We report an investigation of the electronic structure and chemical bonding of AuH 2 À using photoelectron spectroscopy and ab initio calculations. We obtained vibrationally resolved photoelectron spectra of AuH 2 À at several photon energies. Six electronic states of AuH 2 were observed and assigned according to the theoretical calculations. The ground state of AuH 2 À is known to be linear, while that of neutral AuH 2 is bent with a :H-Au-H equilibrium bond angle of 129 . This large geometry change results in a very broad bending vibrational progression in the photoelectron spectra for the ground-state transition. The electron affinity of AuH 2 is measured to be 3.030 AE 0.020 eV. A short bending vibrational progression is also observed in the second photodetachment band, suggesting a slightly bent structure for the first excited state of AuH 2 . The linear geometry is a saddle point for the ground and first excited states of AuH 2 , resulting in double-well potentials for these states along the bending coordinate. Spectroscopic evidence is observed for the detachment transitions to the doublewell potentials of the ground and first excited states of AuH 2 . Higher excited states of AuH 2 due to detachment from the nonbonding Au 5d electrons are all linear, similar to the anion ground state. Kohn-Sham molecular orbital analyses reveal surprising participation of H 2p orbitals in the Au-H chemical bonding and an unprecedented weak Au 5dp to H 2pp back donation. The simplicity of the linear AuH 2 À anion and its novel spectroscopic features make it a textbook example for understanding the covalent bonding properties and relativistic effects of Au.

We present the development of the multistate multireference second-order perturbation theory (CASPT2) with multiroot references, which are described using the density matrix renormalization group (DMRG) method to handle a large active space. The multistate first-order wave functions are expanded into the internally contracted (IC) basis of the single-state single-reference (SS-SR) scheme, which is shown to be the most feasible variant to use DMRG references. The feasibility of the SS-SR scheme comes from two factors: first, it formally does not require the fourth-order transition reduced density matrix (TRDM) and second, the computational complexity scales linearly with the number of the reference states. The extended multistate (XMS) treatment is further incorporated, giving suited treatment of the zeroth-order Hamiltonian despite the fact that the SS-SR based IC basis is not invariant with respect to the XMS rotation. In addition, the state-specific fourth-order reduced density matrix (RDM) is eliminated in an approximate fashion using the cumulant reconstruction formula, as also done in the previous state-specific DMRG-cu(4)-CASPT2 approach. The resultant method, referred to as DMRG-cu(4)-XMS-CASPT2, uses the RDMs and TRDMs of up to third-order provided by the DMRG calculation. The multistate potential energy curves of the photoisomerization of diarylethene derivatives with CAS(26e,24o) are presented to illustrate the applicability of our theoretical approach.

Gold compounds, clusters, and nanoparticles are widely used as catalysts and therapeutic medicines; the interactions between gold and its ligands in these systems play important roles in their chemical properties and functionalities. In order to elucidate the nature of the chemical interactions between Au(I) and its ligands, herein we use several theoretical methods to study the chemical bonding in a variety of linear [AuX2](-) complexes, where X = halogen atoms (F, Cl, Br, I, At and Uus), H, OH, SH, OCH3, SCH3, CN and SCN. It is shown that the most important bonding orbitals in these systems have significant contributions from the Au sd hybridized atomic orbitals. The ubiquitous linear or quasi-linear structures of [AuX2](-) are attributed to the well-balanced optimal overlap in both σ and π bonding orbitals and minimal repulsion between the two negatively charged ligands. The stability of these complexes is related to the covalency of the Au-X bond and a periodic trend is found in the evolution of covalency along the halogen group ligands. The special stability of [Au(CN)2](-) is a result of strong covalent and ionic interactions. For the superheavy element Uus, the covalency of Au-Uus is enhanced through the spin-orbit interactions.