Quantum Chemical Methods for the Investigation of Photoinitiated Processes in Biological Systems: Theory and Applications

Abstract

With the advent of modern computers and advances in the development of efficient quantum chemical computer codes, the meaningful computation of large molecular systems at a quantum mechanical level became feasible. Recent experimental effort to understand photoinitiated processes in biological systems, for instance photosynthesis or vision, at a molecular level also triggered theoretical investigations in this field. In this Minireview, standard quantum chemical methods are presented that are applicable and recently used for the calculation of excited states of photoinitiated processes in biological molecular systems. These methods comprise configuration interaction singles, the complete active space self-consistent field method, and time-dependent density functional theory and its variants. Semiempirical approaches are also covered. Their basic theoretical concepts and mathematical equations are briefly outlined, and their properties and limitations are discussed. Recent successful applications of the methods to photoinitiated processes in biological systems are described and theoretical tools for the analysis of excited states are presented.

The low-lying excited states of neutral polyacenes and their radical cations: a quantum chemical study employing the algebraic diagrammatic construction scheme of second order

“…The algebraic expressions of the matrix elements M IJ (eq. (4)) are typical perturbation theoretical expressions, which have been derived in detail for ADC (2) and ADC(3) in Refs. [34][35][36].…”

“…These UADC schemes can be employed to compute excited states of open-shell radicals. The major changes in the derivation occur in the choice of the excitation operators in equation (2), where now attention has to be paid that successive annihilation and creation operators act on spin-orbitals with the same spin, thereby excluding so-called spin-flip excitations in which the spin state of the excited electron is changed. Furthermore, UMP2 must be chosen as ground state reference to eventually derive UADC (2).…”

“…The low-lying excited states of neutral polyacenes and their radical cations: a quantum chemical study employing the algebraic diagrammatic construction scheme of second order The development of efficient quantum chemical methods for the accurate calculation of excited state properties of large molecular systems is one of the greatest challenges of modern theoretical chemistry [1,2]. Today, several excited state methods are already available for the investigation of large molecules, among which time-dependent density functional theory (TDDFT) [3][4][5] is by far the most widely used.…”

“…Complete active-space self-consistent field (CASSCF) methods [6,7], symmetry-adapted cluster configuration interaction (SAC-CI) [8] and also approximate linear-response coupled-cluster theory of second order (RI-CC2) [9][10][11][12] are powerful wavefunction-based approaches. Nevertheless, all existing methods possess individual weaknesses and drawbacks [1,2]. The success of the available methods is manifested by ample excellent applications in the fields of photochemistry and photobiology [13][14][15][16][17][18][19][20][21][22][23][24].…”

“…The recent development of efficient computer codes for ADC is a prerequisite for its routine application to molecular systems of chemical and biophysical interest [40][41][42][43]. Although the ADC scheme is general and can be expanded to arbitrarily high-orders in perturbation theory, only second order ADC (ADC (2)) is currently computationally efficient enough to be applicable to large and medium-sized molecules on present-day computers. Recently, the ADC(2)-x scheme has been extended to an unrestricted spin-orbital formalism allowing for the calculation of excited states of open-shell radicals [42,43].…”