Single Excitation Theories in Q-Chem

CIS Methods

• Excited states are computed starting from a Hartree-Fock reference

• Provides qualitatively correct descriptions of singly excited states

• Geometries and frequencies comparable to ground-state Hartree-Fock results

• Efficient, direct algorithm for computing energies, analytic gradients, and second derivatives

• Perturbative doubles correction via CIS(D) and SOS-CIS(D)

  • Reduces the errors in CIS by a factor of two or more (to roughly that of MP2)

  • RI-CIS(D) and RI-CIS(D0) methods for faster correlated excited-state calculations

Time-Dependent DFT (TDDFT)

• Excited-state energies computed from a ground state Kohn-Sham reference

• Provides a marked improvement over CIS, at about the same cost

• Captures correlation effects

• Optimal tuning of range-separated hybrid functionals improves performance for charge-transfer states

• Spin-flip density functional theory (SF-DFT)

  • Extends TDDFT to states with doubly excited character and conical intersections

  • Useful for bond-breaking, diradicals, and single-molecule magnets

  • Implementation of non-collinear formulation extends SF-TDDFT to a broader set of functionals and improves its accuracy

• Analytic gradients and Hessians of excited states are available

• Improved accuracy with asymptotically corrected exchange-correlation potential (TDDFT/TDA with LB94)

• TDDFT within a reduced single-excitation space for special cases

• Localizing diabatic states with Boys or Edmiston-Ruedenberg localization scheme for charge or energy transfer

• Methods for electronic couplings (non-adiabatic and spin-orbit couplings, charge-transfer couplings)