VASPT2

VASPT2: a method for targeted quantum dynamics of hydrogen transfer reactions

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 Obiettivo del progetto (Objective)

'In this proposal, we develop a cutting-edge computational tool for simulating molecular dynamics at the quantum level that will be applied to understand the dynamics of various chemical phenomena, especially for hydrogen-transfer reactions.

Hydrogen-transfer reactions are one of the most basic elementary chemical reactions, and play vital roles in a number of industrially or biologically intriguing systems. Despite their importance, experimental studies of hydrogen-transfer reactions have faced several difficulties. Experimentally, it is difficult to observe hydrogen-transfer reactions directly, owing to their short time scale. In addition, the analysis of experimental results is often challenging, because the current interesting systems are usually large systems (e.g., enzymes).

There is thus a strong demand for computational software that can accurately simulate hydrogen-transfer reactions. At this moment, all existing molecular dynamics methods do not satisfy this demand. The methods that can precisely describe hydrogen-transfer reactions are only applicable for small systems. Others suffer from unreliability owing to neglected or approximated quantum effects.

In this context, we propose a novel ab initio vibrational wave-function theory to describe general chemical reactions in large molecules with a conclusive accuracy. We call this the Vibrational Active Space Second-order Perturbation Theory (VASPT2). This method exploits a variational method for strong quantum effects among small degrees of freedom, and employs a perturbative method for weak quantum effects in a whole system to achieve quantitative results. The proposed method compromises the applicability and reliability (accuracy) by a well-balanced manner. As an application of this method, we will describe the hydrogen-transfer reaction of AADH (aromatic amine dehydrogenase) to answer the question how quantum tunneling effects are important in enzymes.'

Introduzione (Teaser)

Reactions that transfer hydrogen or protons (hydrogen ions) are among the most elementary yet important reactions in industrial and biological systems. A new computational framework describes them efficiently and accurately for the first time.

Descrizione progetto (Article)

Studying hydrogen transfer reactions experimentally is difficult because the reactions occur on ultrafast femtosecond time scales and the systems of enzymes are too large to obtain fine spectra. Describing them theoretically in large systems such as enzymes is challenging due to a proton's quantum dynamics.

Accurate computational algorithms require a balance between accounting for strong quantum effects among small degrees of freedom and weak quantum effects in the large system as a whole. Scientists have now developed such a method with EU support of the project 'VASPT2: A method for targeted quantum dynamics of hydrogen transfer reactions' (VASPT2).

Researchers divided the system into active regions (small and local) and bath regions (large and global). The team then applied a computationally heavy approach to the active regions and treated the rest of the system and coupling between two regions with a mean-field approach. The latter focuses on one particle or entity and replaces all interactions with the other entities with an average (mean) interaction. The new method was applied to formic acid, a prototype system with weak and strong correlations. Theoretical predictions of vibrational wave functions (fundamental spectral bands) were shown to match experimental values quite well.

The team also developed a method to describe semi-global potential energy surfaces related to hydrogen transfer reactions. Again, there is a trade-off between computational load and the need to describe quantum dynamics. VASPT2 members used a novel linear regression approach to fit the semi-global potential energy surface that minimises 'over-fitting' but does not create unphysical holes.

Implemented in a new programme suite for quantum dynamics called DYNAMOL, the novel frameworks provide computationally efficient and accurate descriptions of hydrogen transfer reactions. They are expected to help answer one of the most important open questions in biochemistry, namely whether or not quantum effects are important for enzymatic reactions. VASPT2 has thus made an invaluable contribution to the design of improved catalysis that is so important to many industrially relevant reactions.