A. Sánchez de Merás

Current density maps, magnetizability, and nuclear magnetic shielding tensors of bis-heteropentalenes. I. Di-hydro-pyrrolo–pyrrole isomers

Magnetic susceptibility and nuclear magnetic shielding at the nuclei of bis-heteropentalenes formed by two pyrrole units ([2,3−b], [3,2−b], [3,4−b], and [3,4−c] isomers) have been evaluated by a series of different approximations and a large Gaussian basis set. An ab initio model of magnetic field induced current density was obtained for four isomeric systems, showing that strong diamagnetic flow takes place within the π electrons. The π currents are responsible for exalted magnetic anisotropy and proton deshielding. The theoretical findings are used to assess a diatropicity scale for these molecules.

Assessment for the mean value total dressing method: Comparison with coupled cluster including triples methods for BF, NO+,CN+,C2, BeO, NH3,CH2,H2O, BH, HF, SiH2,Li2, LiNa, LiBe+,NeH+, and O3

Limited previous experience with the mean value total dressing (MVTD) method had shown that MVTD energies for closed shell systems are generally better than CCSD(T) ones compared to FCI. The method, previously published as total dressing 2′(td-2′), is based on the single reference intermediate Hamiltonian theory. It is not a CC method but deals in a great part with the same physical effects that CC methods that incorporate amplitudes of triples such as CCSDT or its CCSDT-1n approaches. A number of test calculations comparing to diverse CC methods, as well as FCI and experiment when available, have been performed. The tests concern equilibrium energies in NH3 and CH2, equilibrium energies and distances in some diatomics (BF, NO+, CN+, C2, BeO), different bond breaking situations (H2O, BH, HF, SiH2) and spectroscopic properties of different bonding conditions (Li2, LiNa, LiBe+, NeH+, and O3). The results are in general closer to the full CCSDT ones in the equilibrium regions and close to CCSDT-1 along most ...

Multi-scale theoretical investigation of molecular hydrogen adsorption over graphene: coronene as a case study

The physisorption of molecular hydrogen onto coronene is studied using a multi-scale theoretical approach with Density Functional Theory (DFT) calculations and Molecular Dynamics (MD) simulations. We consider two different kinds of model conformation for the approach of hydrogen towards the coronene i.e., systematic and random. For the systematic attack of hydrogen over coronene, the resulting potential energy profiles from DFT analysis are further found to resemble the Morse potential, and even the highly flexible Murrell–Sorbie (M–S) potential. The resulting M–S fitting also shows a zero-point energy correction of ∼16–17%. On the other hand, the potential energies from the random approach have been implemented into the Improved Lennard-Jones (ILJ) force field of the DL_POLY package following a prior statistical treatment. The MD simulations have been performed at different temperatures from 10 to 390 K. For the interaction of seven hydrogen molecules with coronene, the DFT method shows an average interaction energy of −3.85 kJ mol−1 per H2, which is slightly smaller than the Coupled Cluster value (CCSD(T)) of −4.71 kJ mol−1 that was calculated for a single molecule in the most favorable situation. Moreover, the MD calculations reveal a mean interaction energy of −3.69 kJ mol−1 per H2 (a gross mean Ecfg of −25.98 kJ mol−1 at T = 299.97 K), which is again in good agreement with the aforementioned DFT results, proving the quality of the approach used for the study of van der Waals interactions between hydrogen and graphene.

Diatropicity of tetraazanaphthalenes

Tetraazanaphthalenes are diatropic molecules, whose magnetic response to a magnetic field perpendicular to the molecular plane closely resembles that of naphthalene. The out‐of‐plane component of the magnetic susceptibility tensor and its strong anisotropy can be used as quantifiers of magnetic aromaticity. Maps showing streamlines and modulus of the current density field provide clear evidence for diatropicity of these systems. They also explain the strong anisotropy of carbon and nitrogen magnetic shielding, which is determined by the big out‐of‐plane component of the nuclear shielding tensor. The electronic ring currents observed in the map deshield the nuclei of ring hydrogens by enforcing the local magnetic field and diminishing the out‐of‐plane component of proton shielding.

Theoretical spectroscopic parameters of the alkali monofluorides LiF, NaF and KF

Abstract Multireference configuration interaction and second-order perturbation theory are used to determine accurate spectroscopic parameters for the ground state of the alkali monofluorides from LiF to KF. Systematic saturation of the spdf subspaces of the atomic basis sets for the electron affinity of fluorine and the ionization potential of the metals have been shown to be more efficient than for the approximate dissociation energies in the molecule. the calculated rotational and vibrational constants and transitions for the three systems show excellent agreement with the expeirmental data.

A MRCI PS and CASSCF study of the ground state MgO dissociation energy

Abstract Ab initio calculations at CASSCF and MRCI PS levels are used to determine the dissociation energy for the X 1 Σ + state of MgO, which adiabatically dissociates to the ground state 1 S g of magnesium and to the excited 1 D g state of oxygen, as well as other spectroscopic parameters. Emphasis is placed upon the problem of properly selecting an adequate active space in CASSCF calculations and upon the improvements obtained in MRCI by selecting perturbatively the most important contributions to the total wavefunction and evaluating the remaining ones only by perturbational method. Through a procedure based on stabilizing the computed dissociation energy, values of 3.87 eV (MRCI PS) and 4.20 eV (CASSCF) are obtained. These values compare with the experimental value of 3.76±0.13 eV.

The electronic spectrum of SiH4: Jahn-Teller Rydberg series.

The aim of the present theoretical work is to provide data necessary for a better understanding of the electronic spectrum of the silane molecule, which is affected by the Jahn-Teller effect. By selecting an adequate distorted C(2v) geometry of SiH(4), the three lower Koopmans ionization potentials are evaluated with the equation of motion coupled cluster of singles and doubles method. Vertical excitation energies for the different Rydberg series converging to the three Jahn-Teller components are inferred from ab initio coupled cluster linear response calculations. Absorption oscillator strengths for dipole-allowed electronic transitions are also determined with the molecular-adapted quantum defect orbital methodology. Predictions of new spectroscopic data on SiH(4) are reported.

Assessment of the CTOCD-DZ method in a hierarchy of coupled cluster methods

Gauge origin independent calculations of nuclear magnetic shielding tensors are carried out inside the formalism of the continuous transformation of the origin of the current density leading to formal annihilation of its diamagnetic contribution (CTOCD-DZ). We employ the unrelaxed linear response approach with a hierarchy of different coupled cluster methods in order to assess the importance of the level of approximation in the coupled cluster expansion. The basis set dependence of the computed nuclear magnetic shielding constants is also analyzed in the series of correlation consistent basis sets, with the aim of designing optimized basis sets of relatively small size.

MP2 Study of Physisorption of Molecular Hydrogen onto Defective Nanotubes: Cooperative Effect in Stone-Wales Defects.

We use large-scale MP2 calculations to investigate the physisorption of molecular hydrogen on (9,0) defective carbon nanotubes (CNTs) of C72H18. These large (supra)molecular systems are typically studied using conventional DFT methods, which do not describe well the van der Waals interactions responsible for this process. Here we use CCSD(T)-calibrated MP2 calculations to estimate binding energies by considering four defective structures (hydrogenated divacancy, octagon-pentagon, and two Stone-Wales defects). The largest physisorption energies for the nondefective CNT are for configurations in which H2 points toward the center of one ring. The computed interaction energies for defect-free CNT are in the range 5.7 to 5.9 kJ/mol, in good agreement with the experimental value of 5.98 kJ/mol. The defects introduced in the (9,0)-CNT increase the surface area of the nanotube, such that the largest surface in found in the 55-77 Stone-Wales defective CNT that furthermore is the most aromatic. Only that defect enlarges the physisorption binding energy, which can become >25% larger. Moreover, a cooperative effect in the adsorption of H2 not appearing in the regular structure is found.

Reply to comment on “A MRCI PS and CASSCF study of the ground state MgO dissociation energy”

To compute the dissociation energy of MgO, the relationship among the size of the active space in CASSCF wavefunctions, the computed De and the continuity of ∂E/∂r is studied. Basis set influence is also considered. Finally, it is concluded that the dissociation energy of MgO referred to ground state atoms is 2.32±0.1 eV.