TD-CCSD

In addition, all the ketene states discussed above are open-shell singlets, which require at least two determinants in a zeroth-order description as discussed earlier. An approach for an in-depth study specific to one of these states is offered by the two-determinant, TD-CCSD, multireference method. In ketene, with a DZP basis the TD-CCSD vertical excitation energy is 3.83 for the 1A2 state in excellent agreement with experiment, while the l Aj state is at... 

The S-T splitting is obtained as a difference of the two separate individual energy calculations for the singlet and triplet. Those individual singlet and triplet calculations do not necessarily have to be by the same method, and in fact, the majority of the results in Table 1 use composite methods in which the singlet and triplet are treated by two different methods (primarily the singlet as a MR state while the triplet is SR). RMR indicates the reduced multi-reference approach of Li and Paldus [58]. The TD-CCSD of Baikova and Bartlett [54], an early SU-CCSD application, provides two roots simultaneously at each geometry. This SU-CCSD calculation introduced GVB CCSD as a two-determinant reference. The MR-BW is a state-specific MR... 

The TD-CCSD calculations are done with a DZP basis set (for comparison with full Cl) and ANO basis set of Widmark et al. See Ref. [54] for details (the cited result in the table is for ANO basis)... 

The predicted energy differences differ from the experimental results by up to 0.2 kcal/mol, despite basis saturation. In the sense of agreement to experiment, the two-determinant CCSD would appear to perform very well. The TD-CCSD (GVB) SU-CCSD calculation done used a large ANO basis. It appears to have the right mix of MR character for the and Bi states. 

The first approach is two-determinant CC theory, described above. Analytical derivatives have been implemented for the TD-CCSD method in ACES II, and these can be used to search excited state potential energy surfaces and to compute first-order properties of excited states. 

Cluster Transition character TD-B3LYP EOM-CCSD MRDCI Exp. 

Table 4.7. Excitation energies (in eV) for transitions into the lowest excited states in the X2SiOz cluster (X = OSiH3) calculated at the optimized geometry of the 3 A electronic state using TD-B3LYP, EOM-CCSD, and CASPT2 methods...

ASCFa (DFT) TD-DFTb (B3LYP) CASSCF/ CASPT2C MR-CId SAC-CI6 EOM-CCSD (Ext-STEOM)f Exp.g... 

Vertical excitation energies for the 1S2L and 2S1L structures, calculated by TD-DFT and EOM-CCSD, using the respective geometries are shown in Tables 3 and 4, respectively. (A dehnite decision on which C2V isomer is more stable cannot be made due to the small difference in stabilization energies.) VEEs for the lowest excited state... 

AIMD = ab initio molecular dynamics B-LYP = Becke-Lee-Yang-Parr CCSD = coupled cluster single double excitations CVC = core-valence correlation ECP = effective core potential DF = density functional GDA = gradient corrected density approximation MCLR = multiconfigurational linear response MP2 = M0ller-Plesset second-order (MRD)CI = multi-reference double-excitation configuration interaction RPA = random phase approximation TD-MCSCF = time-dependent multiconfigurational self-consistent field TD-SCF = time-dependent self-consistent field. 

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