Ab initio-based double many-body expansion potential energy surface for the first excited triplet state of the ammonia molecule

Accurate High-Level Ab Initio-Based Global Potential Energy Surface and Quantum Dynamics Calculation for the First Excited State of CH2. J Phys Chem A 2021;125:5490-5498. [PMID: 34137628 DOI: 10.1021/acs.jpca.1c02413]

A full three-dimensional global potential energy surface (PES), covering the whole configuration space, is reported first for the title system by fitting high-level ab initio energies at the multireference configuration interaction level with the aug-cc-pV6Z basis set. In this work, the many-body expansion method is invoked to fit the innate character of the CH₂⁺(1²A″) PES. The topographical features are examined in detail based on the new global PES and in accordance with the other calculations from the ab initio energies, which show the correct behavior at the C⁺(²P) + H₂(X¹Σ_g⁺) and CH⁺(a³Π) + H(²S) dissociation limits. Using a time-dependent wave packet method, we provide insights into the dynamics behavior for reaction of C⁺(²P) + H₂(X¹Σ_g⁺) → CH⁺(a³Π) + H(²S). The integral cross sections and reaction probabilities increase monotonically in terms of the collision energy.

Accurate global potential energy surface by extrapolation to the complete basis set limit and dynamics studies for ground state of H2S. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2019;219:267-273. [PMID: 31048256 DOI: 10.1016/j.saa.2019.04.051]

The atomic ions with hydrogen reactions play important role in biological chemistry, atmospheric and combustion chemistry. In this work, a full three-dimensional global many-body expansion potential energy surface (PES) for the ground state of H₂S^-(²A') is reported for the first time, since there is almost no investigations can accurately and completely describe the PES. It is obtained using many-body expansion theory and an extensive set of accurate ab initio energies extrapolated to the complete basis set limit, for improving the accuracy of the potential energy surface. The topographical features of the new three-dimensional global potential energy surface are examined in detail, and found to be in good agreement with theoretical and experimental results. To better test the reaction, such a surface was applied to quasi classical trajectory calculations for S^-(²P)+H₂(¹Σ_g⁺)→SH^-(¹Σ)+H(²S) reaction. The integral cross sections, differential cross sections and the rate coefficients have been computed. Results indicate that such a work can be recommended for corresponding atmospheric dynamics studies and as building blocks for constructing the many body expansion potential energy surface of larger S^-/H containing systems.

Accurate global potential energy surface for the ground state of CH2+ by extrapolation to the complete basis set limit

A full three-dimensional global potential energy surface is reported for the ground state of CH₂⁺ by fitting accurate multireference configuration interaction energies calculated using aug-cc-pVQZ and aug-cc-pV5Z basis sets with extrapolation of the electron correlation energy to the complete basis set limit.

Globally accurate potential energy surface for the ground-state HCS(X2A') and its use in reaction dynamics

A globally accurate many-body expansion potential energy surface is reported for HCS(X²A') by fitting a wealth of accurate ab initio energies calculated at the multireference configuration interaction level using aug-cc-pVQZ and aug-cc-pV5Z basis sets via extrapolation to the complete basis set limit. The topographical features of the present potential energy surface are examined in detail and is in good agreement with the raw ab initio results, as well as other theoretical results available in literatures. By utilizing the potential energy surface of HCS(X²A'), the dynamic studies of the C(³P) + SH(X²Π) → H(²S) + CS(X¹∑⁺) reaction has been carried out using quasi-classical trajectory method.

A new potential energy surface for the H2S system and dynamics study on the S((1)D) + H2(X(1)Σg(+)) reaction

We constructed a new global potential energy surface (PES) for the electronic ground state ((1)A') of H2S based on 21,300 accurate ab initio energy points over a large configuration space. The ab initio energies are obtained from multireference configuration interaction calculations with a Davidson correction using basis sets of quadruple zeta quality. The neural network method is applied to fit the PES, and the root mean square error of fitting is small (1.68 meV). Time-dependent wave packet studies for the S((1)D) + H2(X(1)Σg(+)) → H((2)S) + SH(X(2)Π) reaction on the new PES are conducted to study the reaction dynamics. The calculated integral cross sections decrease with increasing collision energy and remain fairly constant within the high collision energy range. Both forward and backward scatterings can be observed as expected for a barrierless reaction with a deep well on the PES. The calculated integral cross sections and differential cross sections are in good agreement with the experimental results.

A new potential energy surface for the ground electronic state of the LiH2 system, and dynamics studies on the H((2)S) + LiH(X(1)Σ(+)) → Li((2)S) + H2(X(1)Σg(+)) reaction

A new global potential energy surface (PES) is obtained for the ground electronic state of the LiH2 system based on high-level energies. The energy points are calculated at the multireference configuration interaction level with aug-cc-pVXZ (X = Q, 5) basis sets, and these energies are extrapolated to the complete basis set limit. The neural network method and hierarchical construction scheme are applied in the fitting process and the root mean square error of the fitting result is very small (0.004 eV). The dissociation energies and equilibrium distances for LiH(X(1)Σ(+)) and H2(X(1)Σg(+)) obtained from the new PES are in good agreement with the experimental data. On the new PES, time-dependent wave packet studies for the H((2)S) + LiH(X(1)Σ(+)) → Li((2)S) + H2(X(1)Σg(+)) reaction have been carried out. In this reaction, no threshold is found due to the absence of an energy barrier on the minimum energy path. The calculated integral cross sections are high at low collision energy and will decrease with the increase of the collision energy. The product molecule H2 tends to be forward scattering due to direct reactive collisions, which becomes more evident at higher collision energies.

Accurate high level ab initio-based global potential energy surface and dynamics calculations for ground state of CH2(+)

A global many-body expansion potential energy surface is reported for the electronic ground state of CH2 (+) by fitting high level ab initio energies calculated at the multireference configuration interaction level with the aug-cc-pV6Z basis set. The topographical features of the new global potential energy surface are examined in detail and found to be in good agreement with those calculated directly from the raw ab initio energies, as well as previous calculations available in the literature. In turn, in order to validate the potential energy surface, a test theoretical study of the reaction CH(+)(X(1)Σ(+))+H((2)S)→C(+)((2)P)+H2(X(1)Σg (+)) has been carried out with the method of time dependent wavepacket on the title potential energy surface. The total integral cross sections and the rate coefficients have been calculated; the results determined that the new potential energy surface can both be recommended for dynamics studies of any type and as building blocks for constructing the potential energy surfaces of larger C(+)/H containing systems.

Accurate ab initio-based adiabatic global potential energy surface for the 2(2)A" state of NH2 by extrapolation to the complete basis set limit

A full three-dimensional global potential energy surface is reported first time for the title system, which is important for the photodissociation processes. It is obtained using double many-body expansion theory and an extensive set of accurate ab initio energies extrapolated to the complete basis set limit. Such a work can be recommended for dynamics studies of the N((2)D) + H2 reaction, a reliable theoretical treatment of the photodissociation dynamics and as building blocks for constructing the double many-body expansion potential energy surface of larger nitrogen/hydrogen containing systems. In turn, a preliminary theoretical study of the reaction N((2)D)+H2(X(1)Σg (+))(ν=0,j=0)→NH(a(1)Δ)+H((2)S) has been carried out with the method of quasi-classical trajectory on the new potential energy surface. Integral cross sections and thermal rate constants have been calculated, providing perhaps the most reliable estimate of the integral cross sections and the rate constants known thus far for such a reaction.

Accurate double many-body expansion potential energy surface by extrapolation to the complete basis set limit and dynamics calculations for ground state of NH2

An accurate single-sheeted double many-body expansion potential energy surface is reported for the title system. A switching function formalism has been used to warrant the correct behavior at the H2(X1Σg+)+N(2D) and NH (X3Σ-)+H(2S) dissociation channels involving nitrogen in the ground N(4S) and first excited N(2D) states. The topographical features of the novel global potential energy surface are examined in detail, and found to be in good agreement with those calculated directly from the raw ab initio energies, as well as previous calculations available in the literature. The novel surface can be using to treat well the Renner-Teller degeneracy of the 12A″ and 12A' states of NH 2. Such a work can both be recommended for dynamics studies of the N(2D)+H2 reaction and as building blocks for constructing the double many-body expansion potential energy surface of larger nitrogen/hydrogen-containing systems. In turn, a test theoretical study of the reaction N(2D)+H2(X1Σg+)(ν=0,j=0)→NH (X3Σ-)+H(2S) has been carried out with the method of quantum wave packet on the new potential energy surface. Reaction probabilities, integral cross sections, and differential cross sections have been calculated. Threshold exists because of the energy barrier (68.5 meV) along the minimum energy path. On the curve of reaction probability for total angular momentum J = 0, there are two sharp peaks just above threshold. The value of integral cross section increases quickly from zero to maximum with the increase of collision energy, and then stays stable with small oscillations. The differential cross section result shows that the reaction is a typical forward and backward scatter in agreement with experimental measurement result.