Electronic structure methods for excited states

The electronic structure method used to provide the energies and gradients of the states is crucial in photochemistry and photophysics. Ab initio electronic structure methods have been used for many years. Treating closed shell systems in their ground state is a problem that, in many cases, can now be solved routinely by chemists using standardized methods and computer packages. In order to obtain quantitative results, electron correlation (also referred to as dynamical correlation) should be included in the model and there are many methods available for doing this based on either variational or perturbation principles [41], 

In order to improve the quality of excited state description, correlation needs to be included. The methods having been developed can be categorized into two groups, the multireference methods, and the single-reference based methods. 

Multireference methods provide a straightforward way to treat excited states, since studying excited states requires the equivalent treatment of these states. Multireference methods are extensions of the single reference Hartree-Fock or configuration interaction (Cl) methods, where many configurations are used instead of a single configuration, 

The basis of the expansion, ifra, are configuration state functions (CSF), which are linear combinations of Slater determinants that are eigenfunctions of the spin operator and have the correct spatial symmetry and total spin of the electronic state under investigation [42], 

The Multi-Configuration Self-Consistent Field (MCSCF) method includes configurations created by excitations of electrons within an active space. Both the coefficients ca of the expansion in terms of CSFs and the expansion coefficients of the 

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In order to be able to characterize the PES of excited states, locate conical intersections, and derive mechanisms for photophysics and photochemistry, efficient electronic structure methods for excited states are required. In the following section we give a brief overview of the current state of methodological developments in electronic structure methods applicable to excited states. 

Krylov AI (2006) Spin-flip equation-of-motion coupled-cluster electronic structure method for a description of excited states, bond breaking, diradicals, and triradicals. Acc Chem Res 39 83-91... 

Our present focus is on correlated electronic structure methods for describing molecular systems interacting with a structured environment where the electronic wavefunction for the molecule is given by a multiconfigurational self-consistent field wavefunction. Using the MCSCF structured environment response method it is possible to determine molecular properties such as (i) frequency-dependent polarizabilities, (ii) excitation and deexcitation energies, (iii) transition moments, (iv) two-photon matrix elements, (v) frequency-dependent first hyperpolarizability tensors, (vi) frequency-dependent polarizabilities of excited states, (vii) frequency-dependent second hyperpolarizabilities (y), (viii) three-photon absorptions, and (ix) two-photon absorption between excited states. 

Krylov, A. 1. [2006]. Spin-Flip Equation-of-Motion Coupled-Cluster Electronic Structure Method for a Description of Excited States, Bond Breaking, Diradicals, and Triradicals, Acc. Chem. Res., 39,83-91. 

As discussed earlier, thymine is very similar to uracil in its excited states pattern. This is also true for its radiationless decay mechanism except from the fact that the excited state lifetime in thymine is somewhat longer than in uracil. Theoretically the mechanism for radiationless decay has been studied using CASPT2 electronic structure methods [150, 152],... 

The photolysis experiments have also received theoretical attention, with electronic structure methods used to calculate the nature of the excited states (44), as well as the potential energy curves for loss of CO (45). Theoretical models for the excited-state dynamics leading to dissociation have also been proposed (46). 

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