NMR Shielding Tensors and Chemical Shifts in Scalar-Relativistic Local Exact Two-Component Theory

An efficient formulation of scalar-relativistic NMR shielding tensors based on (one-electron) spin-free exact two-component theory (X2C) is presented. It utilizes the diagonal local approximation to the unitary decoupling matrix (DLU), which we recently applied to analytical derivatives [J. Chem. Phys. 2018, 148, 104110]. This allows for routine calculations of large molecules with heavy atoms. Here, the computation times of the NMR shielding tensors of all nuclei formally scale cubically with the size of the system, while memory demands increase quadratically. Efficiency is demonstrated for heavy-element clusters and organometallic complexes with more than 120 atoms and 3200 contracted basis functions. The accuracy of the DLU scheme is evaluated based on C-13, O-17, Si-29, Ge-73, Sn-119, Xe-129, W-183, and Pb-207 NMR shielding constants and chemical shifts using different basis sets. The finite nucleus model is available throughout.