Mark A. Vincent

Unified theoretical treatment of analytic first and second energy derivatives in open-shell Hartree—Fock theory

Abstract Second derivatives of polyatomic potential energy hypersurfaces are of widespread importance to problems in theoretical chemistry. A formalism is presented which allows the analytic evaluation of energy second derivatives from restricted Hartree—Fock wavefunctions for arbitrary closed- or open-shell molecular systems. The present method makes use of previously reported integral second-derivative techniques and earlier advances in the solution of the open-shell coupled perturbed Hartree—Fock equations. The applicability of the new method is demonstrated in studies of the first excited singlet state of formaldehyde, for which the equilibrium geometry and vibrational frequencies have been determined with two different basis sets.

Analytic second derivative techniques for self-consistent-field wave functions. A new approach to the solution of the coupled perturbed hartree-fock equations

Abstract A new method for the solution of the coupled perturbed Hartree-Fock equations is outlined which avoids the four-index transformation of electron repulsion integrals. This allows the extension of analytic energy second derivative methods to large molecular systems. The new approach has been implemented for both closed- and open-shell molecules and a number of test cases considered, including as many as 96 contracted gaussian basis functions. The time required for the determination of all cartesian force constants varies from 8% to 65% of that required for analytic gradient techniques.

Computer simulation of zeolite structure and reactivity using embedded cluster methods

The use of bare cluster models to understand the nature of zeolite–substrate interactions may be improved to take account of the environment of the Bronsted acid site. We consider two models for introducing the electrostatic effects of the zeolite lattice. The first involves generating a specialised correction potential by fitting a non-periodic array of ca. 60 point charges to the difference between the bare cluster and periodic potentials. The second starts by fitting a periodic array of atomic charges to the potential of the infinite lattice and then builds up a classical cluster of ca. 2000 atoms into which the QM cluster is embedded. Such embedded cluster calculations, employing a T3 cluster, with electron correlation at the density functional theory level, are described, to model the interaction of water at a Bronsted acid site. Structures of the water–zeolite complex, and associated vibrational frequencies and 1H NMR shifts are calculated and compared with calculations of bare clusters of varying size and with experimental data. We then describe a mixed quantum mechanical–molecular mechanical (QM–MM) model derived by combining charges from the second model with a standard aluminosilicate force field. We report preliminary results on the effect of embedding on the energetics of a prototypical hydrocarbon cracking reaction; the methyl-shift reaction of a propenium ion coordinated to the acid site.

Cooperative effects in the structuring of fluoride water clusters: Ab initio hybrid quantum mechanical/molecular mechanical model incorporating polarizable fluctuating charge solvent

A new hybrid quantum mechanical/molecular mechanical model of solvation is developed and used to describe the structure and dynamics of small fluoride/water clusters, using an ab initio wave function to model the ion and a fluctuating charge potential to model the waters. Appropriate parameters for the water–water and fluoride–water interactions are derived, with the fluoride anion being described by density functional theory and a large Gaussian basis. The role of solvent polarization in determining the structure and energetics of F(H2O)4− clusters is investigated, predicting a slightly greater stability of the interior compared to the surface structure, in agreement with ab initio studies. An extended Lagrangian treatment of the polarizable water, in which the water atomic charges fluctuate dynamically, is used to study the dynamics of F(H2O)4− cluster. A simulation using a fixed solvent charge distribution indicates principally interior, solvated states for the cluster. However, a preponderance of tris...

What is the initiation step of the Grubbs-Hoveyda olefin metathesis catalyst?

Density function theory calculations reveal that the Grubbs-Hoveyda olefin metathesis pre-catalyst is activated by the formation of a complex in which the incoming alkene substrate and outgoing alkoxy ligand are both clearly associated with the ruthenium centre. The computed energies for reaction are in good agreement with the experimental values, reported here.

Density functional and semiempirical molecular orbital methods including dispersion corrections for the accurate description of noncovalent interactions involving sulfur-containing molecules

We describe the use of density functional theory (DFT-D) and semiempirical (AM1-D and PM3-D) methods having an added empirical dispersion correction, to treat noncovalent interactions between molecules involving sulfur atoms. The DFT-D method, with the BLYP and B3LYP functionals, was judged against a small-molecule database involving sulfur-π, S-H···S, and C-H···S interactions for which high-level MP2 or CCSD(T) estimates of the structures and binding or interaction energies are available. This database was also used to develop appropriate AM1-D and PM3-D parameters for sulfur. The DFT-D, AM1-D, and PM3-D methods were further assessed by calculating the structures and binding energies for a set of eight sulfur-containing base pairs, for which high-level ab initio data are available. The mean absolute deviations (MAD) for both sets of structures shown by the DFT-D methods are 0.04 Å for the intermolecular distances and less than 0.7 kcal mol(-)(1) for the binding and interaction energies. The corresponding values are 0.3 Å and 1.5 kcal mol(-)(1) for the semiempirical methods. For the complexes studied, the dispersion contributions to the overall binding and interaction energies are shown to be important, particularly for the complexes involving sulfur-π interactions.

Carbohydrate–aromatic π interactions: a test of density functionals and the DFT-D method

The performance of a number of computational approaches based upon density functional theory (DFT) for the accurate description of carbohydrate-pi interactions is described. A database containing interaction energies of a small number of representative complexes, computed at a high ab initio level, is described, and is used to judge 18 different density functionals including the M05 and M06 families as well as the DFT method augmented with empirical dispersive corrections (DFT-D). The DFT-D method and the M06 functionals are found to perform particularly well, whilst traditional functionals such as B3LYP perform poorly. The interaction energies for 23 sugar-aromatic complexes calculated by the DFT-D method are compared with the values from the 18 functionals. Again, the M06 class of functional is found to be superior.

The geometric structures, vibrational frequencies and redox properties of the actinyl coordination complexes ([AnO2(L)n]m; An = U, Pu, Np; L = H2O, Cl−, CO32−, CH3CO2−, OH−) in aqueous solution, studied by density functional theory methods

The geometric and electronic structures of the aqua, chloro, acetato, hydroxo and carbonato complexes of U, Np and Pu in both their (VI) and (V) oxidation states, and in an aqueous environment, have been studied using density functional theory methods. We have obtained micro-solvated structures derived from molecular dynamics simulations and included the bulk solvent using a continuum model. We find that two different hydrogen bonding patterns involving the axial actinyl oxygen atoms are sometimes possible, and may give rise to different An-O bond lengths and vibrational frequencies. These alternative structures are reflected in the experimental An-O bond lengths of the aqua and carbonato complexes. The variation of the redox potential of the uranyl complexes with the different ligands has been studied using both BP86 and B3LYP functionals. The relative values for the four uranium complexes having anionic ligands are in surprisingly good agreement with experiment, although the absolute values are in error by approximately 1 eV. The absolute error for the aqua species is much less, leading to an incorrect order of the redox potentials of the aqua and chloro species.

The interaction of carbohydrates and amino acids with aromatic systems studied by density functional and semi-empirical molecular orbital calculations with dispersion corrections.

Density functional theory (DFT-D) and semi-empirical (PM3-D) methods having an added dispersion correction have been used to study stabilising carbohydrate-aromatic and amino acid-aromatic interactions. The interaction energy for three simple sugars in different conformations with benzene, all give interaction energies close to 5 kcal mol(-1). Our original parameterization of PM3 (PM3-D) seriously overestimates this value, and has prompted a reparametrization which includes a modified core-core interaction term. With two additional parameters, the carbohydrate complexes, as well as the S22 data set, are well reproduced. The new PM3 scheme (PM3-D*) is found to describe the peptide bond-aromatic ring interactions accurately and, together with the DFT-D method, it is used to investigate the interaction of six amino acids with pyrene. Whilst the peptide backbone can adopt both stacked and T-shaped structures in the complexes with similar interaction energies, there is a preference for the unsaturated ring to adopt a stacked structure. Thus, peptides in which the latter interactions are maximised are likely to be the most effective for the functionalisation of carbon nanotubes.

A new theoretical prediction of the infrared spectra of cytosine tautomers

Abstract The infrared spectra of the cytosine amino-oxo and amino-hydroxy tautomers predicted theoretically at the ab initio Hartree—Fock level with a 6-31G** basis set are reported. These are compared with the experimental spectra obtained in an argon low-temperature matrix. The IR spectra computed at this level reproduce the experimental spectra closer than the previous predictions.