M. Malagoli

Density functional theory study of the geometric structure and energetics of triphenylamine-based hole-transporting molecules

Abstract We compare the geometric and electronic structures of triphenylamine and N,N,′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD), two widely used models for hole-transporting molecules. It is seen that triphenylamine and TPD display significantly different properties, the biphenyl segment playing a major role in the latter. We also evaluate, for the two compounds, the reorganization energy involved in the electron-transfer process from a neutral molecule to a hole-containing molecule (radical-cation).

Basis set consistent revision of the S22 test set of noncovalent interaction energies.

The S22 test set of interaction energies for small model complexes [Phys. Chem. Chem. Phys. 8, 1985 (2006)] has been very valuable for benchmarking new and existing methods for noncovalent interactions. However, the basis sets utilized to compute the CCSD(T) interaction energies for some of the dimers are insufficient to obtain converged results. Here we consistently extrapolate all CCSD(T)/complete basis set (CBS) interaction energies using larger basis sets for the CCSD(T) component of the computation. The revised values, which we designate S22A, represent the most accurate results to date for this set of dimers. The new values appear to be within a few hundredths of 1 kcal mol(-1) of the true CCSD(T)/CBS limit at the given geometries, but the former S22 values are off by as much as 0.6 kcal mol(-1) compared to the revised values. Because some of the most promising methods for noncovalent interactions are already achieving this level of agreement (or better) compared to the S22 data, more accurate benchmark values would clearly be helpful. The MP2, SCS-MP2, SCS-CCSD, SCS(MI)-MP2, and B2PLYP-D methods have been tested against the more accurate benchmark set. The B2PLYP-D method outperforms all other methods tested here, with a mean average deviation of only 0.12 kcal mol(-1). However, the consistent, slight underestimation of the interaction energies computed by the SCS-CCSD method (an overall mean absolute deviation and mean deviation of 0.24 and -0.23 kcal mol(-1), respectively) suggests that the SCS-CCSD method has the potential to become even more accurate with a reoptimization of its parameters for noncovalent interactions.

Computational approach to molecular magnetic properties by continuous transformation of the origin of the current density

Abstract The continuous set of gauge transformations method (CSGT) for calculating magnetic susceptibilities and nuclear magnetic shieldings reported by Keith and Bader is analyzed. To avoid numerical integration, analytical expressions are obtained for direct computation of the magnetic properties, which are written in closed form as a sum of the conventional paramagnetic terms and cotributions which, in the limit of exact eigenfunctions to a model Hamiltonian, reduce to the conventional diamagnetic terms. The latter can also be expressed as the expectation value of certain commutators over the reference electronic state. It is shown that the CSGT method is related to the Geertsen procedure. The magnetic shielding within the CSGT method is origin independent, but the CSGT susceptibility depends on the origin of the vector potential, whereas the Geertsen average susceptibility is independent of the origin of the vector potential, but depends on the origin of the reference frame.

On CHF calculations of second-order magnetic properties using the method of continuous transformation of origin of the current density

SummaryA fully analytical formulation is outlined for computing molecular magnetic susceptibilities and nuclear magnetic shieldings via a continuous change of origin of the electronic current density induced by an external magnetic field. The change of origin is described in terms of a (continuous) arbitrary shift functiond(r). Coupled Hartree-Fock second-order magnetic properties of CH4 and CO2 molecules have been computed, using the special choiced(r)=r as generating function. A detailed analysis of results obtained with a variety of basis sets reveals that such a method is not as good as previously suggested. Large basis sets must be used to obtain accurate magnetic properties. On the other hand, all the components of theoretical nuclear magnetic shielding evaluated via this approach are independent of the origin of the vector potential. In general, theoretical magnetic susceptibilities depend linearly on the distance between different coordinate frames, but are origin independent for centre-symmetric molecules.

A multimode analysis of the gas-phase photoelectron spectra in oligoacenes

We present a multimode vibrational analysis of the gas-phase ultraviolet photoelectron spectra of the first ionization in anthracene, tetracene, and pentacene, using electron-vibration constants computed at the density functional theory level. The first ionization of each molecule exhibits a high-frequency vibronic structure; it is shown that this regularly spaced feature is actually the consequence of the collective action of several vibrational modes rather than the result of the interaction with a single mode. We interpret this feature in terms of the missing mode effect. We also discuss the vibronic coupling constants and relaxation energies obtained from the fit of the photoelectron spectra with the linear vibronic model.

MOLECULAR MAGNETIC PROPERTIES WITHIN CONTINUOUS TRANSFORMATIONS OF ORIGIN OF THE CURRENT DENSITY

A new method for the calculation of molecular magnetic susceptibility and nuclear magnetic shielding is presented. It is based on continuous transformations of origin of the current density which make the transverse part of the paramagnetic current vanish. For any molecule all the components of the nuclear magnetic shielding tensor provided by the new method are independent on the origin of the gauge, whereas the components of the magnetizability tensor are translationally invariant only for center‐symmetric molecules (they, in general, show a linear dependence on the shift of origin). This method, termed CTOCD‐PZ, has been implemented for the theoretical determination of molecular magnetic properties via numerical integration techniques and, from preliminar results obtained for methane and carbon dioxide molecules, reveals reliable.

Structure and properties of C70

Abstract Electric and magnetic properties are calculated for C 70 at the optimised geometry in the STO-3G basis. The calculation gives a pictorial interpretation of the NMR spectrum of C 70 ; upper and lower region of the molecule have bond alternation and average chemical shift within 4 ppm of C 60 but the five central rings are benzenoid and the equatorial carbons lie 2 ppm downfield of benzene.

Quantum chemistry in parallel with PQS

This article describes the capabilities and performance of the latest release (version 4.0) of the Parallel Quantum Solutions (PQS) ab initio program package. The program was first released in 1998 and evolved from the TEXAS program package developed by Pulay and coworkers in the late 1970s. PQS was designed from the start to run on Linux‐based clusters (which at the time were just becoming popular) with all major functionality being (a) fully parallel; and (b) capable of carrying out calculations on large—by ab initio standards—molecules, our initial aim being at least 100 atoms and 1000 basis functions with only modest memory requirements. With modern hardware and recent algorithmic developments, full accuracy, high‐level calculations (DFT, MP2, CI, and Coupled‐Cluster) can be performed on systems with up to several thousand basis functions on small (4‐32 node) Linux clusters. We have also developed a graphical user interface with a model builder, job input preparation, parallel job submission, and post‐job visualization and display.

Magnetic properties of C60 and C70

Abstract The published 13 C NMR spectra of C 60 give an absolute carbon shielding of σ = 43 ppm, i.e. 14 ppm less than for carbon in benzene. Extrapolation from basis ab initio coupled Hartree—Fock calculations with the origin of the vector potential at the nucleus gives σ=46 ppm as the basis-set limit. Minimal ab initio calculations on C 70 predict greatest diamagnetic shielding at the equator and least near the poles of this spheroidal cluster, in good agreement with the experimental assignment of the 13 C NMR spectrum.

Coupled Hartree–Fock calculations of origin‐independent magnetic properties of benzene molecule

Conventional coupled Hartree–Fock approach and two computational methods based on continuous transformation of origin for the electronic current density induced by a magnetic field have been employed to calculate magnetic susceptibilities and nuclear magnetic shieldings of carbon and hydrogen in benzene. Near Hartree–Fock estimates have been obtained by using a basis set of 492 gaugeless contracted Gaussians containing ad hoc polarization functions. Theoretical magnetic susceptibility and nuclear shieldings evaluated in the present work via pointwise coordinate transformation are independent of the origin of the reference system. A series of sum rules for gauge independence of computed results and charge‐current conservation has been tested to document the high accuracy of the calculation of magnetic properties.