Single Excitation Theories in Q-Chem
CIS Methods
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Excited states are computed starting from a Hartree-Fock reference
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Provides qualitatively correct descriptions of singly excited states
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Geometries and frequencies comparable to ground-state Hartree-Fock results
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Efficient, direct algorithm for computing energies, analytic gradients, and second derivatives
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Perturbative doubles correction via CIS(D) and SOS-CIS(D)
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Reduces the errors in CIS by a factor of two or more (to roughly that of MP2)
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RI-CIS(D) and RI-CIS(D0) methods for faster correlated excited-state calculations
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Time-Dependent DFT (TDDFT)
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Excited-state energies computed from a ground state Kohn-Sham reference
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Provides a marked improvement over CIS, at about the same cost
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Captures correlation effects
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Optimal tuning of range-separated hybrid functionals improves performance for charge-transfer states
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Spin-flip density functional theory (SF-DFT)
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Extends TDDFT to states with doubly excited character and conical intersections
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Useful for bond-breaking, diradicals, and single-molecule magnets
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Implementation of non-collinear formulation extends SF-TDDFT to a broader set of functionals and improves its accuracy
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Analytic gradients and Hessians of excited states are available
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Improved accuracy with asymptotically corrected exchange-correlation potential (TDDFT/TDA with LB94)
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TDDFT within a reduced single-excitation space for special cases
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Methods for electronic couplings (non-adiabatic and spin-orbit couplings, charge-transfer couplings)