arXiv subjects

Tamar Stein

Publications and source records attributed to Tamar Stein.

5 recordsLinked to original sources

arXiv2026

Functionalization of Benzene Ices by Atomic Oxygen

Small aromatic molecules, including functionalized derivatives of benzene, are known to be present throughout the different stages of star and planet formation. In particular, oxygen-bearing monosubstituted aromatics, likely including phenol, have been identified in the coma of comet 67P. This suggests that, earlier in the star and planet formation evolution, icy grains may act as both reservoirs and sites of functionalization for these small aromatics. We investigate the ice-phase reactivity of singlet oxygen atoms (O($^1$D)) with benzene, using ozone as a precursor that is readily photodissociated by relatively low-energy. Our experiments show that O($^1$D) efficiently reacts with benzene, forming phenol, benzene oxide, and oxepine as the main products. Phenol formation is temperature-independent, consistent with a barrierless insertion mechanism. In contrast, the formation of benzene oxide/oxepine shows a slight temperature dependence, suggesting that additional reaction pathways involving either ground-state or excited-state oxygen atoms may contribute. In H$_2$O and \COO ice matrices we find that dilution does not suppress formation of phenol. We extrapolate an experimental upper limit for the benzene-to-phenol conversion fraction of 27-44$\%$ during the lifetime of an interstellar cloud, assuming O($^1$D) production rates based on CO$_2$ ice abundances and a cosmic-ray induced UV field. We compare these estimates with a new analysis of data from the comet 67P, where the C$_6$H$_6$O/C$_6$H$_6$ ratio is 20$\pm$6$\%$. This value lies within our estimated range, suggesting that O($^1$D)-mediated chemistry is a viable pathway for producing oxygenated aromatics in cold astrophysical ices, potentially enriching icy planetesimals with phenol and other biorelevant compounds.

astro-ph.EP↗

arXiv2024

Spin migration in density functional theory: energy, potential and density perspectives

Spin is a fundamental property of any many-electron system. The ability of density functional theory to accurately predict the physical properties of a system, while varying its spin, is crucial for describing magnetic materials and high-spin molecules, spin flip, magnetization and demagnetization processes. Within density functional theory, when using various exchange-correlation approximations, the exact dependence of the energy on the spin often deviates from the exact constant or piecewise-linear behavior, which is directly related to the problem of strong (static) correlation and challenges the description of molecular dissociation. In this paper, we study the behavior of the energy, the frontier Kohn-Sham (KS) orbitals, the KS potentials and the electron density, with respect to fractional spin, in different atomic systems. We analyze five standard exchange-correlation functionals and find three main scenarios of deviation from the expected exact results. We clearly recognize a jump in the frontier orbital energies upon spin variation in the exact exchange and in hybrid functionals, and the related plateau in the corresponding KS potential. Our results are instrumental for the assessment of the quality of existing approximations from a new perspective and for the development of advanced functionals with sensitivity to magnetic properties.

physics.chem-ph↗

arXiv2014

Seniority-based coupled cluster theory

Doubly occupied configuration interaction (DOCI) with optimized orbitals often accurately describes strong correlations while working in a Hilbert space much smaller than that needed for full configuration interaction. However, the scaling of such calculations remains combinatorial with system size. Pair coupled cluster doubles (pCCD) is very successful in reproducing DOCI energetically, but can do so with low polynomial scaling ($N^3$, disregarding the two-electron integral transformation from atomic to molecular orbitals). We show here several examples illustrating the success of pCCD in reproducing both the DOCI energy and wave function, and show how this success frequently comes about. What DOCI and pCCD lack are an effective treatment of dynamic correlations, which we here add by including higher-seniority cluster amplitudes which are excluded from pCCD. This frozen pair coupled cluster approach is comparable in cost to traditional closed-shell coupled cluster methods with results that are competitive for weakly correlated systems and often superior for the description of strongly correlated systems.

physics.chem-ph↗

arXiv2012

Curvature and frontier orbital energies in density functional theory

Perdew et al. [Phys. Rev. Lett 49, 1691 (1982)] discovered and proved two different properties of exact Kohn-Sham density functional theory (DFT): (i) The exact total energy versus particle number is a series of linear segments between integer electron points; (ii) Across an integer number of electrons, the exchange-correlation potential may "jump" by a constant, known as the derivative discontinuity (DD). Here, we show analytically that in both the original and the generalized Kohn-Sham formulation of DFT, the two are in fact two sides of the same coin. Absence of a derivative discontinuity necessitates deviation from piecewise linearity, and the latter can be used to correct for the former, thereby restoring the physical meaning of the orbital energies. Using selected small molecules, we show that this results in a simple correction scheme for any underlying functional, including semi-local and hybrid functionals as well as Hartree-Fock theory, suggesting a practical correction for the infamous gap problem of DFT. Moreover, we show that optimally-tuned range-separated hybrid functionals can inherently minimize both DD and curvature, thus requiring no correction, and show that this can be used as a sound theoretical basis for novel tuning strategies.

cond-mat.other↗

arXiv2010

Fundamental gaps of finite systems from the eigenvalues of a generalized Kohn-Sham method

We present a broadly-applicable, physically-motivated first-principles approach to determining the fundamental gap of finite systems. The approach is based on using a range-separated hybrid functional within the generalized Kohn-Sham approach to density functional theory. Its key element is the choice of a range-separation parameter such that Koopmans' theorem for both the neutral and anionic is obeyed as closely as possible. We demonstrate the validity, accuracy, and advantages of this approach on first, second, and third row atoms, the oligoacene family of molecules, and a set of hydrogen-passivated silicon nanocrystals. This extends the quantitative usage of density functional theory to an area long believed to be outside its reach.

cond-mat.mtrl-sci↗