Jürg Hutter

Separable dual-space Gaussian pseudopotentials

We present pseudopotential coefficients for the first two rows of the Periodic Table. The pseudopotential is of an analytic form that gives optimal efficiency in numerical calculations using plane waves as a basis set. At most, seven coefficients are necessary to specify its analytic form. It is separable and has optimal decay properties in both real and Fourier space. Because of this property, the application of the nonlocal part of the pseudopotential to a wave function can be done efficiently on a grid in real space. Real space integration is much faster for large systems than ordinary multiplication in Fourier space, since it shows only quadratic scaling with respect to the size of the system. We systematically verify the high accuracy of these pseudopotentials by extensive atomic and molecular test calculations. \textcopyright{} 1996 The American Physical Society.

Relativistic separable dual-space Gaussian pseudopotentials from H to Rn

We generalize the concept of separable dual-space Gaussian pseudopotentials to the relativistic case. This allows us to construct this type of pseudopotential for the whole Periodic Table, and we present a complete table of pseudopotential parameters for all the elements from H to Rn. The relativistic version of this pseudopotential retains all the advantages of its nonrelativistic version. It is separable by construction, it is optimal for integration on a real-space grid, it is highly accurate, and, due to its analytic form, it can be specified by a very small number of parameters. The accuracy of the pseudopotential is illustrated by an extensive series of molecular calculations.

The nature of the hydrated excess proton in water

Explanations for the anomalously high mobility of protons in liquid water began with Grotthusss idea, of ‘structural diffusion’ nearly two centuries ago. Subsequent explanations have refined this concept by invoking thermal hopping, , proton tunnelling, or solvation effects. More recently, two main structural models have emerged for the hydrated proton. Eigen, proposed the formation of an H9O4+ complex in which an H3O+ core is strongly hydrogen-bonded to three H2O molecules. Zundel, , meanwhile, supported the notion of an H5O2+ complex in which the proton isshared between two H2O molecules. Here we use ab initio path integral simulations to address this question. These simulations include time-independent equilibrium thermal and quantum fluctuations of all nuclei, and determine interatomic interactions from the electronic structure. We find that the hydrated proton forms a fluxional defect in the hydrogen-bonded network, with both H9O4+ and H5O2+ occurring only in thesense of ‘limiting’ or ‘ideal’ structures. The defect can become delocalized over several hydrogen bonds owing to quantum fluctuations. Solvent polarization induces a small barrier to proton transfer, which is washed out by zero-point motion. The proton can consequently be considered part of a ‘low-barrier hydrogen bond’, , in which tunnelling is negligible and the simplest concepts of transition-state theory do not apply. The rate of proton diffusion is determined by thermally induced hydrogen-bond breaking in the second solvation shell.

A hybrid Gaussian and plane wave density functional scheme

A density functional theory-based algorithm for periodic and non-periodic ab initio calculations is presented. This scheme uses pseudopotentials in order to integrate out the core electrons from the problem. The valence pseudo-wavefunctions are expanded in Gaussian-type orbitals and the density is represented in a plane wave auxiliary basis. The Gaussian basis functions make it possible to use the efficient analytical integration schemes and screening algorithms of quantum chemistry. Novel recursion relations are developed for the calculation of the matrix elements of the density-dependent Kohn-Sham self-consistent potential. At the same time the use of a plane wave basis for the electron density permits efficient calculation of the Hartree energy using fast Fourier transforms, thus circumventing one of the major bottlenecks of standard Gaussian based calculations. Furthermore, this algorithm avoids the fitting procedures that go along with intermediate basis sets for the charge density. The performance a...

Ab initio molecular dynamics simulation of liquid water: Comparison of three gradient‐corrected density functionals

Three frequently used gradient‐corrected density functionals (B, BP, and BLYP) are applied in an ab initio molecular dynamics simulation of liquid water in order to evaluate their performance for the description of condensed aqueous systems. A comparison of structural characteristics (radial distribution functions) and dynamical properties (vibrational spectra, orientational relaxation, and self‐diffusion) leads to the conclusion that hydrogen bonding is too weak in the usual local density approximation corrected for exchange only according to Becke (B), whereas adding the gradient correction for correlation according to Perdew (BP) yields effective hydrogen bonds in the liquid that are too strong. The combination of B with the semilocal correlation functional according to Lee, Yang, and Parr (BLYP) yields the best agreement with experiment. The computational method, which is the basis for the determination of (adiabatic) electronic structure in the ab initio molecular dynamics simulation, has been valida...

A hybrid method for solutes in complex solvents: Density functional theory combined with empirical force fields

We present a hybrid method for molecular dynamics simulations of solutes in complex solvents as represented, for example, by substrates within enzymes. The method combines a quantum mechanical (QM) description of the solute with a molecular mechanics (MM) approach for the solvent. The QM fragment of a simulation system is treated by ab initio density functional theory (DFT) based on plane-wave expansions. Long-range Coulomb interactions within the MM fragment and between the QM and the MM fragment are treated by a computationally efficient fast multipole method. For the description of covalent bonds between the two fragments, we introduce the scaled position link atom method (SPLAM), which removes the shortcomings of related procedures. The various aspects of the hybrid method are scrutinized through test calculations on liquid water, the water dimer, ethane and a small molecule related to the retinal Schiff base. In particular, the extent to which vibrational spectra obtained by DFT for the solute can be...

MOLECULAR DYNAMICS IN LOW-SPIN EXCITED STATES

A Kohn–Sham-like formalism is introduced for the treatment of excited singlet states. Motivated by ideas of Ziegler’s sum method and of restricted open-shell Hartree–Fock theory, a self-consistent scheme is developed that allows the efficient and accurate calculation of excited state geometries. Vertical as well as adiabatic excitation energies for the n→π* transitions of several small molecules are obtained with reasonable accuracy. As is demonstrated for the cis-trans isomerization of formaldimine, our scheme is suited to perform molecular dynamics in the excited singlet state. This represents a first step towards the simulation of photochemical reactions of large systems.

Density functional study of structure and bonding in lithium clusters Lin and their oxides LinO

Density functional (DF) calculations have been performed for lithium clusters Lin and their monoxides LinO with up to ten atoms. There are numerous stable structures, and new isomers have been found in each family. The structural patterns of the homonuclear and oxide clusters are quite distinct. The combination of DF calculations with molecular dynamics (MD) sheds light on the observed pseudorotation of Li3 and Li5. We compare with available experimental data and discuss the bonding and structural patterns in the clusters and their oxides, which are often described as “hyperlithiated.”

Ab initio molecular dynamics simulation of methanol adsorbed in chabazite

Abstract Ab initio molecular dynamics simulations based on density functional theory together with a plane wave basis set and Vanderbilt pseudopotentials are performed to explore the potential energy surface of a methanol molecule interacting with the Bronsted site of zeolite chabazite. In agreement with a recent study a stationary point is located which corresponds to a chemisorbed methoxonium species. However, molecular dynamics simulations at 400 K show that configurations corresponding to this structure are of minor probability. Instead, the most stable structure is a physisorbed complex in which the strongly interacting Bronsted proton is significantly delocalized in the region between the methanol and the framework oxygen of the zeolite catalyst. A stationary point of the potential energy surface corresponding to such a structure proves to be more stable than the ion pair complex by about 18 kJ/mol.

Ab initio analysis of proton transfer dynamics in (H2O)3H

Abstract We have harvested ab initio trajectories of proton transfer in (H 2 O) 3 H + by combining Car–Parrinello molecular dynamics (CPMD) with the transition path sampling method. Two transition state regions contribute to these dynamics, with saddle points similar to those identified by Geissler, Dellago, and Chandler for an empirical model of the same cluster [Phys. Chem. Chem. Phys. 1 (1999) 1317]. As in that model, the location of a transition state along a finite-temperature trajectory indicates that proton transfer is driven by reorganization of the oxygen ring. From vibrational properties it is estimated that the characteristic time for proton transfer is ∼1 ns at a temperature of 300 K.