Evaluation Of Potential Energy Surface Research Articles

Exploring Accurate Potential Energy Surfaces via Integrating Variational Quantum Eigensolver with Machine Learning.

The potential energy surface (PES) is crucial for interpreting a variety of chemical reaction processes. However, predicting accurate PESs with high-level electronic structure methods is a challenging task due to the high computational cost. As an appealing application of quantum computing, we show in this work that variational quantum algorithms can be integrated with machine learning (ML) techniques as a promising scheme for exploring accurate PESs. Different from using a ML model to represent the potential energy, we encode the molecular geometry information into a deep neural network (DNN) to represent parameters of the variational quantum eigensolver (VQE), leaving the PES to the wave function ansatz. Once the DNN model is trained, the variational optimization procedure that hinders the application of the VQE to complex systems is avoided, and thus the evaluation of PESs is significantly accelerated. Numerical results demonstrate that a simple DNN model is able to reproduce accurate PESs for small molecules.

What Can We Learn from First Principles Multi-Scale Models in Catalysis? The Role of the Ni/Al₂O₃ Interface in Water-Gas Shift and Dry Reforming as a Case Study.

Computational first principles models based on density functional theory (DFT) have emerged as an important tool to address reaction mechanisms and active sites in metal nanoparticle catalysis. However, the common evaluation of potential energy surfaces for selected reaction steps contrasts with the complexity of reaction networks under operating conditions, where the interplay of adsorbate populations and competing routes at reaction conditions determine the most relevant states for catalyst activity and selectivity. Here, we discuss how the use of a multi-scale first principles approach combining DFT calculations at the atomistic level with kinetic models may be used to understand reactions catalyzed by metal nanoparticles. The potential of such an approach is illustrated for the case of Al₂O₃-supported Ni nanoparticle catalysts in the water-gas shift and dry reforming reactions. In these systems, both Ni nanoparticle (metal) as well as metal/oxide interface sites are available and may play a role in catalysis, which depends not only on the energy for critical reaction steps, as captured by DFT, but also on the reaction temperature and adsorbate populations, as shown by microkinetic modelling and experiments.

CO methanation on ruthenium flat and stepped surfaces: Key role of H-transfers and entropy revealed by ab initio molecular dynamics

The methanation reaction converting CO into CH4 at ca. 400 °C is proposed to take place on uncoordinated surface sites (e.g. step-edges) of Ru catalysts because of the higher measured reactivity of stepped vs. flat single crystal metal surfaces. However, the reaction mechanisms occurring at step-edges remain unclear due to the limitations in modelling high temperatures processes by the common evaluation of potential energy surfaces by density functional theory calculations. Here, we show using ab initio molecular dynamics simulations on CO-covered Ru flat and stepped surfaces models at 400 °C that hydrogen-transfers favoured at elevated temperatures are crucial for both CO activation and CHx (x = 1, 2, 3) hydrogenation steps of the methanation reaction, especially at step-edges.

Improved on-the-Fly MCTDH Simulations with Many-Body-Potential Tensor Decomposition and Projection Diabatization.

We have recently demonstrated how potential energy surface (PES) interpolation methods, such as kernel ridge regression (KRR), can be combined with accurate wave function time-propagation methods, specifically the multiconfiguration time-dependent Hartree (MCTDH) method, to generate a new "on-the-fly" MCTDH scheme (DD-MCTDH) that does not require the pre-fitting of the PES, which is normally required by MCTDH. Specifically, we have shown how our DD-MCTDH strategy can be used to model non-adiabatic dynamics in a 4-mode/2-state model of pyrazine, with ab initio electronic structure calculations performed directly during propagation, requiring around 100 h of computer wall-time. In this Article, we show how the efficiency and accuracy of DD-MCTDH can be dramatically improved further still by (i) using systematic tensor decompositions of the KRR PES, and (ii) using a novel scheme for diabatization within the framework of configuration interaction (CI) methods which only requires local adiabatic electronic states, rather than non-adiabatic coupling matrix elements. The result of these improvements is that our latest version of DD-MCTDH can perform a 12-mode/2-state simulation of pyrazine, with PES evaluations at CAS level, in just 29-90 h on a standard desktop computer; this work therefore represents an enormous step towards direct quantum dynamics with MCTDH.

Theoretical study of the mechanism of stable phosphorus ylides derived from 2-aminothiophenol in the presence of different dialkyl acetyelenedicarboxylates

GRAPHICAL ABSTRACT

EXPERIMENTAL AND THEORETICAL STUDY OF STABLE PHOSPHORUS YLIDES DERIVED FROM INDAZOLE IN THE PRESENCE OF DIFFERENT DIALKYL ACETYELENEDICARBOXYLATES: FURTHER INSIGHTS INTO THE REACTION MECHANISM

In the recent work, kinetics studies were made of the reactions between triphenylphosphine 1, dialkyl acetylenedicarboxylates 2 in the presence of NH-acid, such as indazole 3. The kinetic parameters of all reactions determined by UV spectrophotometry. The second order fits were drawn and the values of the second order rate constant (k) were calculated using standard equations. All reactions repeated at different solvents and temperatures. Furthermore, useful information was obtained from studies of the effect of solvent, the structure of the reactants (dialkyl acetylenedicarboxylates) and also the concentration of reactants on the reaction rates. Theoretical studies were performed for evaluation of potential energy surfaces of all structures participated in the reaction mechanism. The first step of all reactions was recognized as a rate-determining step in the reaction mechanism, on the basis of experimental and theoretical data. Theoretical studies performed for evaluation of potential energy surfaces of all structures participated in the reaction mechanism. Quantum mechanical calculations were clarified that how the ylides exist, in solution, as a mixture of two geometrical Z- and E-isomers as a minor or major form.

Calculation of chemical reaction rate constants using on-the-fly high level electronic structure computations with account of multidimensional tunneling

The semiclassical instanton approach to the calculation of reaction rate constants at arbitrary temperatures in multiatomic systems is combined with high-level ab initio calculations of reactive potential energy surface (PES). The number of required ab initio calculations weakly depends on system size and allows on-the-fly evaluations of PES with high accuracy. The approach can be efficiently parallelized and provides a practical way of calculating quantum reaction rate constants with account of nuclear quantum effects such as multidimensional tunneling and zero point energies, which are rigorously incorporated in the theory. An algorithm for the search of instanton trajectories is explained. Application of the approach is illustrated for H + H(2) → H(2) + H and D + D(2) → D(2) + D bimolecular reactions in the wide temperature range with on-the-fly evaluation of PES at the ab initio full configuration interaction (FCI), coupled-cluster single double (CCSD), and single and double excitation configuration interaction (CISD) levels.

Deciphering the photophysics of 5-chlorosalicylic acid: evidence for excited-state intramolecular proton transfer

A detailed photophysical study of a pharmaceutically important chlorine-substituted derivative of salicylic acid viz., 5-chlorosalicylic acid (5ClSA), has been carried out by steady-state absorption, emission and time-resolved fluorescence spectroscopy and compared with its parent molecule salicylic acid (SA). A large Stokes-shifted emission band with negligible solvent polarity dependency indicates the spectroscopic signature of excited-state intramolecular proton transfer reaction. Presence of various ground-state species has been confirmed by a thorough investigation in different solvents and by pH variation experiments. Quantum chemical calculation by ab initio Hartree-Fock (HF) and Density Functional Theory (DFT) methods yields results consistent with experimental findings. Particularly, evaluation of potential energy surfaces for the S(0) and S(1) states across the proton transfer coordinate reveals distinct theoretical support for the inoperativeness of the GSIPT reaction as well as the occurrence of ESIPT process. The less favourable ESIPT reaction in 5ClSA is due to reduction of ground-state intramolecular hydrogen bond strength and excited-state negative charge density at the acceptor (-COOH) oxygen atom and increase of the excited-state barrier with respect to SA.

An integrated approach for the interpretation of emission fluorescence of DMABN-Crown derivatives in polar environments

We analyze the behavior in several solvents of a perspective metal sensor, namely benzonitrile (DMABN-Crown5), a derivative of 4-( N, N-dimethylamino) benzonitrile (DMABN) that exhibits a dual fluorescence due to a twisted intra-molecular charge transfer process. To this end, we extend a stochastic modeling previously employed for DMABN, in which internal degrees of freedom are coupled with an effective solvent relaxation variable. Evaluation of potential energy surfaces using advanced QM approach and estimates of dissipative parameters based on hydrodynamic arguments are discussed. Emission fluorescence is calculated by solving a diffusion/sink/source equation for the stationary population of excited state, and compared to experimentally measured emission fluorescence of DMABN-Crown5.

Multi-reference-state perturbation theory for computation of potential-energy surfaces

The implementation of multi-reference-state Rayleigh-Schrodinger perturbation theory for the evaluation of potential-energy surfaces is reviewed. The organization of the computation and the basic algorithms are outlined. A major difficulty is the quadratic increase in computing sources as the reference space is expanded. Truncating the space of nonreference determinants is suggested as a solution for the problem. The method applies a numerical cutoff criterion for the weights of the first-order wave function. The truncation reduces the computing time significantly with negligible sacrifice in the quality of the results.

Molecular orbital calculations on the conformation of polypeptides and proteins: I. Preliminary investigations and simple dipeptides

The quantum-mechanical method PCILO based on a perturbative configuration interaction using localized orbitals, particularly efficient for the calculation of ground state energies of molecules, is employed for the evaluation of potential energy surfaces and preferred conformations of two dipeptides (glycyl and alanyl residues). It represents an independent procedure for the determintion of these properties, omitting the usual partitioning of the potential energy into empirically determined components. In distinction to such empirical calculations, the computations predict as the most stable conformation of the two dipeptides a folded seven-membered hydrogen bonded ring. The prediction is in agreement with very recent experimental findings. The method is rapid enough to be applicable to large polymeric systems.