Browsing by Subject "Quantum Theory"

Now showing 1 - 5 of 5

Open Access
Accelerating self-consistent field convergence with the augmented Roothaan-Hall energy function.
(J Chem Phys, 2010-02-07) Hu, Xiangqian; Yang, Weitao
Based on Pulay's direct inversion iterative subspace (DIIS) approach, we present a method to accelerate self-consistent field (SCF) convergence. In this method, the quadratic augmented Roothaan-Hall (ARH) energy function, proposed recently by Høst and co-workers [J. Chem. Phys. 129, 124106 (2008)], is used as the object of minimization for obtaining the linear coefficients of Fock matrices within DIIS. This differs from the traditional DIIS of Pulay, which uses an object function derived from the commutator of the density and Fock matrices. Our results show that the present algorithm, abbreviated ADIIS, is more robust and efficient than the energy-DIIS (EDIIS) approach. In particular, several examples demonstrate that the combination of ADIIS and DIIS ("ADIIS+DIIS") is highly reliable and efficient in accelerating SCF convergence.
Open Access
Conductive junctions with parallel graphene sheets.
(J Chem Phys, 2010-03-21) Zheng, Xiao; Ke, San-Huang; Yang, Weitao
The establishment of conductive graphene-molecule-graphene junction is investigated through first-principles electronic structure calculations and quantum transport calculations. The junction consists of a conjugated molecule connecting two parallel graphene sheets. The effects of molecular electronic states, structure relaxation, and molecule-graphene contact on the conductance of the junction are explored. A conductance as large as 0.38 conductance quantum is found achievable with an appropriately oriented dithiophene bridge. This work elucidates the designing principles of promising nanoelectronic devices based on conductive graphene-molecule-graphene junctions.
Open Access
Einstein’s New Physics in James Joyce’s Finnegans Wake: The Role of Irish Nationalism
(2010-05-27T12:29:26Z) Sypek, Alexis
In previous centuries imaginations were ignited by scientific advancement. Inspired by astronomical developments, probed to questioning by the discovery of new worlds, and excited by technological advances that asserted humanity’s authority over nature, there is a long-standing tradition of science acting as muse for innumerable literary endeavors. This relationship held true for James Joyce as he was creating Finnegans Wake from 1923 to 1939. However, scholarship that connects 20th century literary production and the rapidly advancing science based on Einstein’s groundbreaking work, first published in 1905, is not as developed as that of previous centuries. Also, while Joycean scholarship abounds, Joyce’s precise exposure to the highly creative science of Einstein is often only briefly discussed or altogether absent. Thus, a unique challenge presents itself. In Finnegans Wake Joyce problematizes time and temporal sequence yet he does this independently of his response to the nationalist question. While the two experiments in both political commentary and Einsteinian physics may not have been consciously linked, the aim of this thesis is to provide a new voice in exploring the use of relativity and quantum theory within Joyce’s national narrative. To trace the role historical events played in the transference from laboratory to bookshelf, it is first necessary to explore Joyce’s precise attitude towards the movement that held his attention the most: Irish nationalism. Once Joyce’s unique political temperament is understood, the next step is to tease out Joyce’s understanding of new physics. Finally, the two distinct trends of Joyce’s nationalism and his scientific exposure must be brought together by the text itself.
Open Access
Elucidating solvent contributions to solution reactions with ab initio QM/MM methods.
(J Phys Chem B, 2010-03-04) Hu, Hao; Yang, Weitao
Computer simulations of reaction processes in solution in general rely on the definition of a reaction coordinate and the determination of the thermodynamic changes of the system along the reaction coordinate. The reaction coordinate often is constituted of characteristic geometrical properties of the reactive solute species, while the contributions of solvent molecules are implicitly included in the thermodynamics of the solute degrees of freedoms. However, solvent dynamics can provide the driving force for the reaction process, and in such cases explicit description of the solvent contribution in the free energy of the reaction process becomes necessary. We report here a method that can be used to analyze the solvent contributions to the reaction activation free energies from the combined QM/MM minimum free-energy path simulations. The method was applied to the self-exchange S(N)2 reaction of CH(3)Cl + Cl(-), showing that the importance of solvent-solute interactions to the reaction process. The results were further discussed in the context of coupling between solvent and solute molecules in reaction processes.
Open Access
Probing near-infrared photorelaxation pathways in eumelanins and pheomelanins.
(J Phys Chem A, 2010-11-04) Piletic, Ivan R; Matthews, Thomas E; Warren, Warren S
Ultraviolet-visible spectroscopy readily discerns the two types of melanin pigments (eumelanin and pheomelanin), although fundamental details regarding the optical properties and pigment heterogeneity are more difficult to disentangle via analysis of the broad featureless absorption spectrum alone. We employed nonlinear transient absorption spectroscopy to study different melanin pigments at near-infrared wavelengths. Excited-state absorption, ground-state depletion, and stimulated emission signal contributions were distinguished for natural and synthetic eumelanins and pheomelanins. A starker contrast among the pigments is observed in the nonlinear excitation regime because they all exhibit distinct transient absorptive amplitudes, phase shifts, and nonexponential population dynamics spanning the femtosecond-nanosecond range. In this manner, different pigments within the pheomelanin subclass were distinguished in synthetic and human hair samples. These results highlight the potential of nonlinear spectroscopies to deliver an in situ analysis of natural melanins in tissue that are otherwise difficult to extract and purify.