First-Principles Photoemission Spectroscopy of DNA and RNA Nucleobases from Koopmans-Compliant Functionals

Abstract

The need to interpret ultraviolet photoemission data strongly motivates the refinement of firstprinciples techniques able to accurately predict spectral properties. In this work we employ Koopmans-compliant functionals, constructed to enforce piecewise linearity in approximate density functionals, to calculate the structural and electronic properties of DNA and RNA nucleobases. Our results show that not only ionization potentials and electron affinities are accurately predicted with mean absolute errors < 0.1 eV, but also that calculated photoemission spectra are in excellent agreement with experimental ultraviolet photoemission spectra. In particular, the role and contribution of different tautomers to the photoemission spectra are highlighted and discussed in detail. The structural properties of nucleobases are also investigated, showing an improved description with respect to local and semilocal density-functional theory. Methodologically, our results further consolidate the role of Koopmans-compliant functionals in providing, through orbital-density-dependent potentials, accurate electronic and spectral properties.

Koopmans-Compliant Spectral Functionals for Extended Systems

“…(1). As mentioned earlier, for small molecules [31,32], it is typically sufficient to compute two values of α, one to be used for all the occupied orbitals, and the other to be used for all empty orbitals; these two values can be chosen by enforcing that the HOMO eigenvalue of a neutral molecule be equal to the LUMO eigenvalue of the respective cation, and that the HOMO eigenvalue of an anion molecule be equal to the LUMO eigenvalue of the neutral one, respectively. However, determining the screening on frontier (canonical) orbitals is only meant to act as an average measure of the response of the electronic system at hand.…”

“…In previous work [29,31,32], we presented the performance of KC functionals in predicting frontier energies, ultraviolet photoemission spectra, and orbital tomography momentum maps for different classes of molecules, while also arguing that these functionals provide accurate quasiparticle approximations to the exact spectral potential [33,39]. In fact, we typically find very good agreement with experiments, comparable or sometimes even better than state-of-the-art MBPT methods, while preserving moderate computational costs and the quality of the potential energy surface of the underlying base functional [30] (or even improving on it when using the KIPZ implementation of KC functionals [32]). In this work, we discuss how the framework of KC functionals extends to the case of solids.…”

“…The criterion of piecewise linearity was chosen in particular as a key feature by some of us when introducing the class of Koopmans-compliant (KC) functionals [26][27][28][29][30] that enforce a generalized criterion of PWL; in these approaches, every orbital energy is independent from the occupation of the orbital itself (we note in passing that these functionals generalize to all orbitals the linearization criterion of the DFT þ Hubbard U approach [16,17], which linearizes the energy with respect to the occupation of the Hubbard manifold). The accuracy of KC functionals in reproducing spectral properties is quite remarkable [29][30][31][32] and, in our view, it highlights the role of these functionals as approximations to the exact spectral functional, i.e., the functional able to reproduce spectral properties in addition to total energies [33]. Last, we also mention that the potential energy surface of KC functionals preserves exactly or slightly improves [32] the base functional upon which they are constructed (typically, the PBE [34] approximation is used).…”

“…The accuracy of KC functionals in reproducing spectral properties is quite remarkable [29][30][31][32] and, in our view, it highlights the role of these functionals as approximations to the exact spectral functional, i.e., the functional able to reproduce spectral properties in addition to total energies [33]. Last, we also mention that the potential energy surface of KC functionals preserves exactly or slightly improves [32] the base functional upon which they are constructed (typically, the PBE [34] approximation is used). We briefly summarize and expand here the concepts alluded to above; a detailed description of the KC spectral functionals, first introduced in Refs.…”

Green’s function coupled cluster simulation of the near-valence ionizations of DNA-fragments

“…up to three G−C base pairs in Reference 11). a) Electronic mail: peng398@pnnl.gov b) Electronic mail: karol.kowalski@pnnl.gov For higher VIEs, the DFT approach with Koopmans-Compliant functionals 15 has been employed to simulate the photoemission spectroscopy of single nucleobases corresponding up to ∼20 eV VIEs showing good agreements with the experimental data, while the computation of the ionizations of longer sequence has rarely been reported in the similar theoretical framework. In the meanwhile, it should be realized that the quality of the DFT results heavily depends on the choice of density functionals, and the employed density functionals need to be validated by more accurate and predictable methods to attenuate the large self-interaction energy (SIE) and correct the overdelocalized charge density.…”