Propagator matrices as matrices of power''s series. II. It''s relationship with HF''s stability problem and alternative solutions

Some non-singlet quasi-instabilities (QIs) cases that arise in the calculation of NMR-J parameters are analyzed within response theory. The relationship between ‘very close to zero’ eigenvalues of the principal propagator and the rate of convergency for specific coupling pathways is shown by a power series implemented to calculate the principal propagator matrix. A natural criterion for the analysis of the stability problem emerges from that series. This is more general and accurate compared with previous proposals. Its relationship with π-type molecular orbitals is given. We present an alternative scheme to minimize the effects of non-singlet QIs in such a way that the NMR-J parameters become close to the best theoretical calculations for H₂CX (X=CH₂, NH and O).     

Optical absorption of a green fluorescent protein variant: environment effects in a density functional study

We present an ab initio study of the optical absorption properties of a particularly interesting fluorescent protein (E2GFP), whose complex photophysics still escapes elucidation. In particular, we focus on the role of the protein environment, showing that the effects of both nearby residues and the external field due to residues not accounted for explicitly are needed to properly reproduce the experimental data. The spectra calculated taking such contributions into account provide for the first time a robust identification of the states relevant for the photophysics of this system.     

Noniterative Doubles Corrections to the Random Phase and Higher Random Phase Approximations: Singlet and Triplet Excitation Energies

The second-order noniterative doubles-corrected random phase approximation (RPA) method has been extended to triplet excitation energies and the doubles-corrected higher RPA method as well as a shifted version for calculating singlet and triplet excitation energies are presented here for the first time. A benchmark set consisting of 20 molecules with a total of 117 singlet and 71 triplet excited states has been used to test the performance of the new methods by comparison with previous results obtained with the second-order polarization propagator approximation (SOPPA) and the third order approximate coupled cluster singles, doubles and triples model CC3. In general, the second-order doubles corrections to RPA and HRPA significantly reduce both the mean deviation as well as the standard deviation of the errors compared to the CC3 results. The accuracy of the new methods approaches the accuracy of the SOPPA method while using only 10–60% of the calculation time. © 2019 The Authors. Journal of Computational Chemistry published by Wiley Periodicals, Inc.     

Theoretical study of HCN and HNC neutral and charged clusters

A theoretical study of linear and cyclic clusters of (HCN)n and (HNC)n (up to n = 10) has been carried out by means of DFT and MP2 ab initio methods. The transition states linking the cyclic clusters show high energetic barriers that prevent the spontaneous transformation of the high-energy clusters, (HNC)n, into the low-energy ones, (HCN)n. The effect of the protonation/deprotonation of the linear clusters has also been explored. The results show that (HNC)n clusters with n values larger than six are thermodynamically more stable as charged systems than as neutral ones. The geometrical results have been analyzed using a Steiner-Limbach plot. The electron density and its Laplacian at the bond critical points correlate with the corresponding bond distances by means of two exponential functions, one for the open shell and another for the closed shell cases.     

A computational study of 2J(HH)(gem) indirect spin-spin coupling constants in simple hydrides of the second and third periods

Several theoretical methods have been used to compute (2)J(HH) in neutral, anionic and cationic HXH hydrides, X being the 14 nuclei from Li to Cl (28 molecules). Since the calculations also provide (1)J(XH) spin-spin coupling constants (SSCC), these have also been analyzed. The best results were obtained using Second-order polarization propagator approximation (SOPPA)/sadJ. The geminal coupling constants appear to be dependent on the electronegativity of the X-atom.     

Electric field effects on nuclear magnetic shielding of the 1:1 and 2:1 (homo and heterochiral) complexes of XOOX' (X, X' = H, CH3) with lithium cation and their chiral discrimination

The set of 1:1 and 2:1 complexes of XOOX' (X, X' = H, CH(3)) with lithium cation has been studied to determine if they are suitable candidates for chiral discrimination in an isotropic medium via nuclear magnetic resonance spectroscopy. Conventional nuclear magnetic resonance is unable to distinguish between enantiomers in the absence of a chiral solvent. The criterion for experimental detection is valuated by the isotropic part of nuclear shielding polarisability tensors, related to a pseudoscalar of opposite sign for two enantiomers. The study includes calculations at coupled Hartree-Fock and density functional theory schemes for (17)O nucleus in each compound. Additional calculations for (1)H are also included for some compounds. A huge static homogeneous electric field, perpendicular to the magnetic field of the spectromer, as big as ≈1.7 × 10(8) V m(-1) should be applied to observe a shift of ≈1 ppm for (17)O magnetic shielding in the proposed set of complexes.     

Investigation of the resonance‐assisted hydrogen bond in model β‐diketones through localized molecular orbital analysis of the spin–spin coupling constants related to the O–H · · · O hydrogen bond

The resonance‐assisted hydrogen bond (HB) phenomenon has been studied theoretically by a localized molecular orbital (LMO) decomposition of the spin–spin coupling constants between atoms either involved or close to the O–H · · · O system of some β‐diketones and their saturated counterparts. The analysis, carried out at the level of the second‐order polarization propagator approximation, shows that the contributions in terms of LMO to the paramagnetic spin orbital and the spin dipolar Ramsey terms proof the importance of the delocalized π‐electron structure supporting the idea of the existence of the resonance‐assisted HB phenomenon phenomenon. The LMO contributions to the Fermi contact term indicate mainly the presence of the HB that may or not be linked to the π‐electrons. Copyright © 2014 John Wiley & Sons, Ltd.     

Study on particle removal efficiency of an impinging jet by an image-processing method

 An image-processing method is proposed to obtain the distribution of the removal efficiency of particles on a plate by an air jet. This method can be used to measure particle removal from a flat surface by processing the image of the reflected light from the surface. Factors affecting the particle removal efficiency such as air pressure, distance between the nozzle and the impinging surface and the impinging angle are discussed. Optimal conditions are determined to obtain the most effective particle removal by the air jet. Received: 10 April 2001 / Accepted: 2 August 2001