Theoretical study on the potential energy surface and vibrational energy levels of HXeI

The potential energy surface for the electronic ground state of the HXeI molecule is constructed by using the internally contracted multi-reference configuration interaction with the Davidson correction（icMRCI＋Q）method and large basis sets. The stabilities and dissociation barriers are identified from the potential energy surfaces.The three-body dissociation channel is found to be the dominate dissociation channel for HXeI.Based on the obtained potentials,vibrational energy levels of HXeI are calculated using the Lanczos algorithm.Our theoretical results are in excellent agreement with the available observed values.

Theoretical Studies on the Potential Energy Surface and Vibrational Energy Levels of HXeBr

The potential energy surface for the electronic ground state of the HXeBr molecule is constructed from more than 4200 ab initio points calculated using the internally contracted multi-reference configuration interaction method with the Davidson correction （icMRCI ＋ Q）. The stabilities and dissociation barriers are identified from the potential energy surface. The three-body dissociation channel is found to be the dominant dissociation channel for HXeBr. Low-lying vibrational energy levels of HXeBr calculated using the Lanczos algorithm are found to be in good agreement with the available experimental band origins.

Benchmark Calculations on the Atomization Enthalpy,Geometry and Vibrational Frequencies of UF_6 with Relativistic DFT Methods

Benchmark calculations on the molar atomization enthalpy, geometry, and vibrational frequencies of uranium hexafluoride （UF6） have been performed by using relativistic density functional theory （DFT） with various levels of relativistic effects, different types of basis sets, and exchange-correlation functionals. Scalar relativistic effects are shown to be critical for the structural properties. The spin-orbit coupling effects are important for the calculated energies, but are much less important for other calculated ground-state properties of closed-shell UF6. We conclude through systematic investigations that ZORA- and RECP-based relativistic DPT methods are both appropriate for incorporating relativistic effects. Comparisons of different types of basis sets （Slater, Gaussian, and plane-wave types） and various levels of theoretical approximation of the exchange-correlation functionals were also made.

Adiabatic Potential Energy Surfaces and Photodissociation Mechanisms for Highly Excited States of H_(2)O

Full-dimensional adiabatic potential energy surfaces of the electronic ground state X and nine excited states A,I,B,C,D,D',D'',E' and F of H_(2)O molecule are developed at the level of internally contracted multireference configuration interaction with the Davidson correction.The potential energy surfaces are fitted by using Gaussian process regression combining permutation invariant polynomials.With a large selected active space and extra diffuse basis set to describe these Rydberg states,the calculated vertical excited energies and equilibrium geometries are in good agreement with the previous theoretical and experimental values.Compared with the well-investigated photodissociation of the first three low-lying states,both theoretical and experimental studies on higher states are still limited.In this work,we focus on all the three channels of the highly excited state,which are directly involved in the vacuum ultraviolet photodissociation of water.In particular,some conical intersections of D-E',E'-F,A-I and I-C states are clearly illustrated for the first time based on the newly developed potential energy surfaces(PESs).The nonadiabatic dissociation pathways for these excited states are discussed in detail,which may shed light on the photodissociation mechanisms for these highly excited states.

Generalized Energy-Based Fragmentation Approach for Accurate Binding Energies and Raman Spectra of Methane Hydrate Clusters

Methane hydrates(MHs)play important roles in the fields of chemistry,energy,environmental sciences,etc.In this work,we employ the generalized energy-based fragmentation(GEBF)approach to compute the binding energies and Raman spectra of various MH clusters.For the GEBF binding energies of various MH clusters,we first evaluated the various functionals of density functional theory(DFT),and compared them with the results of explicitly correlated combined coupled-cluster singles and doubles with noniterative triples corrections[CCSD(T)(F12^(*))]method.Our results show that the two best functionals are B3PW91-D3 and B97D,with mean absolute errors of only 0.27 and 0.47 kcal/mol,respectively.Then we employed GEBF-B3PW91-D3 to obtain the structures and Raman spectra of MH clusters with mono-and double-cages.Our results show that the B3PW91-D3 functional can well reproduce the experimental C-H stretching Raman spectra of methane in MH crystals,with errors less than 3 cm^(-1).As the size of the water cages increased,the C-H stretching Raman spectra exhibited a redshift,which is also in agreement with the experimental“loose cage-tight cage”model.In addition,the Raman spectra are only slightly affected by the neighboring environment(cages)of methane.The blueshifts of C-H stretching frequencies are no larger than 3 cm^(-1) for CH_(4) from monocages to doublecages.The Raman spectra of the MH clusters could be combined with the experimental Raman spectra to investigate the structures of methane hydrates in the ocean bottom or in the interior of interstellar icy bodies.Based on the B3PW91-D3 or B97D functional and machine learning models,molecular dynamics simulations could be applied to the nucleation and growth mechanisms,and the phase transitions of methane hydrates.

Full-Dimensional Potential Energy Surfaces of Ground(X^(2)A′)and Excited(A^(2)A″)Electronic States of HCO and Absorption Spectrum

In this work,high-fidelity full-dimensional potential energy surfaces(PESs)of the ground(X^(2)A′)and first doublet excited(A^(2)A″)electronic states of HCO were constructed using neural network method.In total,4624 high-level ab initio points have been used which were calculated at Davidson corrected internally contracted MRCI-F12 level of theory with a quite large basis set(ACV5Z)without any scaling scheme.Compared with the results obtained from the scaled PESs of Ndenguéet al.,the absorption spectrum based on our PESs has slightly larger intensity,and the peak positions are shifted to smaller energy for dozens of wavenumbers.It is indicated that the scaling of potential energy may make some unpredictable difference on the dynamical results.However,the resonance energies based on those scaled PESs are slightly closer to the current available experimental values than ours.Nevertheless,the unscaled high-level PESs developed in this work might provide a platform for further experimental and theoretical photodissociation and collisional dynamic studies for HCO system.

Quantum Dynamics of Oxyhydrogen Complex-Forming Reactions for the HO2 and HO3 Systems

Complex-forming reactions widely exist in gas-phase chemical reactions. Various complexforming bimolecular reactions have been investigated and interesting phenomena have been discovered. The complex-forming reactions usually have small or no barrier in the entrance channel, which leads to obvious differences in kinetic and dynamic characteristics compared with direct reactions. Theoretically, quantum state-resolved reaction dynamics can provide the most detailed microscopic dynamic mechanisms and is now feasible for a direct reaction with only one potential barrier. However, it is of great challenge to construct accurate potential energy surfaces and perform accurate quantum dynamics calculations for a complex polyatomic reaction involving deep potential wells and multi-channels. This paper reviews the most recent progress in two prototypical oxyhydrogen complex-forming reaction systems, HO2 and HO3, which are significant in combustion, atmospheric, and interstellar chemistry. We will present a brief survey of both computational and experimental work and emphasize on some unsolved problems existing in these systems.

Accurate Calculation of Equilibrium Reduced Density Matrix for the System-Bath Model:a Multilayer Multiconfiguration Time-Dependent Hartree Approach and its Comparison to a Multi-Electronic-State Path Integral Molecular Dynamics Approach

An efficient and accurate method for computing the equilibriurn reduced density matrix is presented for treating open quantum systems characterized by the systern-bath model. The method employs the rnultilayer nmlticonfiguration tirne-dependent Hartree theory for imag- inary time propagation and an importance sampling procedure for calculating the quantum mechanical trace. The method is applied to the spin-boson Harniltonian, which leads to ac- curate results in agreement with those produced by the rnulti-electronic-state path integral molecular dynamics method.

An ab initio potential energy surface and vibrational energy levels of HXeBr

A three-dimensional global potential energy surface for the electronic ground state of HXeBr molecule is constructed from more than 4200 ab initio points. These points are generated using an internally contracted multi-reference configuration interaction method with the Davidson correction （icMRCI ＋ Q） and large basis sets. The stabilities and dissociation barriers are identified from the potential energy surfaces. The three-body dissociation channel is found to be the dominate dissociation channel for HXeBr. Based on the obtained potentials, low-lying vibrational energy levels of HXeBr calculated using the Lanczos algorithm is found to be in good agreement with the available experimental band origins.

Path Integral Liouville Dynamics Simulations of Vibrational Spectra of Formaldehyde and Hydrogen Peroxide

Formaldehyde and hydrogen peroxide are two important realistic molecules in atmospheric chemistry.We implement path integral Liouville dynamics(PILD)to calculate the dipolederivative autocorrelation function for obtaining the infrared spectrum.In comparison to exact vibrational frequencies,PILD faithfully captures most nuclear quantum effects in vibrational dynamics as temperature changes and as the isotopic substitution occurs.

Theoretical Study on the Rotational Spectra of Ar-D232S Complex

We report a theoretical study on the rotational spectra of Ar-D232S. The intermolecular po- tential energy surface was transformed from our latest ab initio three-dimensional potential of Ar-H232S. The rotational energy levels and wavefunctions of the complex were calcu- lated by using the radial discrete variable representation/angular finite basis representation method and the Lanczos algorithm. The calculated rotational transition frequencies and structural parameters were found to be in good agreement with the available experimental values.

Ultralow-Energy-Barrier H_(2)O_(2)Dissociation on Coordinatively Unsaturated Metal Centers in Binary Ce-Fe Prussian Blue Analogue for Efficient and Stable Photo-Fenton Catalysis

The low intrinsic activity of Fenton catalytic site and high demand for light-energy input inhibit the organic-pollution control efficiency of photo-Fenton process.Here,through structural design with density functional theory(DFT)calculations,Ce is predicted to enable the construction of coordinatively unsaturated metal centers(CUCs)in Prussian blue analogue(PBA),which can strongly adsorb H_(2)O_(2)and donate sufficient electrons for directly splitting the O-O bond to produceOH.Using a substitution-co-assembly strategy,binary Ce-Fe PBA is then prepared,which rapidly degrades sulfamethoxazole with the pseudo-first-order kinetic rate constant exceeding reported values by 1-2 orders of magnitude.Meanwhile,the photogenerated electrons reduce Fe(Ⅲ)and Ce(Ⅳ)to promote the metal valence cycle in CUCs and make sulfamethoxazole degradation efficiency only lose 6.04%in 5 runs.Overall,by introducing rare earth metals into transition metal-organic frameworks,this work guides the whole process for highly active CUCs from design and construction to mechanism exploration with DFT calculations,enabling ultrafast and stable photo-Fenton catalysis.

Theoretical study of interactions between human adult hemoglobin and acetate ion by polarizable force field and fragmentation quantum chemistry methods

A series of theoretical approaches,including conventional FF03 and FF03-based polarization model,as well as the generalized energy-based fragmentation(GEBF) quantum chemistry method,have been applied to investigate the interactions between acetate ion(CH3COO-) and the α-subunit of human adult hemoglobin(designated as Hb-α) at four binding sites(Lys16,Lys90,Arg92,and Lys127),respectively.The FF03-based polarizable force fields show that the interaction energies between the CH3COO-group and Hb-α follow the trend of Arg92>Lys127>Lys90>Lys16.The complexation of CH3COO-with Hb-α is governed by the long-range electrostatic interactions and steric effect.

The Role of Hydrogen Bond in Catalytic Triad of Serine Proteases

In order to investigate the origin of catalytic power for serine proteases,the role of the hydrogen bond in the catalytic triad was studied in the proteolysis process of the peptides chymotrypsin inhibitor 2(CI2),MCTI-A,and a hexapeptide(SUB),respectively.We first calculated the free energy profile of the proton transfer between His and Asp residues of the catalytic triad in the enzyme-substrate state and transition state by employing QM/MM molecular dynamics simulations.The results show that a low-barrier hydrogen bond(LBHB)only forms in the transition state of the acylation of CI2,while it is a normal hydrogen bond in the acylation of MCTI-A or SUB.In addition,the change of the hydrogen bond strength is much larger in CI2 and SUB systems than in MCTI-A system,which decreases the acylation energy barrier significantly for CI2 and SUB.Clearly,a LBHB formed in the transition state region helps accelerate the acylation reaction.But to our surprise,a normal hydrogen bond can also help to decrease the energy barrier.The key to reducing the reaction barrier is the increment of hydrogen bond strength in the transition state state,whether it is a LBHB or not.Our studies cast new light on the role of the hydrogen bond in the catalytic triad,and help to understand the catalytic triad of serine proteases.

Constrained Density Functional Theory Plus the Hubbard U Correction Approach for the Electronic Polaron Mobility:A Case Study of TiO_(2)

The formation and migration of polarons have important influences on physical and chemical properties of transition metal oxides.Density functional theory plus the Hubbard U correction(DFT+U)and constrained density functional theory(cDFT)are often used to obtain the transfer properties for small polarons.In this work we have implemented the cDFT plus the Hubbard U correction method in the projector augmented wave(PAW)framework,andapplied it to study polaron transfer in the bulk phases of TiO_(2).We have confirmed that the parameter U can have significant impact on theoretical prediction of polaronic properties.It was found that using the Hubbard U calculated by the cDFT method with the same orbital projection as used in DFT+U,one can obtain theoretical prediction of polaronic properties of rutile and anatase phases of TiO_(2) in good agreement with experiment.This work indicates that the cDFT+U method with consistently evaluated U is a promising first-principles approach to polaronic properties of transition metal oxides without empirical input.

Theoretical Study on Quantum Dynamics for Ar-HF Inelastic Collision

The integral cross sections and rate constants of pure rotational and ro-vibrational energy transfer processes for the Ar-HF system are thoroughly studied by using the timeindependent close coupling method based on our newly constructed potential energy surface. Compared to previous theoretical results, pure rotational transitions in this work achieve better agreement with the experimental data. For ro-vibrational energy transfer, it is found that quasi-resonant transitions dominate the cross sections in all cases. Furthermore, the vibrational-resolved rate constant of transition v=1→v=0 increases very quickly with the temperature from 100K to 1500K and is also in good agreement with the available experimental results.

Generalized Fourth-Order Decompositions of Imaginary Time Path Integral:Implications of the Harmonic Oscillator

The imaginary time path integral formalism offers a powerful numerical tool for simulating thermodynamic properties of realistic systems.We show that,when second-order and fourth-order decompositions are employed,they share a remarkable unified analytic form for the partition function of the harmonic oscillator.We are then able to obtain the expression of the thermodynamic property and the leading error terms as well.In order to obtain reasonably optimal values of the free parameters in the generalized symmetric fourth-order decomposition scheme,we eliminate the leading error terms to achieve the accuracy of desired order for the thermodynamic property of the harmonic system.Such a strategy leads to an efficient fourth-order decomposition that produces thirdorder accurate thermodynamic properties for general systems.

Extensive Numerical Tests of Leapfrog Integrator in Middle Thermostat Scheme in Molecular Simulations

Accurate and efficient integration of the equations of motion is indispensable for molecular dynamics(MD)simulations.Despite the massive use of the conventional leapfrog(LF)integrator in modern computational tools within the framework of MD propagation,further development for better performance is still possible.The alternative version of LF in the middle thermostat scheme(LFmiddle)achieves a higher order of accuracy and efficiency and maintains stable dynamics even with the integration time stepsize extended by several folds.In this work,we perform a benchmark test of the two integrators(LF and LF-middle)in extensive conventional and enhanced sampling simulations,aiming at quantifying the time-stepsizeinduced variations of global properties(e.g.,detailed potential energy terms)as well as of local observables(e.g.,free energy changes or bondlengths)in practical simulations of complex systems.The test set is composed of six chemically and biologically relevant systems,including the conformational change of dihedral flipping in the N-methylacetamide and an AT(AdenineThymine)tract,the intra-molecular proton transfer inside malonaldehyde,the binding free energy calculations of benzene and phenol targeting T4 lysozyme L99A,the hydroxyl bond variations in ethaline deep eutectic solvent,and the potential energy of the blue-light using flavin photoreceptor.It is observed that the time-step-induced error is smaller for the LFmiddle scheme.The outperformance of LF-middle over the conventional LF integrator is much more significant for global properties than local observables.Overall,the current work demonstrates that the LF-middle scheme should be preferably applied to obtain accurate thermodynamics in the simulation of practical chemical and biological systems.

Theoretical study of the intermolecular hydrogen bond interaction for furan-HCl and furan-CHCl_(3) complexes

The nature of the intermolecular hydrogen bond for the furan-HCl and furan-CHCI3 complexes has been studied using ab initio calculations with MP2 level of theory. The new hydrogen bond type of C(CI)-H...O and π interactions are studied also. It is shown that, for the optimized geometries of furan-CHCI3, C-H bond lengths contract and vibrational frequencies are blue-shifted, while for the furan-HCl complex, H-CI bond lengths elongate and vibrational frequencies are red-shifted. In addition, the NBO analysis indicates that, for the furan-CHCI3 complex, the charge transfers from the lone pair of the proton acceptor to both σ *(CH) antibonding MO and lone pairs of CI atom.