Dynamic optimization of a dead-end filtration trajectory: Non-ideal cake filtration

A control strategy aimed at minimizing energy consumption is formulated for non-ideal dead-end cake filtration with an inside-out hollow fiber ultrafiltration membrane system. The non-ideal behavior was assumed to originate from cake compression, non-linear cake resistance and a variable pump efficiency.

Constant gross power, constant flux and constant pressure filtration were considered as alternatives for the optimal operating strategy. It was found that the ratio between the initial and final total resistance determines whether a large difference between these strategies occurs. This is mainly determined by the specific cake resistance, the final state and the membrane resistance.

When there is a large difference between the operating strategies, the pump characteristics determine which suboptimal strategies are attractive. For a pump with a low head and a large capacity, constant flux filtration is nearly optimal, whereas for a pump with a large head and a small capacity optimal operation is closer to constant pressure filtration. Under the investigated conditions there was no significant difference (< 0.5%) between the constant gross power and the optimal operating strategy.     

High-resolution investigation of the 112Sn(p,d)111Sn reaction

The ¹¹²Sn(p, d)¹¹¹Sn reaction was studied at a proton energy of 27.45 MeV. The outgoing deuterons were momentum analyzed with an Enge split-pole spectrograph and recorded with position-sensitive solid-state detectors with a total resolution between 12 and 16 keV. Angular distributions were compared with DWBA calculations in order to extract l-values, spectroscopic factors, single-quasiparticle energies and occupation probabilities. In the gross structure between 3 and 6 MeV, which was interpreted as being due to the pickup of deeply bound neutrons, several discrete peaks were also found with an angular distribution characteristic for l = 1 or 4 transfer. The excitation of core-coupled states was investigated by performing two-step DWBA calculations. The results of the present experiment are compared with previous experimental results and with number-projected BCS calculations.     

Flexible, ab initio potential, and dipole moment surfaces for water. I. Tests and applications for clusters up to the 22-mer

We report full-dimensional, ab initio potential energy and dipole moment surfaces, denoted PES and DMS, respectively, for arbitrary numbers of water monomers. The PES is a sum of 1-, 2-, and 3-body potentials which can also be augmented by semiempirical long-range higher-body interactions. The 1-body potential is a spectroscopically accurate monomer potential, and the 2- and 3-body potentials are permutationally invariant fits to tens of thousands of CCSD(T)/aug-cc-pVTZ and MP2/aug-cc-pVTZ electronic energies, respectively. The DMS is a sum of 1- and 2-body DMS, which are covariant fits to tens of thousands MP2/aug-cc-pVTZ dipole moment data. We present the details of these new 2- and 3-body potentials and then extensive applications and tests of this PES are made to the structures, classical binding energies, and harmonic frequencies of water clusters up to the 22-mer. In addition, we report the dipole moment for these clusters at various minima and compare the results against available and new ab initio calculations.     

Gradient-induced longitudinal relaxation of hyperpolarized noble gases in the fringe fields of superconducting magnets used for magnetic resonance

When hyperpolarized noble gases are brought into the bore of a superconducting magnet for magnetic resonance imaging (MRI) or spectroscopy studies, the gases must pass through substantial field gradients, which can cause rapid longitudinal relaxation. In this communication, we present a means of calculating this spatially dependent relaxation rate in the fringe field of typical magnets. We then compare these predictions to experimental measurements of (3)He relaxation at various positions near a medium-bore 2-T small animal MRI system. The calculated and measured relaxation rates on the central axis of the magnet agree well and show a maximum (3)He relaxation rate of 3.83×10(-3) s(-1) (T(1)=4.4 min) at a distance of 47 cm from the magnet isocenter. We also show that if this magnet were self-shielded, its minimum T(1) would drop to 1.2 min. In contrast, a typical self-shielded 1.5-T clinical MRI scanner will induce a minimum on-axis T(1) of 12 min. Additionally, we show that the cylindrically symmetric fields of these magnets enable gradient-induced relaxation to be calculated using only knowledge of the on-axis longitudinal field, which can either be measured directly or calculated from a simple field model. Thus, while most MRI magnets employ complex and proprietary current configurations, we show that their fringe fields and the resulting gradient-induced relaxation are well approximated by simple solenoid models. Finally, our modeling also demonstrates that relaxation rates can increase by nearly an order of magnitude at radial distances equivalent to the solenoid radius.     

Probing the structure of CH5+ by dissociative charge exchange

Dissociative charge exchange of CH5+ with Cs, coupled with quasiclassical trajectory calculations on an ab initio PES for CH5, has been used to probe the structure of the CH5+ cation. Product kinetic energy release distributions and branching ratios for CH5 --> CH4 + H and CH5 --> CH3 + H2 have been compared. The agreement of the product branching ratios provides evidence for the fluxional nature of CH5+.     

Ex vivo quantification of mTHPC concentration in tissue: influence of chemical extraction on the optical properties

A method for the quantification of the concentration of the photosensitizer meso-tetra(hydroxyphenyl) chlorin (mTHPC) in tissue samples is presented. The technique is an extension of a previously published method based on alkaline hydrolysis of tissue, using Solvable as a tissue solubilizer. mTHPC quantification was achieved by subsequent fluorescence spectroscopy. Since the original extraction method involved multiple steps in which water dilution of the sample was implemented, we studied the spectral characteristics of mTHPC in different Solvable/water mixtures. Using UV-VIS absorption and fluorescence spectroscopy, it was demonstrated that the spectral characteristics of mTHPC vary for different Solvable concentrations. In the range of 20-100% Solvable, the fluorescence intensity of mTHPC did not change, while dramatic changes in the mTHPC fluorescence intensity were observed for lower Solvable concentrations (< 20%) due to increasing hydrophilicity of the environment, combined with pH alterations. We also demonstrated that the absorption and fluorescence spectra of the dissolved tissue were time-dependent. Longer incubation of the samples resulted in a significant increase of the native tissue chromophore fluorescence. This implies that for the correct quantification of photosensitizer concentrations, the fluorescence of native tissue chromophores must be accounted for.     

Full-dimensional quantum calculations of ground-state tunneling splitting of malonaldehyde using an accurate ab initio potential energy surface

Quantum calculations of the ground vibrational state tunneling splitting of H-atom and D-atom transfer in malonaldehyde are performed on a full-dimensional ab initio potential energy surface (PES). The PES is a fit to 11 147 near basis-set-limit frozen-core CCSD(T) electronic energies. This surface properly describes the invariance of the potential with respect to all permutations of identical atoms. The saddle-point barrier for the H-atom transfer on the PES is 4.1 kcalmol, in excellent agreement with the reported ab initio value. Model one-dimensional and "exact" full-dimensional calculations of the splitting for H- and D-atom transfer are done using this PES. The tunneling splittings in full dimensionality are calculated using the unbiased "fixed-node" diffusion Monte Carlo (DMC) method in Cartesian and saddle-point normal coordinates. The ground-state tunneling splitting is found to be 21.6 cm(-1) in Cartesian coordinates and 22.6 cm(-1) in normal coordinates, with an uncertainty of 2-3 cm(-1). This splitting is also calculated based on a model which makes use of the exact single-well zero-point energy (ZPE) obtained with the MULTIMODE code and DMC ZPE and this calculation gives a tunneling splitting of 21-22 cm(-1). The corresponding computed splittings for the D-atom transfer are 3.0, 3.1, and 2-3 cm(-1). These calculated tunneling splittings agree with each other to within less than the standard uncertainties obtained with the DMC method used, which are between 2 and 3 cm(-1), and agree well with the experimental values of 21.6 and 2.9 cm(-1) for the H and D transfer, respectively.     

Variational calculation of second-order reduced density matrices by strong N-representability conditions and an accurate semidefinite programming solver

The reduced density matrix (RDM) method, which is a variational calculation based on the second-order reduced density matrix, is applied to the ground state energies and the dipole moments for 57 different states of atoms, molecules, and to the ground state energies and the elements of 2-RDM for the Hubbard model. We explore the well-known N-representability conditions (P, Q, and G) together with the more recent and much stronger T1 and T2(') conditions. T2(') condition was recently rederived and it implies T2 condition. Using these N-representability conditions, we can usually calculate correlation energies in percentage ranging from 100% to 101%, whose accuracy is similar to CCSD(T) and even better for high spin states or anion systems where CCSD(T) fails. Highly accurate calculations are carried out by handling equality constraints and/or developing multiple precision arithmetic in the semidefinite programming (SDP) solver. Results show that handling equality constraints correctly improves the accuracy from 0.1 to 0.6 mhartree. Additionally, improvements by replacing T2 condition with T2(') condition are typically of 0.1-0.5 mhartree. The newly developed multiple precision arithmetic version of SDP solver calculates extraordinary accurate energies for the one dimensional Hubbard model and Be atom. It gives at least 16 significant digits for energies, where double precision calculations gives only two to eight digits. It also provides physically meaningful results for the Hubbard model in the high correlation limit.     

New ab initio potential energy surface and the vibration-rotation-tunneling levels of (H2O)2 and (D2O)2

We report a new full-dimensional potential energy surface (PES) for the water dimer, based on fitting energies at roughly 30,000 configurations obtained with the coupled-cluster single and double, and perturbative treatment of triple excitations method using an augmented, correlation consistent, polarized triple zeta basis set. A global dipole moment surface based on Moller-Plesset perturbation theory results at these configurations is also reported. The PES is used in rigorous quantum calculations of intermolecular vibrational frequencies, tunneling splittings, and rotational constants for (H2O)2 and (D2O)2, using the rigid monomer approximation. Agreement with experiment is excellent and is at the highest level reported to date. The validity of this approximation is examined by comparing tunneling barriers within that model with those from fully relaxed calculations.     

Quasiclassical trajectory calculations of acetaldehyde dissociation on a global potential energy surface indicate significant non-transition state dynamics

A recent experimental study [Houston, P. L.; Kable, S. H. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 16079] of the photodissociation of acetaldehyde (CH(3)CHO) has suggested two distinct mechanisms for the production of the molecular products CH(4) + CO. One corresponds to the traditional transition state mechanism and the other to a transition state-skirting path similar to the roaming channel previously reported in formaldehyde. To investigate this theoretically, a full-dimensional potential energy surface (PES) has been constructed. The PES was fit with permutationally invariant polynomials to 135,000 points calculated using coupled cluster theory with single and double excitations and a perturbative treatment of triple excitations [CCSD(T)] and correlation consistent basis sets of double- and triple-zeta quality. To test the accuracy of the PES additional CCSD(T) and multireference configuration interaction calculations were carried out. Quasiclassical trajectory calculations were run on the PES starting at the acetaldehyde equilibrium geometry and also at the conventional transition state (TS) for the molecular products CH(4) + CO. The former calculations agree well with the experimental results of Houston and Kable; however, those from the TS do not. The implications for a non-transition state, roaming mechanism in this molecule are discussed.