r e s e a r c h i n t e r e s t s

CURRENT RESEARCH:

Nonadiabatic Car-Parrinello Molecular Dynamics:
We have developed an extension of Car-Parrinello molecular dynamics for efficient treatment of electronically nonadiabatic transitions. The current approach couples the S₁ restricted open-shell Kohn-Sham excited state to the S₀ ground state using Tully's fewest switches surface hopping algorithm. Explicit evaluation of the nonadiabatic coupling vector is avoided by exploiting the available wavefunction time derivatives. The method makes possible ab initio molecular simulations beyond the Born-Oppenheimer approximation of systems similar in character and complexity as those typically studied by standard Car-Parrinello molecular dynamics. Since the computational cost scales linearly with the number of excited states, the technique is ideally suited to study the photochemistry of large molecules, particularly in solution. As a demonstration, we have investigated the gas phase cis - trans photoisomerisation of formaldimine, the minimal model of a Schiff base, and the trans - cis photoisomerisation of N₂H₂ in aqueous solution. We are also applying this technique to the study of excited state intramolecular proton transfer in OHBA.

UNIVERSITY OF CAMBRIDGE (1998 - 2000):

Car-Parrinello Molecular Dynamics:
Structural and dynamic properties of the phosphorane intermediates occuring in the first step of the hydrolysis of RNA have been studied by means of the Car-Parrinello MD (CPMD) technique focussing on their lifetime and protonation state as a function of the leaving group. Coordination constraint methods are being used to determine the site-specific pK_a values for the simplest case, P(OH)₅, from free energy calculations.
Density Functional Development:
In order to improve the density functional description of liquid water and aqueous chemistry, in particular, new generalised gradient approximation functionals have been developed in collaboration with A. D. Boese and Prof. N. Handy. Calculations on small phosphorus compounds have been performed using the new HCTH-120 density functional to establish its value for the above biochemical applications.
Excited State Dynamics:
Time-dependent density functional theory (TDDFT) has been applied to the study of atomic and molecular electronic excitations. Our plane-wave implementation of TDDFT has proven to give reliable results for the lowest singlet and triplet excitations of alkaline earth atoms and a number of small molecules including N₂, H₂CO, and formamide. As proposed by Handy and Tozer an asymptotic correction to the exchange-correlation potential has been applied for the calculation of Rydberg states.
In an effort to investigate solvent effects on molecular photoabsorption spectra we are currently studying the n - pi^* transition in H₂CO*nH₂O as a function of number of water molecules. Identification of molecular orbitals in solution will be possible through the use of the maximally localized Wannier functions.
We wish to extent the present model to eventually be able to perform excited state - and ultimately nonadiabatic - CPMD simulations.

UNIVERSITY OF BIRMINGHAM (1995 - 1998):

Ab initio Quantum Chemistry:
The heat of formation of CH₂ has been computed using high accuracy ab initio results for its dissociation energy. MRCI calculations were carried out with correlation consistent polarized valence basis sets (cc-pVXZ, X=2 - 6). Core-valence correlation energies were computed using CCSD(T) with correlated consistent polarized core-valence basis sets (cc-pCVXZ, X=2 - 5).
Electronic Structure Modelling:
A multi-state Diatomics-in-Molecules (DIM) model has been constructed to explore structure and dynamics of positively charged argon clusters. Such a model uses data from neutral and ionic dimer potential energy curves only, yet reproduces ab initio results faithfully. The contributions of 3-body nonadditivities to the polyatomic energy were studied in detail.
Molecular Dynamics Calculations:
A semiclassical surface hopping approach including the 'fewest switches' criterion was chosen to carry out nonadiabatic many-body dynamics calculations. The potential energy surfaces obtained by DIM were used to investigate the fragmentation of Ar_n⁺ clusters following their photoexcitation.

UNIVERSITY OF STUTTGART/MAX-PLANCK INSTITUTE (1994 - 1995):

Rapid Thermal Processing of Semiconductors
Spreading Resistance Technique
Computer Modelling of Diffusion Processes

Gold (Au) diffusion in crystalline silicon (Si) was investigated using Rapid Optical Annealing at temperatures in the range of 950^oC to 1250^oC and annealing times from 120 s down to 15 s. The resulting content of substitutional gold was determined by Spreading Resistance measurements and analyzed by comparison with extensive numerical simulations within the framework of the so-called kick-out model. Previous long-time diffusion experiments have yielded the product of equilibrium concentration and diffusivity of Si self-interstitials involved in the interstitial-substitutional exchange of Au. The present short-time diffusion method enables us to determine the effective di ffusion coefficient of interstitial gold.