Where to draw the line: Chasing energy extrapolations, cluster convergence, and molecular trajectories
The adsorption of S on Cu surfaces is studied by density functional theory using both plane-wave and atomic orbital basis sets. Calculations are performed on Cu clusters of increasing sizes, and strong oscillations in the S-Cu binding energy versus cluster size are found. Although expected for small clusters, the oscillations persist even to clusters containing a few hundred atoms. Smearing of the occupancy function in plane-wave DFT, and averaging over clusters of different sizes are presented as possible approaches to approximate bulk results using small to medium sized clusters.
Chemically accurate potential energy curves for the lowest lying singlet states of C2 are obtained using the correlation energy extrapolation by intrinsic scaling (CEEIS) method. The potential energies also include complete basis set extrapolation, core-valence correlation, spin-orbit coupling, and scalar relativistic effects. Our calculated ro-vibrational levels show deviations from experiment of between ~10-20 cm-1, demonstrating near spectroscopic accuracy.
The correlation energy extrapolation by many-body expansion (CEEMBE) method is presented. Like the CEEIS method, CEEMBE approximates configuration interaction (CI) energies using a linear extrapolation from CI calculations with reduced numbers of virtual orbitals. The method also uses a many-body expansion of the CI energy based on separating the valence orbitals into groups. Tests on ozone and F2 potential energy surfaces show that CI energies can be reproduced to within a few millihartree, and in many cases to within less than 1 millihartree. We also present a hybrid methodology, CEEMBE-h, which adds CEEIS style extrapolations to the CEEMBE procedure. CEEMBE-h reproduces the original CEEMBE energies to within 0.1-0.5 millihartree or less.
Nonadiabatic dynamics using spin-flip time-dependent density functional theory (SF-TDDFT) are presented for the penta-2,4-dieniminium cation. We developed an interface between the GAMESS and Newton-X programs for SF-TDDFT dynamics. Time-derivative couplings between SF-TDDFT states are calculated using an approximate wavefunction overlap method. Our comparison with analytical couplings from CASSCF demonstrates that the overlap method for time-derivative couplings is effective for SF-TDDFT. Because of the spin-contamination in SF-TDDFT, the interface includes a state-tracking algorithm to ensure dynamics are propagated on the correct potential energy surface.