ and augmented DZ quality is decreasing with system size. On the augmented DZ level we reproduce canonical, complete basis set limit extrapolated reference values with an accuracy of 80 meV on average for a set of 20 large organic molecules. We anticipate our algorithm, in its current form, to be very useful in the study of single-particle properties of large organic systems such as chromophores and acceptor molecules.
Straightforward and accurate automatic auxiliary basis set generation for molecular calculations with atomic orbital basis sets
密度拟合（英语：Density fitting，DF），又称恒等式解析（英语：Resolution of the identity，RI），是一种在量子化学计算中广泛使用的技术，使用各种类型的原子基组-高斯型轨道、斯莱特型轨道以及数值原子轨道-来加速密度泛函、哈特里-福克和后哈特里-福克计算。传统上，自定义辅助基组是手工优化每个轨道基组，但是，也有人提出了一些自动方案。在这项工作中，我们提出了一个简单的，但数值稳定的自动化计划，形成辅助基集的帮助下，枢轴Cholesky分解，这是适用于任何类型的原子基函数。我们使用高斯基组进行概念验证计算，证明了该方案，并表明当与至少极化三重$ζ$质量的轨道基组一起使用时，所提出的方法导致W 4 -17测试集的非多参考部分的Hartree-Fock（HF）和二阶Møller-Plesset（MP2）总能量的DF/RI误差可以忽略不计。结果是有前途的未来应用采用斯莱特型轨道或数值原子轨道，以及计划的基础上使用本地拟合方法。也可以使用全局拟合方法，在这种情况下，可以截断由本方案产生的高角动量函数以使计算成本最小化。
Analytic energy, gradient, and hessian of electrostatic embedding QM/MM based on electrostatic potential fitted atomic charges scaling linearly with the MM subsystem size
