Extension of multi-residue methodology to include the determination of quinolones in food.

The multi-residue procedure for basic drugs described elsewhere by this laboratory has been evaluated for quinolone and fluoroquinolone antibiotics. The fluoroquinolones are a relatively new class of drug and an increasing number of licensed products containing these compounds are becoming available for use in animal husbandry. This, along with the possibility of the development of antibiotic resistant human pathogens, make it an important class of drug for which methodology is required for the monitoring of residues in food. Validation data are presented for a range of compounds including the quinolones; oxolinic acid and nalidixic acid, and the fluoroquinolones; flumequine, ciprofloxacin, danofloxacin, enoxacin, enrofloxacin, lomefloxacin, marbofloxacin, norfloxacin, ofloxacin and sarafloxacin. Foods for which the method was validated included poultry muscle (chicken and turkey), egg, chicken liver, honey, cattle muscle and pig muscle. This application of the multi-residue procedure further demonstrates the importance and wide-ranging usefulness of the technique.     

Relativistic two-component calculations of electronic g-tensors that include spin polarization

The first two-component relativistic density-functional approach for the calculation of electronic g-tensors is reported that includes spin polarization using noncollinear spin-density functionals. The method is based on the relativistic Douglas-Kroll-Hess Hamiltonian and has been implemented into the ReSpect program package. Using three self-consistent-field calculations with orthogonal orientations of total magnetization J, the full g-matrix may be obtained. In contrast to previous spin-restricted two-component treatments, results with the new approach agree excellently with spin-polarized one-component calculations for light-atom radicals. Additionally, unlike one-component approaches, the method also reproduces successfully the negative deltag(parallel)-values of heavy-atom 2sigma radicals and the negative deltag(perpendicular) components in cysteinyl. The new method removes effectively the dilemma existing up to now regarding the simultaneous inclusion of spin polarization and higher-order spin-orbit effects in g-tensor calculations. It is straightforwardly applicable to higher than doublet spin multiplicities and has been implemented with hybrid functionals.     

Correlation factor approach to the correlation energy functional

A global survey of the correlation factor energy functionals and its application to atomic and molecular properties is made. Its performances are compared with those of the density functional theory (DFT) correlation energy functionals, and some interesting conclusions from previous publications are reinforced here; namely, after removing the one-Slater-determinant hypothesis from the Kohn–Sham method, all DFT correlation functionals are able to provide reasonable results in any circumstance, with an additional restriction, for systems having a quasi-degenerate wave function, the DFT correlation functionals must depend explicitly on the on-top density. Acknowledgement.This work has been done under the support of the Spain DGICYT, project n⁰ BQU2001-0883.     

Cluster expansion of the wavefunction. Pseduo-orbital theory applied to spin correlation

We have discussed the utility of the cluster expansion of the wavefunction and proposed a pseudo-orbital theory which constitutes a generalization of the orbital-theoretic idea. It has been applied to the calculation of spin densities of the first-row atoms. The results are encouraging in comparison with the experimental values.     

Extension of Langmuir kinetics in dilute solutions to include lateral interactions according to regular solution theory and the Kiselev association model

The influence of lateral non-specific and specific interactions on the kinetics in dilute solutions is analyzed within the framework of the Langmuir model. Regular solution theory is used to derive kinetic equations for dilute solutions (RSK model). RSK equations are modified to include simple Kiselev associative interactions and deviations from the regular solution theory (mRSK model) and LF-type energetic heterogeneity (LF-mRSK). Derived models lead to significantly different kinetic behavior than the commonly used FG model or the SRT approach. The influence of the equilibrium uptake u(eq) and coverage θ(eq) on the observed effects of lateral interactions is discussed. A new kind of kinetic plot for data analysis is also presented. The mixed LF-mRSK model is applied to analysis of solute adsorption on mesoporous carbon.     

Electron spin resonance studies of spin labeled alternating functional copolymers

The spin-labeled and spin-probed alternating copolymers of poly(phenylvinyl alkyl ethers) with various alkyl groups (Me, Et, sec-Bu) in the ether moiety were investigated by electron spin resonance (ESR). Measurements were taken in different solvents over a wide range of concentration. Studies reveal that the molecular motion of spin-labels and spin-probes depends on the bulkiness of the alkyl group at the solvent concentration in which the effect of segmental rotation is dominant. The implication of the type of chemical bonding on the nitroxide motion is discussed.     

Performance of the OP correlation functional in relation to its formulation: Influence of the exchange component and the effect of incorporating same‐spin correlations

In the present study, we have investigated two significant features of the OP correlation functional, namely the incorporation of the exchange functional into itself, and the inclusion of only opposite‐spin (OS) effects. To explore the latter feature, we have compared OP with B95 and a new functional introduced in the present study – the OPB method that combines OP with the same‐spin (SS) component of B95. In general, we find that B95 and OPB perform comparably. Our comparisons of the various DFT procedures suggest that the incorporation of a meta‐GGA (e.g., TPSS) into OP and OPB does not necessarily lead to a chemically more accurate procedure than the use of a related GGA (e.g., PBE). An important finding is the more notable (and somewhat more consistent) improvement in performance with the incorporation of SS correlation, particularly for longer‐range chemical properties. Nonetheless, on average across our test sets of over 800 systems, the difference between the performances of OP versus B95 or OPB is not exceedingly large. By drawing a parallel between these DFT methods and the wavefunction scaled‐MP2‐type methods, we reason that one can further develop the OP functional, and perhaps a wider range of correlation functionals by combining it with the technique of range separation. © 2016 Wiley Periodicals, Inc.     

Universal density functional approach to the calculation of correlation energies of atoms

A new local density functional approach for the calculation of correlation energies of many-electron atomic systems is proposed by using the exact results for the correlation energy of a two-electron system bound by a harmonic oscillator external potential. This is motivated by the fact that the correlation energy is a universal functional of the electron density, and the form of this functional is independent of the external potential. The calculated numerical results for the correlation energies show very good agreement with the standard values reported in the literature. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 62: 461–465, 1997     

Free energy functional of the Ising-like model of spin crossover

Abstract  

The model of spin crossover based on the Ising-like Hamiltonian (IHM) has been analysed by deriving the functional of free energy from the mean-field solutions of this Hamiltonian. The contribution of the configurational entropy was found to be identical to that in the functional of the molecular statistical model (MSM) of spin crossover. However, the polynomial expansion over composition (x _B) and degree of order (s _B) in these functionals differ fundamentally due to different ways of accounting for the effects of molecular interactions. It was found that IHM takes into account next-to-nearest neighbour interactions by introducing affinities of sublattices towards molecules of given kinds. This yields a term proportional to the first power of the degree of order in the functional of IHM, whereas the MSM free energy is only proportional to s _B². The choice of formal independent variables does not affect the results of simulations of transition curves provided the functional remains unaltered. This provides for more flexibility in numerical simulations of transition curves.     

Additive density functional correlation corrections to single particle theories

The correlation energy functional E_c of the Hartree-Fock density is investigated. It was previously established that E_c produces the exact ground-state energy when added to the Hartree-Fock energy. Except when certain degeneracies occur, it is here shown that E_c is bounded from below when the coordinates of the Hartree-Fock density are scaled uniformly by λ, as λ → ∞. Consequently, approximations to E_c should display this bounded property, which is a second-order perturbation energy. It is also shown that a corresponding result applies to that correlation energy functional, Ë_c, which is to be added to a completed exact exchange-only (in the OPM sense) density functional calculation. Scaling requirements are presented for each order in the perturbation expansion for Ë_c. For instance, the second-order term is dimensionless. © 1997 John Wiley & Sons, Inc.     

Kinetic energy as functional of the correlation hole

Using the marginal decomposition of the many-body probability distribution the electronic kinetic energy is expressed as the functional of the electron density and correlation hole. The analysis covers both the molecule as a whole and its constituent subsystems. The importance of the Fisher information for locality is emphasized.     

On-top pair-density interpretation of spin density functional theory,with applications to magnetism

The on-top pair density P(r, r) gives the probability that one electron will be found on top of another at position r. We find that the local spin density (LSD) and generalized gradient (GGA) approximations for exchange and correlation predict this quantity with remarkable accuracy. We show how this fact and the usual sum-rule arguments explain the success of these approximations for real atoms, molecules, and solids, where the electron spin densities do not vary slowly over space. Self-consistent LSD or GGA calculations make realistic predictions for the total energy E, the total density n(r), and the on-top pair density P(r,r), even in those strongly “abnormal” systems (such as stretched H₂) where these approximations break symmetries and yield unrealistic spin magnetization densities m(r). We then suggest that ground-state ferromagnetic iron is a “normal” system, for which for LSD or GGA m(r) and the related local spin moment are trustworthy, but that iron above the Curie temperature and antiferromagnetic clusters at all temperatures are abnormal system for which the on-top pair density interpretation is more viable than the standard physical interpretation. As an example of a weakly abnormal system, we consider the four-electron ion with nuclear charge Z → ∞ © 1997 John Wiley & Sons, Inc.     

Applications of effective core potentials and density functional theory to the spin states of iron porphyrin

We investigated the performance of the B3LYP density functional in combination with ab initio effective core potentials (ECPs) that are derived from either Hartree-Fock or Dirac-Fock calculations. The transferability of ab initio ECPs is assessed on the basis of comparison with all-electron density functional calculations. For iron(II) porphyrin in particular, our assessment focused on the relative energetic ordering of five low-lying spin states, 1A1G, 3A1G, 3B2G, 5A2G, and 5B1G, and their properties, including optimized structures, charge distribution, spin density, and vibrational frequencies. Our results show that core electron correlation and core-valence electron correlation do not have significant effects on the relative energetics of the spin states of iron porphyrin. Our calculations suggest that effects of replacing the core electrons with ECPs are less significant than the choice of basis functions. We conclude that ab initio ECPs such as LANL2, RCEP, and MEFIT-R may be combined with the B3LYP density functional theory to provide consistent and accurate results.     

Attempts to include uncorrected bias in the measurement uncertainty

In ISO Guide it is strictly recommended to correct results for the recognised significant bias, but in special cases some analysts find out practical to omit the correction and to enlarge the expanded uncertainty for the uncorrected bias instead. In this paper, four alternatively used methods computing these modified expanded uncertainties are compared according to the levels of confidence, widths and layouts of the obtained uncertainty intervals. The method, which seems to be the best, because it provides the same uncertainty intervals as in the case of the bias correction, has not been applied very much, maybe since these modified uncertainty intervals are not symmetric about the results. The three remaining investigated methods maintain their intervals symmetric, but only two of them provide intervals of the kind, that their levels of confidence reach at least the required value (95%) or a larger one. The third method defines intervals with low levels of confidence (even for small biases). It is proposed a new method, which gives symmetric intervals just with the required level of confidence. These intervals are narrower than those symmetric intervals with the sufficient level of confidence obtained by the two mentioned methods. A mathematical background of the problem and an illustrative example of calculations applying all compared methods are attached.     

Performance of parallel TURBOMOLE for density functional calculations

The parallelization of density functional treatments of molecular electronic energy and first-order gradients is described, and the performance is documented. The quadrature required for exchange correlation terms and the treatment of exact Coulomb interaction scales virtually linearly up to 100 nodes. The RI-J technique to approximate Coulomb interactions (by means of an auxiliary basis set approximation for the electron density) even shows superlinear speedup on distributed memory architectures. The bottleneck is then linear algebra. Demonstrative application examples include molecules with up to 300 atoms and 3000 basis functions that can now be treated in a few hours per geometry optimization cycle in C₁ symmetry. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1746–1757, 1998     

Extension of linear-scaling divide-and-conquer-based correlation method to coupled cluster theory with singles and doubles excitations

This paper describes the extension of the linear-scaling divide-and-conquer (DC)-based correlation method to the coupled cluster with singles and doubles excitations (CCSD) theory. In this DC-CCSD method, the CCSD equations are solved for all subsystems including their buffer regions with the use of the subsystem orbitals, which are obtained by the DC-Hartree-Fock method. Then, the correlation energy of the total system is evaluated by summing up the subsystem contributions other than the buffer regions by the energy density analysis technique. Numerical applications demonstrate that the present DC-CCSD gives highly accurate results with drastically less computational costs with regard to the required computer memory, scratch-disk capacity, and calculation time.     

Local density functional theory applied to spin coupling in Fe6S supercluster

Using density functional theory (DFT), the multiplicity of the ground state was determined for Fe₆S₆Cl ions as well as the order of the excited spin states. A method to determine the exchange integrals J of the Fe₆S cluster is presented based on these results and a spin‐coupling algebra. The following order of the spin states was established with respect to the total spin 5/2, 1/2, 7/2, 3/2, 9/2, 11/2, 15/2, 13/2, and 17/2. We also calculated the Heisenberg coupling parameters J₁, J₂, J₃, and J₄ as 22, 146, −130, and 81 cm⁻¹, respectively. The possibility of the ground state of high multiplicity as well as the necessary conditions for such state are discussed. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 39–51, 1999