Restricted Active Space Spin-Flip (RAS-SF) Configuration Interaction Approach

  • In the RAS-SF approach, the wave function is specified by the definition of an active space and through spin-flip excitations from a Hartree-Fock reference state.
  • RAS-SF can provide a quite well balanced description of nearly degenerate low-lying excited states at moderate computational cost. 
  • Dynamical correlation correction can be obtained through perturbation or short-range correlation density funcitonals.
  • The implementation of RAS-SF in Q-Chem supports both even and odd numbers of electrons, while the use of RI and OpenMP parallelization are also available.
  • RAS-SF is a strong competitor of multiconfigurational approaches, such as CASSCF. Applications include singlet fission, avoided crossings, conical intersections, bond dissociation, excited states in multi-radicals and spin conversion, etc.

Herringbone structure in tetracene crystal. The multiexciton states in tetracene trimers can be studied by RAS-SF.


RAS(8,5)-SF energy surfaces for the two lowest singlet states of ethene along the torsion and piramidalization distortions.