A coupled cluster study of As2

Abstract

Theoretical calculations at the self‐consistent field (SCF) and coupled‐cluster method including all single and double excitations (CCSD) have been carried out for As₂. A variety of basis sets employing both segmented contractions and atomic natural orbitals have been developed and investigated. The predicted equilibrium bond length (r_e=2.107 Å), harmonic vibrational frequency (ω_e=447 cm⁻¹), and ionization potentials (9.75, 10.08, and 14.78 eV) are in excellent agreement with experimental values 2.103 Å, 430 cm⁻¹, and 9.89 eV, 10.22 eV, and 15.32 eV; respectively. The best theoretical estimates for the dissociation energy (71.6 kcal/mol) and for the electron affinity (0.56 eV) were obtained at the CCSD level of theory employing large basis sets. However, given the importance of correlation effects on these two properties (the corresponding SCF values are 12.4 kcal/mol and 0.42 eV), the theoretical predictions could substantially be affected by the inclusion of connected triple excitations not considered in this work. The open‐shell atomic As(⁴S) and diatomic calculations for As⁺₂ and As⁻₂ were carried out using a recently developed CCSD program that is based on restricted Hartree–Fock references and does not include any spin contamination for the correlation energies. The importance of correlating 3d electrons and the basis‐set superposition error for D_e are analyzed. The effect of including d‐ and f‐type polarization functions in the basis set is also discussed.