Restricted open-shell Kohn-Sham

Filatov, M., Shaik, S., 1999, Application of Spin-Restricted Open-Shell Kohn-Sham Method to Atomic and Molecular Multiplet States , J. Chem. Phys., 110, 116. 

Langer H, Doltsinis NL (2003) Excited state tautomerism of the DNA base guanine a restricted open-shell kohn-sham study. J Chem Phys 118 5400-5407... 

Lately, the CP-MD approach has been combined with a mixed QM/MM scheme [10-12] which enables the treatment of chemical reactions in biological systems comprising tens of thousands of atoms [11, 26]. Furthermore, CP-MD and mixed QM/MM CP-MD simulations have also been extended to the treatment of excited states within a restricted open-shell Kohn-Sham approach [16, 17, 27] or within a linear response formulation of TDDFT [16, 18], enabling the study of biological photoreceptors [28] and the in situ design of optimal fluorescence probes with tailored optical properties [32]. Among the latest extensions of this method are also the calculation of NMR chemical shifts [14]. 

ROKS Restricted open-shell Kohn-Sham theory... 

In the present chapter, we will focus on the simulation of the dynamics of photoexcited nucleobases, in particular on the investigation of radiationless decay dynamics and the determination of associated characteristic time constants. We use a nonadiabatic extension of ab initio molecular dynamics (AIMD) [15, 18, 21, 22] which is formulated entirely within the framework of density functional theory. This approach couples the restricted open-shell Kohn-Sham (ROKS) [26-28] first singlet excited state, Su to the Kohn-Sham ground state, S0, by means of the surface hopping method [15, 18, 94-97], The current implementation employs a plane-wave basis set in combination with periodic boundary conditions and is therefore ideally suited to condensed phase applications. Hence, in addition to gas phase reference simulations, we will also present nonadiabatic AIMD (na-AIMD) simulations of nucleobases and base pairs in aqueous solution. 

Kohn-Sham formalism, which suffers from the spin contamination problem, and on the sum-over-states or coupled perturbed Kohn-Sham approaches. In recent articles devoted to computations of elechonic g-tensors we advocated the use of an alternative approach, namely linear response theory based on the spin-restricted open-shell Kohn-Sham formalism, which is free from spin contamination problem (see Theory section). In the following we briefly review the applicability of this approach for some paramagnetic compounds. 

Restricted Open-Shell Kohn-Sham Theory (ROKS)... 

Fig. 7.2 Restricted open shell Kohn-Sham theory (ROKS). (a) Four determinants resulting from HOMO-LUMO transition of one electron (b) the mixed states m ) and m2) can be combined to form a triplet state t3) and the singlet state s). The singlet-triplet splitting Ast corresponds to twice the splitting between triplets and mixed states.

ROKS restricted open shell Kohn-Sham... 

Singlet excited-state geometries of a set of medium-sized molecules, including 1,2,4,5-tetrazine, with different characteristic lowest excitations have been studied with two closely related restricted open-shell Kohn-Sham methods and within linear response to time-dependent DFT (TDDFT). The results are compared to wave function-based methods <2003JMT(630)163>. 

Restricted ensemble Kohn-Sham DFT Alternatively one can perform standard KS-DFT calculations on a collection of determinants with different occupations and take a weighted average of the individual energies to obtain an estimate of the mul-tideterminantal situation. To avoid the independent calculation of several KS determinants, a generalization of this approach was proposed by Filatov and Shaik based on the coupling operator technique developed by Roothaan for restricted open-shell Hartree-Fock. This restricted open-shell Kohn-Sham (ROKS) approach was later extended to situations where fractional occupation numbers are not imposed by the... 

The analysis starts with a restricted open-shell Kohn-Sham (ROKS) calculation on the HS state. If necessary, the magnetic orbitals are transformed to the representation with local orthogonal orbitals a and b, as shown in the first column of Fig. 5.11. Staying within the spin-restricted formalism makes that for each a orbital a orbital can be found which has the same spatial part. In the first step, the direct exchange is estimated from the energy difference of the HS(ROKS) and a BS determinant in which only the spin of one of the unpaired electrons is inverted, but neither the core nor the magnetic orbitals are optimized. 

Grimm S, Nonnenberg C, Frank I (2003) Restricted open-shell Kohn-Sham theory for pi-pi( ) transitions. I. Polyenes, cyanines, and protonated imines. J Chem Phys 119 11574—11584... 

I. Okazaki, F. Sato, T. Yoshihiro, T. Ueno, and H. Kashiwagi,/. Mol. Struct. (TLPEOCHEM), 451, 109-119 (1998). Development of a Restricted Open Shell Kohn-Sham Program and Its Application to a Model Heme Complex. 

---