The electronic structure of the triiodide ion from relativistic correlated calculations: A comparison of different methodologies
Abstract
The triiodide ion I3- exhibits a complex photodissociation behavior, the dynamics of which are not yet fully understood. As a first step toward determining the full potential energy surfaces of this species for subsequent simulations of its dissociation processes, we investigate the performance of different electronic structure methods [time-dependent density functional theory, complete active space perturbation theory to second order (CASPT2), Fock-space coupled cluster and multireference configuration interaction] in describing the ground and excited states of the triiodide ion along the symmetrical dissociation path. All methods apart from CASPT2 include scalar relativity and spin-orbit coupling in the orbital optimization, providing useful benchmark data for the more common two-step approaches in which spin-orbit coupling is introduced in the configuration interaction. Time-dependent density functional theory with the statistical averaging of model orbital potential functional is off the mark for this system. Another choice of functional may improve performance with respect to vertical excitation energies and spectroscopic constants, but all functionals are likely to face instability problems away from the equilibrium region. The Fock-space coupled cluster method was shown to perform clearly best in regions not too far from equilibrium but is plagued by convergence problems toward the dissociation limit due to intruder states. CASPT2 shows good performance at significantly lower computational cost, but is quite sensitive to symmetry breaking. We furthermore observe spikes in the CASPT2 potential curves away from equilibrium, signaling intruder state problems that we were unable to curb through the use of level shifts. Multireference configuration interaction is, in principle, a viable option, but its computational cost in the present case prohibits use other than for benchmarking purposes.
- Publication:
-
Journal of Chemical Physics
- Pub Date:
- August 2010
- DOI:
- 10.1063/1.3474571
- Bibcode:
- 2010JChPh.133f4305G
- Keywords:
-
- configuration interactions;
- coupled cluster calculations;
- density functional theory;
- excited states;
- ground states;
- iodine;
- molecular electronic states;
- molecule-photon collisions;
- negative ions;
- perturbation theory;
- photodissociation;
- potential energy surfaces;
- relativistic corrections;
- spin-orbit interactions;
- 33.80.Gj;
- 31.50.-x;
- 31.30.J-;
- 31.15.V-;
- 31.15.ee;
- 31.15.bw;
- Diffuse spectra;
- predissociation photodissociation;
- Potential energy surfaces;
- Relativistic and quantum electrodynamic effects in atoms molecules and ions;
- Electron correlation calculations for atoms ions and molecules;
- Time-dependent density functional theory;
- Clusters: electronic properties equilibrium geometries coupled-cluster theory