CHAPTER 5: Computing Accurate Net Atomic Charges, Atomic Spin Moments, and Effective Bond Orders in Complex Materials

We summarize the computation of net atomic charges (NACs) and atomic spin moments (ASMs) using the recently developed density derived electrostatic and chemical (DDEC) method. This approach has three primary advantages: (a) it applies to both periodic and non-periodic systems, (b) it applies to systems with collinear or non-collinear magnetism as well as non-magnetic systems, and (c) it simultaneously optimizes the NACs to reproduce atomic chemical states in a material and the electrostatic potential outside the material's electron distribution. These properties make DDEC NACs suitable for constructing force-fields used in atomistic simulations. We also describe existing techniques for computing effective bond orders (EBOs) and the need to develop an improved method for computing EBOs in periodic materials. A variety of examples are used to demonstrate the procedures for computing NACs, ASMs, and EBOs from ab initio methods such as density functional theory (DFT) and coupled-cluster theory.