Ground-state functional for quantum spin chains with bond defects

We have developed a nonlocal functional of the exchange interaction for the ground-state energy of quantum spin chains described by the Heisenberg Hamiltonian. An alternating chain is used to obtain the correlation energy and a local unit-cell approximation is defined in the context of the density-functional theory. The agreement with our exact numerical data, for small chains, is significantly better than a previous formulation, even for chains with several ferromagnetic or antiferromagnetic bond defects. The results can be particularly relevant in the study of finite spin-1/2 Heisenberg chains, with exchange couplings changing, magnitude, or even sign, from bond-to-bond.

Full-dimensional analytical ab initio potential energy surface of the ground state of HOI

Extensive ab initio calculations using a complete active space second-order perturbation theory wavefunction, including scalar and spin-orbit relativistic effects with a quadruple-zeta quality basis set were used to construct an analytical potential energy surface (PES) of the ground state of the [H, O, I] system. A total of 5344 points were fit to a three-dimensional function of the internuclear distances, with a global root-mean-square error of 1.26 kcal mol(-1). The resulting PES describes accurately the main features of this system: the HOI and HIO isomers, the transition state between them, and all dissociation asymptotes. After a small adjustment, using a scaling factor on the internal coordinates of HOI, the frequencies calculated in this work agree with the experimental data available within 10 cm(-1). (C) 2011 American Institute of Physics. [doi: 10.1063/1.3615545]

Excitation energies from ground-state density-functionals by means of generator coordinates

The generator-coordinate method is a flexible and powerful reformulation of the variational principle. Here we show that by introducing a generator coordinate in the Kohn-Sham equation of density-functional theory, excitation energies can be obtained from ground-state density functionals. As a viability test, the method is applied to ground-state energies and various types of excited-state energies of atoms and ions from the He and the Li isoelectronic series. Results are compared to a variety of alternative DFT-based approaches to excited states, in particular time-dependent density-functional theory with exact and approximate potentials.

Ground-state energy of a classical artificial molecule

We study the ground-state energy of a classical artificial molecule formed by two-dimensional clusters (artificial atoms) of N/2 charged particles separated by a distance d. For the small molecules of N = 2 and 4, we obtain analytical expressions for this energy. For the larger ones, we calculate the ground-state energy using molecular dynamics simulation for N up to 128. From our numerical results, we are able to find out a function to approximate the ground-state energy of the molecules covering the range from atoms to molecules for any inter-atom distance d and for particle number from N = 8 to 128 within a difference less than one percent from the MD data.

Schrodinger-Poisson equations without Ambrosetti-Rabinowitz condition

We prove the existence of ground state solutions for a stationary Schrodinger-Poisson equation in R(3). The proof is based on the mountain pass theorem and it does not require the Ambrosetti-Rabinowitz condition. (C) 2010 Elsevier Inc. All rights reserved.

Photophysics and photostability of adenine in aqueous solution: A theoretical study

The sequential Monte Carlo/CASPT2 approach was employed to investigate deactivation and emission processes from the lowest-lying pi pi * and n pi * excited states of 9H-adenine in aqueous solution. It is found that conical intersections connecting the pi pi* and n pi* states with the ground state are also present in solution, whereas the barriers for the deactivation paths are significantly smaller on solvated conditions. The large destabilization of the n pi* state found in solution possibly prevents its involvement in the deactivation photophysics and explains the change from a bi- to a mono-exponential decay for the molecule in the gas phase and solution, respectively. (C) 2010 Elsevier B.V. All rights reserved.

A three-state model for the photophysics of guanine

The nonadiabatic photochemistry of the guanine molecule (2-amino-6-oxopurine) and some of its tautomers has been studied by means of the high-level theoretical ab initio quantum chemistry methods CASSCF and CASPT2. Accurate computations, based by the first time on minimum energy reaction paths, states minima, transition states, reaction barriers, and conical intersections on the potential energy hypersurfaces of the molecules lead to interpret the photochemistry of guanine and derivatives within a three-state model. As in the other purine DNA nucleobase, adenine, the ultrafast subpicosecond fluorescence decay measured in guanine is attributed to the barrierless character of the path leading from the initially populated (1)(pi pi* L-a) spectroscopic state of the molecule toward the low-lying methanamine-like conical intersection (gs/pi pi* L-a)(CI). On the contrary, other tautomers are shown to have a reaction energy barrier along the main relaxation profile. A second, slower decay is attributed to a path involving switches toward two other states, (1)(pi pi* L-b) and, in particular, (1)(n(o)pi*), ultimately leading to conical intersections with the ground state. A common framework for the ultrafast relaxation of the natural nucleobases is obtained in which the predominant role of a pi pi*-type state is confirmed.

Energy of the first excited state of (43)Ar

Results of proton-proton-gamma coincidence measurements using the (36)S+(9)Be reaction revealed a gamma ray of 201.27 +/- 0.16 keV that most probably corresponds to the transition between the predicted 7/2(-) first excited state to the 5/2(-) ground state of (43)Ar.

Electronic Structure of the Ground and Low-Lying Electronic States of MoB and MoB(+)

Multiconfigurational second-order perturbation theory (CASSCF//CASPT2) and quadruple-zeta ANO-RCC basis sets were employed to investigate the ground and low-lying electronic states of MoB and MoB(+). Spectroscopic constants, potential energy curves, wavefunctions, Mulliken population analyses, and ionization energies are given. The ground state of MoB is of X(6)Pi symmetry (R(e) = 1.968 angstrom, omega(e) = 664 cm(-1), and mu = 2.7 D), giving rise to a Omega = 7/2 ground state after including spin-orbit coupling. For MoB(+), the ground state is computed to be of X(7)Sigma(+) symmetry (R(e) = 2.224 angstrom, omega(e) = 141 cm(-1), and mu = 1.2 D), with an adiabatic ionization energy of 7.19 eV and a vertical one of 7.53 eV. (C) 2011 Wiley Periodicals, Inc. Int J Quantum Chem 111: 3362-3370, 2011

Picosecond Dynamics of Proton Transfer of a 7-Hydroxyflavylium Salt in Aqueous-Organic Solvent Mixtures

The intermediacy of the geminate base proton pair (A*center dot center dot center dot H(+)) in excited-state proton-transfer (ESPT) reactions (two-step mechanism) has been investigated employing the synthetic flavylium salt 7-hydroxy-4-methyl-flavylium chloride (HMF). In aqueous solution, the ESPT mechanism involves solely the excited acid AH* and base A* forms of HMF as indicated by the fluorescence spectra and double-exponential fluorescence decays (two species, two decay times). However, upon addition of either 1,4-dioxane or 1,2-propylene glycol, the decays become triple-exponential with a term consistent with the presence of the geminate base proton pair A*center dot center dot center dot H(+). The geminate pair becomes detectable because of the increase in the recombination rate constant, k(rec), of (A*center dot center dot center dot H(+)) with increasing the mole fraction of added organic cosolvent. Because the two-step ESPT mechanism splits the intrinsic prototropic reaction rates (deprotonation of AH(+)*, k(d), and recombination, k(rec) of A*center dot center dot center dot H(+)) from the diffusion controlled rates (dissociation, k(diss) and formation, k(diff)[H(+)], of A*center dot center dot center dot H+), the experimental detection of the geminate pair provides a wealth of information on the proton-transfer reaction (k(d) and k(rec)) as well as on proton diffusion/migration (k(diss) and k(diff)).

Electronic structure and chemical bonding in the ground states of Tc(2) and Re(2)

Multiconfiguration second-order perturbation theory, including relativistic effects and spin-orbit coupling, has been employed to investigate the nature of the chemical bonding in the ground state of Tc(2) and Re(2). The Tc(2) ground state is found to be a 0(g)(+) state, with an effective bond order (EBO) of 4.4, and a dissociation energy of 3.25 eV. The Re(2) ground state is a 1(g) state, with EBO = 4.3. Almost degenerate to it, is a 0(g)(+) state (T(e) = 77 cm(-1)), with EBO = 4.1. Experimental evidence also indicates that the ground state is of 1(g) nature. The dissociation energy is computed to be 5.0 eV in agreement with an experimental estimate of 4 +/- 1 eV.

Electronic Structure and Chemical Bonding in the Ground and Low-Lying Electronic States of Ta(2)

The electronic structure and chemical bonding of the ground and low-lying Lambda - S and Omega states of Ta(2) were investigated at the multiconfiguration second-order perturbation theory (CASSCF//CASPT2) level. The ground state of Ta(2) is computed to be a X(3)Sigma(-)(g) state (R(e) = 2.120 angstrom, omega(e) = 323 cm(-1), and D(e) = 4.65 eV), with two low-lying singlet states close to it (a(1) Sigma(+)(g) : T(e) = 409 cm(-1), R(e) = 2.131 angstrom, and omega(e) = 313 cm(-1); b(1) Gamma(g): T(e) = 1, 038 cm(-1), R(e) = 2.127 angstrom, and omega(e) = 316 cm(-1)). These electronic states are derived from the same electronic configuration: vertical bar 13 sigma(2)(g)14 sigma(2)(g)7 delta(2)(g)13 pi(4)(u)>. The effective bond order of the X(3) Sigma(-)(g) state is 4.52, which indicates that the Ta atoms are bound by a quintuple chemical bond. The a(1) Sigma(+)(g) state interacts strongly with the X(3)Sigma(-)(g) g ground state by a second-order spin-orbit interaction, giving rise to the (1)0(g)(+) (ground state) (dominated by the X(3)Sigma(-)(g) Lambda - S ground state) and (9)0(g)(+) (dominated by the a(1) Sigma(+)(g) Lambda - S state) Omega states. These results are in line with those reported for the group 5B homonuclear transition metal diatomics. (C) 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111: 1306-1315, 2011

On the Relaxation Mechanisms of 6-Azauracil

The nonadiabatic photochemistry of 6-azauracil has been studied by means of the CASPT2//CASSCF protocol and double-zeta plus polarization ANO basis sets. Minimum energy states, transition states, minimum energy paths, and surface intersections have been computed in order to obtain an accurate description of several potential energy hypersurfaces. It is concluded that, after absorption of ultraviolet radiation (248 nm), two main relaxation mechanisms may occur, via which the lowest (3)(pi pi*) state can be populated. The first one takes place via a conical intersection involving the bright (1)(pi pi*) and the lowest (1)(n pi*) states, ((1)pi pi*/(1)n pi*)(CI), from which a low energy singlet-triplet crossing, ((1)n pi*/(3)pi pi*)(STC), connecting the (1)(n pi*) state to the lowest (3)(pi pi*) triplet state is accessible. The second mechanism arises via a singlet-triplet crossing, ((1)pi pi*/(3)n pi*)(STC), leading to a conical intersection in the triplet manifold, ((3)n pi*/(3)pi pi*)(CI), evolving to the lowest (3)(pi pi*) state. Further radiationless decay to the ground state is possible through a (gs/(3)pi pi*)(STC).

Photodegradation of Folate Sensitized by Riboflavin

Folate is shown to react with singlet-excited state of riboflavin in a diffusion controlled reaction and with triplet-excited state of riboflavin in a somewhat slower reaction with (3)k(q) = 4.8 x 10(8) L mol(-1) s(-1) in aqueous phosphate buffer at pH 7.4, ionic strength of 0.2 mol L(-1), and 25 degrees C. Singlet quenching is assigned as photo-induced reductive electron transfer from ground state folate to singlet-excited riboflavin, while triplet quenching is assigned as one-electron transfer rather than hydrogen atom transfer from folate to triplet-excited riboflavin, as the reaction quantum yield, phi = 0.32, is hardly influenced by solvent change from water to deuterium oxide, phi = 0.37. Cyclic voltammetry showed an irreversible two-electron anodic process for folate, E = 1.14 V versus NHE at a scan-rate of 50 mV s(-1), which appears to be kinetically controlled by the heterogeneous electron transfer from the substrates to the electrode. Main products of folate photooxidation sensitized by riboflavin were pterin-6-carboxylic acid and p-aminobenzoyl-L-glutamic acid as shown by liquid chromatographic ion-trap mass spectrometry (LC-IT-MS).

Color-flavor locked strange matter and strangelets at finite temperature

It is possible that a system composed of up, down, and strange quarks exists as the true ground state of nuclear matter at high densities and low temperatures. This exotic plasma, called strange quark matter (SQM), seems to be even more favorable energetically if quarks are in a superconducting state, the so-called color-flavor locked state. Here we present calculations made on the basis of the MIT bag model, considering the influence of finite temperature on the allowed parameters characterizing the system for stability of bulk SQM (the so-called stability windows) and also for strangelets, small lumps of SQM, both in the color-flavor locking scenario. We compare these results with the unpaired SQM and also briefly discuss some astrophysical implications of them. Also, the issue of the strangelet's electric charge is discussed. The effects of dynamical screening, though important for nonpaired SQM strangelets, are not relevant when considering pairing among all three flavors and colors of quarks.