GEOMETRIES AND ENERGY SEPARATIONS OF LOW-LYING ELECTRONIC STATES OF AG-4 AND CU-4

Complete active space self-conslstent-field calculations (CASSCF) followed by POLCI and multireference singles + doubles CI (MRSDCI) calculations, which included all (n - 1)d10ns1 electrons, are carried out on 14 electronic states of Ag4 and the three low-lying electronic states of Cu4. The ground states of both Cu4 and Ag4 are found to be of 1A1 symmetry with rhombic geometries. The MRSDCI calculations included up to 430000 configurations. The linear 1Σg+ state of Ag4 was found to be 0.95 eV above the ground state. Two stationary points were found in the 3B2 surface with rhombic and square geometries. The nature of the various states was analyzed through the CI wave functions and the Mulliken population analyses of the MRSDCI natural orbitals. The Mulliken populations of both Cu4 and Ag4 reveal considerable participation of the 4p and 5p orbitals of the metal atoms in addition to the s orbitals of the metal atom in the bonding. The periodic trends in the geometries, binding energies, and ionization potentials of Cu4, Ag4, and Au4 show that relativistic effects stabilize the metal-metal bonding to a considerable extent in Au4. The effects of spin-orbit coupling are also obtained through the relativistic CI methodology. The barrier to convert the acute rhombic structure to the obtuse structure in the 1A1 surface was calculated as 1.11 eV for Ag4. © 1990 American Chemical Society.