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It is shown that the Born–Oppenheimer–Huang non-adiabatic coupling terms (NACTs) in a molecular Hamiltonian can be considered as equivalent to a friction force in classical mechanics and consequently responsible for the formation of triatomic species like HeH 2 + . The classical equation of motion is solved accordingly for a test case of a helium atom approaching an H 2 + molecule at its potential minimum along the collinear axis and lines parallel to it and also for the C 2 v geometry. It is shown that the singular NACTs between the first two excited states slow down the motion of the three quasi-ions (He, H, H) + while approaching each other and result in substantial loss of kinetic energy in the collision process. These results have profound implications for the formation of di- and triatomic species in the early universe, during the chemical synthesis epoch following the nucleosynthesis that followed the Big Bang. [ABSTRACT FROM AUTHOR]

By computing the non-adiabatic coupling terms and the adiabatic-to-diabatic transformation angle along closed contours in nuclear configuration space using the CASSCF method and the aug-cc-pVTZ basis set for the lowest three electronic states of HeH2+, we explore the conical intersection between states in near collinear and noncollinear geometries, particularly in the C2v geometry. [ABSTRACT FROM AUTHOR]

We review our development on beyond Born–Oppenheimer (BBO) theory and its implementation on various models and realistic molecular processes as carried out over the last 15 years. The theoretical formulation leading to the BBO equations are thoroughly discussed with ab initio calculations. We have employed first principle based BBO theory not only to formulate single surface extended Born–Oppenheimer equation to understand the nature of nonadiabatic effect but also to construct accurate diabatic potential energy surfaces (PESs) for important spectroscopic systems, namely, NO 2 radical, Na 3 and K 3 clusters, NO 3 radical, benzene and 1,3,5-trifluorobenzene radical cations ( C 6 H 6 + and C 6 H 3 F 3 + ) as well as triatomic reactive scattering systems like H + + H 2 and F + H 2 . The nonadiabatic phenomena like Jahn–Teller (JT), Renner–Teller, pseudo Jahn–Teller effects and the accidental conical intersections are the key players in dictating spectroscopic and reactive scattering profiles. The nature of diabatic coupling elements derived from ab initio data with BBO theory for molecular processes in Franck-Condon region has been analysed in the context of linearly and bilinearly coupled JT model Hamiltonian. The results obtained from quantum dynamical calculations on BBO based diabatic PESs of the above molecular systems are found to be in accord with available experimental outcomes. [ABSTRACT FROM AUTHOR]

The present study is devoted to the possibility that tri-atomic molecules were formed during or shortly after the Big Bang. For this purpose, we consider the ordinary H3+ and H3 molecular systems and the primitive tri-atomic molecular system, H3++, which, as is shown, behaves differently. The study is carried out by comparing the topological features of these systems as they are reflected through their non-adiabatic coupling terms. Although H3++ is not known to exist as a molecule, we found that it behaves as such at intermediate distances. However, this illusion breaks down as its asymptotic region is reached. Our study indicates that whereas H3+ and H3 dissociate smoothly, the H3++ does not seem to do so. Nevertheless, the fact that H3++ is capable of living as a molecule on borrowed time enables it to catch an electron and form a molecule via the reaction H3++ + e → H3+ that may dissociate properly: Thus, the two unique features acquired by H3++, namely, that it is the most primitive system formed by three protons and one electron and topologically, still remain for an instant a molecule, may make it the sole candidate for becoming the cornerstone for creating the molecules. [ABSTRACT FROM AUTHOR]

The Time Dependent Discrete Variable Representation (TDDVR) method was initiated by Adhikari and Billing considering time dependent Gauss-Hermite basis functions, where all the parameters were assumed to be time dependent. Adhikari et al. had reformulated the TDDVR approach considering the width parameter as time independent, whereas the equation of motion for time dependent parameters (center of wave packet and its momentum) are derived from Dirac-Frenkel variational principle. Such a method is computationally efficient due to its inherent parallelizable nature to perform multistate (electronic) multidimensional (vibrational) quantum dynamics for well-converged results within reasonably fast computation time, where the complexity of the Hamiltonian is not a matter of concern. Its parallel version is computationally efficient as compared to other quantum dynamical method like the multiconfiguration time dependent Hartree (MCTDH). The parallelized version of this method has also been employed to different complex dynamical systems to calculate transition probabilities, tunnelling probabilities, inelastic surface scattering, bi-molecular reactive scattering and photoexcitation. We have also made use of TDDVR methodology successfully to different diatom (H2/D2)-metal surface (Cu/Ni) scattering processes and triatomic reaction dynamics by using 3D time dependent wave packet approach in hyperspherical coordinates to calculate state-to-state reaction probabilities of D+H2 reaction for J=0 case. [ABSTRACT FROM AUTHOR]

In order to explore spectroscopic properties of a floppy molecule, it is necessary to compute accurate diabatic potential energy surfaces (PESs) around the Franck–Condon region of nuclear configuration space. In such cases, it is desirable to include the non-adiabatic coupling terms (NACTs) through Jahn–Teller (JT) and pseudo Jahn–Teller (PJT) effects as well as other accidental conical intersections (CIs) prevailing among the electronic states of the molecule. In this work, we investigate NACTs among the five lowest doublet electronic states of NO3radical [and 22B2)], in which all those effects (JT, PJT and CIs) have prominent contributions. The study is carried out in terms of normal modes (Qis) of NO3and the CIs (JT and accidental) are located in a plane of nuclear configuration space of two such coordinates (QiandQj). We have calculatedab initioadiabatic PESs and NACTs of those five states of NO3at the MRCI level as functions of pairwiseQis. Further, to explore the nature of a given CI in terms of normal coordinates contributing to NACTs within a particular planeQi-Qj, we have performed CP-MCSCF study computing analytic NACTs and characterised their contributions along variousQks. [ABSTRACT FROM PUBLISHER]

A theoretical investigation of selective bond dissociation of O–H or O–D bond of HOD molecule is carried out by optimally designed electromagnetic field where optimisation is performed by Genetic Algorithm (GA). Two strategies depending upon the objective function and variable space for optimisation have been followed to achieve selective photodissociation. InStrategy Iflux along a particular channel (JH + O-D/JD + O-H) in the repulsive excited state of HOD is considered in defining the objective function with a polychromatic IR pulse of eight components and a UV radiation of two components being optimally found out by GA. The polychromatic IR pulse distributes the population among the low quanta vibrational states of O–H or O–D stretching mode in ground electronic state and the subsequent UV pulse transfers the population to the excited state where photodissociation occurs. According to the direction of population along O–H or O–D stretch in ground electronic state, fluxes in the channels may be expected. We have obtained a maximum value of 92.38% and 74.12% alongJH + O-DandJD + O-Hchannels, respectively. TheStrategy IIis the conventional strategy of selective vibrational excitation followed by population transfer to excited state by single UV pulse. In this case, the polychromatic IR fields are optimised by GA to achieve selective vibrational excitation on |1, 0⟩, |2, 0⟩, |0, 1⟩ and |0, 2⟩ states and the matching single UV pulse is fired for electronic excitation. The first two states correspond to the O–H stretch and population transfer from these states to excited state result in predominant flux along H+O–D channel and similar scheme from the last two states result in D+O–H dissociation as they are effectively of O–D character. The best values ofJH + O-DandJD + O-Hare 86.91% and 65.94% obtained by usingStrategy II. [ABSTRACT FROM AUTHOR]

This article continues a previous study (Int. J. Quantum Chem.114, 1645 (2014)) in which is presented a theory that discusses the possibility to induce a novel field – to be called molecular field – via the interaction of an external electro-magnetic (EM) field and a molecular system. Assuming the molecular system is made up of two coupled adiabatic states the theory leads from three time-space curl equations and one time-space divergence equation to a set of decoupled wave equations usually encountered for fields. In the present study, wave equations are derived for an external field having two features: (1) the field intensity is relativelyhigh enough; (2) the duration of the interaction isshort enough. For this situation, the study reveals that the just mentioned interaction creates two fields that coexist within a molecule: one is just a scalar EM field essentially identical to the external EM field and the other, a vectorial field, produced by the non-adiabatic coupling terms. In addition, we mention that the wave velocities related to these two fields are identical to the wave velocity, c, of the external EM field. [ABSTRACT FROM PUBLISHER]

We carry out both four-dimensional (4D×2D) and six-dimensional (6D) quantum dynamics on a parametrically time- and temperature-dependent effective Hamiltonian for H2/D2(v= 0,j= 0)–Ni(100) collision process. Such an effective potential was derived within a theoretical framework of mean-field approximation by considering weakly correlated interaction between molecular degrees of freedom, phonon modes and electron– hole pair (elhp) coupling through a Hartree-product-type wave function, where the initial state distribution of the surface modes and elhp coupling were introduced through Bose– Einstein and Fermi– Dirac probability factor, respectively. The temperature-dependent dissociation and state-to-state transition probabilities for H2/D2(v= 0,j= 0)–Ni(100) system are depicted as a function of initial kinetic energ of the incoming diatom. Though such effect appears negligibly small for H2(v= 0,j= 0)–Ni(100) system, it is prominent in the case of D2(v= 0,j= 0)–Ni(100) collision. It appears that the change of dissociation and transition probabilities of D2with the increase of surface temperature is exclusively dictated by the phonon modes directed alongZ-axis, but the effect of elhp coupling particularly for transition probabilities is insignificant. [ABSTRACT FROM AUTHOR]

We are investigating the molecular dynamics of the allene system using a parallelised Time Dependent Discrete Variable Representation (TDDVR) methodology by employing a corresponding cation ( [image omitted]) where its two electronic surfaces (A2E and B2B2) are vibronically coupled with each other. In fact the allene radical cation exhibits a three-surface system due to the presence of degeneracy in the A2E state. Our initial investigation is carried out on a linear vibronically coupled model Hamiltonian consisting of 11 vibrationally active modes with two potential energy surfaces. We included both the bilinear and quadratic coupled terms in the effective three-surface Hamiltonian of the [image omitted] radical cation. The spectral profiles obtained from the higher order Hamiltonian show better agreement with the experimental spectrum than the result corresponding to the linearly coupled one. Along with the spectral calculation, we also analyse the nuclear cum population dynamics of the same system. The TDDVR calculated spectral profile as well as the population dynamics show reasonably good agreement with the results calculated by an exact quantum mechanical (multiconfiguration time-dependent Hartree, MCTDH) approach as well as experimental measurement. It appears that the TDDVR approach for those large systems where the quantum mechanical description is needed in a restricted region, is a good compromise between accuracy and speed. [ABSTRACT FROM AUTHOR]

It is shown that the Born–Oppenheimer–Huang non-adiabatic coupling terms (NACTs) in a molecular Hamiltonian can be considered as equivalent to a friction force in classical mechanics and consequently responsible for the formation of triatomic species like HeH 2 + . The classical equation of motion is solved accordingly for a test case of a helium atom approaching an H 2 + molecule at its potential minimum along the collinear axis and lines parallel to it and also for the C 2 v geometry. It is shown that the singular NACTs between the first two excited states slow down the motion of the three quasi-ions (He, H, H) + while approaching each other and result in substantial loss of kinetic energy in the collision process. These results have profound implications for the formation of di- and triatomic species in the early universe, during the chemical synthesis epoch following the nucleosynthesis that followed the Big Bang. [ABSTRACT FROM AUTHOR]

By computing the non-adiabatic coupling terms and the adiabatic-to-diabatic transformation angle along closed contours in nuclear configuration space using the CASSCF method and the aug-cc-pVTZ basis set for the lowest three electronic states of HeH2+, we explore the conical intersection between states in near collinear and noncollinear geometries, particularly in the C2v geometry. [ABSTRACT FROM AUTHOR]