The restricted active space followed by second-order perturbation theory method: Theory and application to the study of CuO2 and Cu2O2 systems

Abstract

A multireference second-order perturbation theory using a restricted active space self-consistent field wave function as reference ͑RASPT2/RASSCF͒ is described. This model is particularly effective for cases where a chemical system requires a balanced orbital active space that is too large to be addressed by the complete active space self-consistent field model with or without second-order perturbation theory ͑CASPT2 or CASSCF, respectively͒. Rather than permitting all possible electronic configurations of the electrons in the active space to appear in the reference wave function, certain orbitals are sequestered into two subspaces that permit a maximum number of occupations or holes, respectively, in any given configuration, thereby reducing the total number of possible configurations. Subsequent second-order perturbation theory captures additional dynamical correlation effects. Applications of the theory to the electronic structure of complexes involved in the activation of molecular oxygen by mono-and binuclear copper complexes are presented. In the mononuclear case, RASPT2 and CASPT2 provide very similar results. In the binuclear cases, however, only RASPT2 proves quantitatively useful, owing to the very large size of the necessary active space.

Viewing the Valence Electronic Structure of Ferric and Ferrous Hexacyanide in Solution from the Fe and Cyanide Perspectives

“…The RASSCF states were optimized within each D2h symmetry class using a state-averaging procedure and a level shift of 1.0. Dynamical correlation effects were taken into account via second-order perturbation theory (PT2) 43 using an imaginary shift of 0.3. Spin-orbit interactions were calculated in the RASSI scheme 44 , including doublet and quartet states for ferricyanide and singlet and triplet states for ferrocyanide (sextet and quintet states were excluded as their effect to spectra was found to be insignificant).…”

Applications to metal K pre-edges of transition metal dimers illustrate the approximate origin independence for the intensities in the length representation

“…[10,11,13] Recently, the restricted active space (RAS) SCF and PT2 methods have been used to model metal L-edge XAS spectra of a variety of transition metal complexes using the electric dipole approximation. [14][15][16][17][18][19][20]. For metal K-edge XAS, in particular the pre edge, the dipole approximation no longer suffices and electric quadrupole and magnetic dipole contributions are therefore also considered.…”

Benchmark of Dynamic Electron Correlation Models for Seniority-Zero Wave Functions and Their Application to Thermochemistry

“…Exceptions are multireference coupled cluster models, some variants of the multi-reference configuration interaction method, and the perturb-then-diagonalize multi-reference perturbation theory approaches that use effective Hamiltonians. To remedy this problem, a posteriori corrections have been developed that are usually based on perturbation theory [11][12][13][14][15], canonical transformation theory [16] or coupled cluster methods [17].…”