Journal of Theoretical and Computational Chemistry

The critical points of the 3-states two-layer Potts model on square lattice for different interlayer couplings (K_x = K_y ≠ K_z) are calculated with high precision using probabilistic cellular automata with Glauber algorithm, where K_x and K_y are the nearest-neighbor interactions within each layer in the x- and y-directions respectively, and K_z is the interlayer coupling. The obtained results are 0.726, 0.807, 0.928, 0.987 and 1.00 for the cases K_z/K_x = 1, 0.5, 0.1, 0.01 and 0.001 respectively, from which the critical curve is been constructed based on this model.

The magnetic properties simulation of extended networks containing quantum spins, by original FORTRAN code "MCIsing", is presented. The computer code is based on Ising model and uses Monte Carlo-Metropolis (MCM) algorithm. The results of magnetic Monte Carlo studies on a garnet type lattice, Ising model ferrimagnet, provide insights into the exchange interactions involved in the Ferrites of Formula 5Fe₂O₃.3Y₂O₃ with Garnet Structure.

A new binaphthalene molecule with two spiropyran units used for chiral molecular switches and logic gates was synthesized and characterized.¹² In this paper, charge and energy transfer in binaphthalene molecule with two spiropyran units are theoretically investigated with quantum chemistry method, as well as 2D and 3D real space analysis methods, since molecule construction with photoinduced electron transfer or charge transfer is one of the most frequently used pathways for building useful sensors and molecular machines. The orientation and strength of transition dipole moment in absorption spectra are obtained by 3D transition density. The orientation and results of intramolecular charge transfer on the excitation are obtained with 3D charge difference densities. The electron-hole coherence and excitation delocalization in absorption spectra are investigated with 2D contour plots of transition density matrix. Overall, the computed results remain in good agreement with the relevant experimental data, and the theoretical results reveal the relationship between the function of sensor and the excited state properties of the structure and transformation of the compound, upon addition of acid and base in absorption spectra.

In the carbon science literature, there have been various reports over the past few decades of potentially novel crystalline forms of carbon emerging as nanometer scale fragments recovered from the explosive remnants of heated, shock compressed graphite and other precursors of C. Two nanometric and crystalline forms of C that are particularly prominent in these studies are known as n-diamond and i-carbon forms. In our previous work, we have shown that the commonly observed diffraction pattern of n-diamond nanocrystals, recorded by several research groups around the world, is consistent with the calculated diffraction pattern of a novel form of carbon that we propose to call glitter. Glitter is a tetragonal allotrope of carbon with a calculated density of ~3.08g/cm³, and the density functional theory (DFT) optimized lattice parameters given as a = 0.2560 nm and c = 0.5925 nm. In addition to the diffraction evidence for n-diamond having the glitter structure, the DFT calculated band structure of glitter shows it to be metallic, like the observed electrical characteristics of n-diamond. In this communication, we report on a comparison of the diffraction pattern observed for nanocrystalline i-carbon by the investigative team of Yamada et al. in 1994, with the calculated diffraction pattern of glitter based upon the optimized lattice parameters. The close fit of the latter dataset to that observed for i-carbon, as reported herein, suggests that indeed i-carbon may be of the same structure as n-diamond, and that they both may have the tetragonal glitter structure.

Possible HF tetramer geometries have been optimized employing the density functional B3LYP method and the aug-cc-pVQZ basis set. Deformation energy has been calculated at the B3LYP/aug-cc-pVQZ level. After the BSSE correction with the CP method, two-body intermolecular interaction energy, three-body nonadditive intermolecular interaction energy, and four-body nonadditive intermolecular interaction energy (ΔE_c[4]) have been obtained at the levels of B3LYP/aug-cc–pVQZ, B3LYP/aug-cc-pVTZ//B3LYP/aug-cc-pVQZ, and MP2/aug-cc-pVTZ//B3LYP/aug-cc-pVQZ. Calculated results show that the three-body nonadditive intermolecular interaction energy is important for the optimized structures of HF tetramer. At the MP2/aug-cc-pVTZ//B3LYP/aug-cc-pVQZ level, the four-body nonadditive intermolecular interaction strength arrives at -4.5kJ/mol in the optimized eight-membered ring structure, but is extremely weak in other optimized structures. The comparison between MP2 and B3LYP calculated intermolecular interaction energies shows that the B3LYP method is applicable to the calculation of the intermolecular interaction energy of HF tetramer when the basis set reaches aug-cc-pVTZ. ΔE_c[4] occupies 8–32% of the total intermolecular interaction energy when the intermolecular distances of the eight-membered ring structure are in the range of 1.06–1.37 Å.

The regioselectivity of hetero Diels–Alder reactions (HDA) of 2-azabutadiene with aldehydes has been elucidated by means of Gazquez–Mendez rules, which are based on the calculation of local softnesses of the four terminal atoms involved in cyclization. The theoretical results obtained with the B3LYP/6-31G(d) method confirm the regioselectivities observed experimentally for all substituents (R=H, CH₃, CN) present in the aldehyde reactant. The regioselectivities of these HDA reactions have been confirmed by the calculation of the activation barriers corresponding to the two cyclization modes, and also by the application of the Houk rule and the maximum hardness principle.

We present a Gaussian wavepacket approach for curve-crossing dynamics that only requires a single Gaussian wavepacket per surface. Unlike other Gaussian wavepacket approaches to curve-crossing dynamics, the present method does not rely upon probability density being built up on a non-adiabatically coupled surface by the break-up of an evolving wavepacket. Thus, our trial solution for the time-dependent Schrodinger equation comprised of a single Gaussian wavepacket per non-adiabatically coupled surface. We also present a generalization of the method to treat multi-dimensional systems that is based on the time-dependent Hartree approximation. We present numerical results for both single and multi-dimensional curve-crossing dynamics and compare them to the exact results.

The performance of the previously proposed double zeta valence quality plus polarization functions (DZP) and augmented DZP (ADZP) basis sets is tested at the Hartree–Fock, second-order Møller–Plesset, and density functional levels of theory for the electronic ground state of the second-row diatomic hydrides. Total energy, second-order correlation energy, dissociation energy, bond length, vibrational frequency, and dipole moment are calculated and compared with results obtained with popular basis sets reported in the literature. It is shown that the DZP and ADZP basis sets in general provides better accuracy for a similar number of basis functions.

Two different hydrolysis mechanisms of metabolic product from tirapazamine's enzymatic catalysis have been studied by density functional theory (DFT) at the B3LYP/6-31G(d) and B3LYP/6-311+G(d) levels. The results indicate that the activation barrier of rate-controling step of Brown's model is smaller than that of Denny's model.

Detailed dynamics of the substantially endothermic reaction K + HF on a new ab initio ground potential energy surface has been studied by means of time-dependent quantum wave packet calculation. The calculations showed that the reaction could be significantly enhanced by vibrational excitation of HF, but not very sensitive to initial rotational excitation. The relative and absolute integral cross sections and the logarithm of the σ(v = 1)/σ(v = 0) ratio have been calculated and compared with available experimental and theoretical results. Relatively good agreement with experiment and other calculations was obtained. Reaction rate constants of this reaction was also calculated and discussed.

We consider the problem of performing imaginary-time propagation of wavefunctions on a grid. We demonstrate that spatially-continuous high-temperature approximations can be discretized in such a way that their convergence order is preserved. Requirements of minimal computational work and reutilization of data then uniquely determine the optimal grid, quadrature technique, and propagation method. It is shown that the optimal propagation technique is O(N), with respect to the grid size. The grid technique is utilized to compare the Monte Carlo efficiency of the Trotter–Suzuki approximation against a recently introduced fourth-order high-temperature approximation, while circumventing the issue of statistical noise, which usually prevents such comparisons from being carried out. We document the appearance of a systematic bias in the Monte Carlo estimators that involve temperature differentiation of the density matrix, bias that is due to the dependence of the eigenvalues on the inverse temperature. This bias is negotiated more successfully by the short-time approximations having higher convergence order, which leads to non-trivial computational savings.

PCN and PNC are possible interstellar species that have not been experimentally characterized. With various ab initio methods, including multireference and restricted open-shell single-reference electronic structure theory, the PCN/PNC species and the transition state for the isomerization reaction PCN ↔ PNC have been studied. The Dunning series of correlation-consistent basis sets, cc-pVXZ and aug-cc-pVXZ (X = T and Q), have been used. Geometries, total energies, dipole moments, harmonic vibrational frequencies, infrared intensities, and zero-point vibrational energies are reported for the PCN/PNC isomers and the transition state. Both PCN and PNC are linear with ³Σ^- ground states, and linear is predicted to lie 13.7 kcal mol^-1 (13.5 kcal mol^-1 with ZPVE correction) above linear at the aug-cc-pVQZ CCSD(T) level of theory. The CN bond distance in is predicted to be 1.174 Å, only 0.002 Å longer than the experimental value of 1.172 Å for diatomic CN (X²Σ⁺, suggesting that CN has triple bond character in . The isomerization transition state is found to be cyclic , with angles θ_e (PCN) = 82.2°, θ_e (CNP) = 63.1°, and θ_e (NPC) = 34.7°. The isomerization barrier is predicted to be 35.7 kcal mol^-1 (34.5 kcal mol^-1 with ZPVE correction) relative to linear . The predicted dipole moments are substantial, 2.79 debye (polarity ⁺PCN^-) and 2.51 debye (polarity ⁺PNC^-).