Nuclear Dynamics Calculations Research Articles

Vibronic coupling in the energetically six lowest electronic states of oxirane radical cation.

Multi-dimensional quantum mechanical simulations are carried out to understand the multi-state and multi-mode vibronic interactions in the first six low-lying viz., X̃2B1, Ã2A1, B̃2B2, C̃2A2, D̃2A1, and Ẽ2B1 electronic states of c-C2H4O·+. Vibronic coupling theory is applied to study interactions among electronic states using symmetry selection rules. A model 6 × 6 diabatic electronic Hamiltonian is constructed. The parameters of the diabatic Hamiltonian are estimated by performing extensive abinitio electronic structure calculations, using the EOM-IP-CCSD method. The nuclear dynamics calculations are performed with both time-independent and time-dependent quantum mechanical methods. The calculated vibronic structures of six electronic states are found to be in excellent agreement with the available experimental findings. Progressions found in the theoretical spectrum are assigned in terms of vibrational modes. It is found that extremely strong vibronic interactions among the X̃2B1-Ã2A1, B̃2B2-C̃2A2, and D̃2A1-Ẽ2B1 electronic states results into highly overlapping vibronic bands due to multiple multi-state conical intersections. The impact of associated nonadiabatic effects on the vibronic structure and dynamics of the mentioned electronic states is examined at length. Interesting comparison is made with the results obtained for the isomeric acetaldehyde radical cation.

Vibronic coupling and ultrafast relaxation dynamics in the first five excited singlet electronic states of bithiophene.

The vibronic structure and nuclear dynamics in the first five excited singlet electronic states of bithiophene (2T) are investigated here. Specifically, considerations are given to comprehend the first two structureless and broad electronic absorption bands and the role of nonadiabatic coupling in the excited state relaxation mechanism of 2T in the gas phase. Associated potential energy surfaces (PESs) are established by constructing a model vibronic coupling Hamiltonian using 18 vibrational degrees of freedom and extensive abinitio electronic structure calculations. The topographies of these PESs are critically examined, and multiple conical intersections are established. The nuclear dynamics calculations are performed by propagating wave packets on the coupled electronic manifold. The present theoretical results are in good agreement with the experimental observations. It is found that strong nonadiabatic coupling between the S1-S4 and S1-S5 states along totally symmetric modes is predominantly responsible for the structureless and broad first absorption band, and overlapping S2, S3, S4, and S5 states form the second absorption band. Photorelaxation from the highly excited S5 to the lowest S1 state takes place through a cascade of diabatic population transfers among the S1-S4-S5 electronic manifold within the first ∼100fs. Totally symmetric C=C stretching, C-S stretching, C-H wagging, ring puckering, and inter-ring bending modes collectively drive such relaxation dynamics.

Photodetachment band of the fluorenyl anion: a theoretical rationalization.

In order to rationalize the experimental photodetachment spectra of the fluorenyl anion, nuclear dynamics studies are performed using adiabatic and non-adiabatic quantum chemistry approaches. The adiabatic dynamics calculations are performed on the basis of FC_Class and Poisson simulations. The outcomes from these simulations explain most of the experimental observations. Later, time-independent non-adiabatic nuclear dynamics calculations using vibronic coupling theory (VCT) are performed to elucidate the origin of a few vibrational peaks that do not appear in the results from adiabatic dynamics calculations. In addition, time-dependent non-adiabatic nuclear dynamics calculations using the same VCT approach are performed to rationalize the lifetime of the excited state of the fluorenyl anion. It is found that the photodetachment band obtained from the non-adiabatic simulations is in better agreement with the experimental findings.

Elucidation of vibronic structure and dynamics of first eight excited electronic states of pentafluorobenzene.

Vibronic coupling in the first eight electronic excited states of Pentafluorobenzene (PFBz) is investigated in this article. In particular, the vibronic coupling between the optically bright ππ* and optically dark πσ* states of PFBz is considered. A model 8 × 8 diabatic Hamiltonian is constructed in terms of normal coordinate of vibrational modes using the standard vibronic coupling theory and symmetry selection rule. The Hamiltonian parameters are estimated with the aid of extensive ab initio quantum chemistry calculations. The topography of the first eight electronic excited states of PFBz is examined at length, and multiple multi-state conical intersections are established. The nuclear dynamics calculations on the coupled electronic surfaces are carried out from first principles by the wave packet propagation method. Theoretical results are found to be in good accord with the available experimental optical absorption spectrum of PFBz.

Vibronic coupling in the first six electronic states of pentafluorobenzene radical cation: Radiative emission and nonradiative decay

Nuclear dynamics in the first six vibronically coupled electronic states of pentafluorobenzene radical cation is studied with the aid of the standard vibronic coupling theory and quantum dynamical methods. A model 6 × 6 vibronic Hamiltonian is constructed in a diabatic electronic basis using symmetry selection rules and a Taylor expansion of the elements of the electronic Hamiltonian in terms of the normal coordinate of vibrational modes. Extensive ab initio quantum chemistry calculations are carried out for the adiabatic electronic energies to establish the diabatic potential energy surfaces and their coupling surfaces. Both time-independent and time-dependent quantum mechanical methods are employed to perform nuclear dynamics calculations. The vibronic spectrum of the electronic states is calculated, assigned, and compared with the available experimental results. Internal conversion dynamics of electronic states is examined to assess the impact of various couplings on the nuclear dynamics. The impact of increasing fluorination of the parent benzene radical cation on its radiative emission is examined and discussed.

Theoretical study of molecular-frame angular emission distributions of electrons emitted by interatomic Coulombic decay from helium dimers

Molecular frame angular distributions of electrons released by interatomic Coulombic decay of ${\mathrm{He}}^{+*}(2\ensuremath{\ell})\text{\ensuremath{-}}\mathrm{He}$ states are studied theoretically by means of electronic structure and nonadiabatic nuclear dynamics calculations. In previous experimental work [Phys. Rev. A 82, 063405 (2010)], distinct variations of the angular emission patterns have been observed for different ranges of kinetic-energy release of the fragment ions. Good agreement between the presently computed and these measured angular distributions can be achieved by assuming nonequal populations of ${}^{2}{\mathrm{\ensuremath{\Sigma}}}_{g/u}^{+}$ and ${}^{2}{\mathrm{\ensuremath{\Pi}}}_{g/u}$ decaying electronic states via initial shake-up ionization of the dimers.

Analytic non-adiabatic derivative coupling terms for spin-orbit MRCI wavefunctions. I. Formalism.

Analytic gradients of electronic eigenvalues require one calculation per nuclear geometry, compared to at least 3n + 1 calculations for finite difference methods, where n is the number of nuclei. Analytic nonadiabatic derivative coupling terms (DCTs), which are calculated in a similar fashion, are used to remove nondiagonal contributions to the kinetic energy operator, leading to more accurate nuclear dynamics calculations than those that employ the Born-Oppenheimer approximation, i.e., that assume off-diagonal contributions are zero. The current methods and underpinnings for calculating both of these quantities, gradients and DCTs, for the State-Averaged MultiReference Configuration Interaction with Singles and Doubles (MRCI-SD) wavefunctions in COLUMBUS are reviewed. Before this work, these methods were not available for wavefunctions of a relativistic MRCI-SD Hamiltonian. Calculation of these terms is critical in successfully modeling the dynamics of systems that depend on transitions between potential energy surfaces split by the spin-orbit operator, such as diode-pumped alkali lasers. A formalism for calculating the transition density matrices and analytic derivative coupling terms for such systems is presented.

Mutual Neutralization in Li+−D− Collisions: A Combined Experimental and Theoretical Study

Abstract We present a combined experimental and theoretical study of the mutual neutralization (MN) process in collisions of lithium ions (Li+) with deuterium anions (D−) at collision energies below 1 eV. We employ a merged-beam apparatus to determine total and state-to-state MN cross sections. We perform nuclear dynamics calculations using the multichannel Landau–Zener model based on accurate ab initio molecular data. We obtain an excellent agreement between the experimental and theoretical results over the energy range covered in this work. We show that the basis sets used in the ab initio calculations have a limited influence on the total cross section, but strongly impacts the results obtained for the partial cross sections or the reaction branching ratios. This demonstrates the important role of high-precision measurements to validate the theoretical approaches used to study gas-phase reactive processes. Finally, we compute MN rate coefficients for Li+ + H− and Li+ + D−, and discuss their significance for astrochemistry models.

Electronic spectroscopy of carbon chains (C2n+1, n = 7–10) of astrophysical importance. II. Quantum dynamics

In continuation with Paper I [S. R. Reddy et al., J. Chem. Phys. 151, 054303 (2019)], the vibronic structure and dynamics of the 1Σu+ electronic state of C15, C17, C19, and C21 chains in the coupled manifold of 1Σu+–1Πg–1Πu– 1Σg+ electronic states have been investigated in this paper. The model vibronic Hamiltonian developed through extensive ab initio quantum chemistry calculations in Paper I is employed, and first principles nuclear dynamics calculations are carried out to obtain the photoabsorption band of the 1Σu+ electronic state. Both time-independent and time-dependent quantum mechanical calculations are carried out to precisely locate the vibrational levels, assign them with the progression of vibrational modes, and elucidate the impact of both Renner-Teller and pseudo-Renner-Teller couplings on them. The nonradiative decay of the 1Σu+ electronic state is studied, and it is found that the decay rate is comparable with the prediction made for them to be qualified as a carrier of diffuse interstellar bands in the literature. The theoretical results are found to be in good accord with the available experimental results.

Numerical convergence of the Sinc discrete variable representation for solving molecular vibrational states with a conical intersection in adiabatic representation

Within the Born-Oppenheimer (BO) approximation, nuclear motions of a molecule are often envisioned to occur on an adiabatic potential energy surface (PES). However, this single PES picture should be reconsidered if a conical intersection (CI) is present, although the energy is well below the CI. The presence of the CI results in two additional terms in the nuclear Hamiltonian in the adiabatic presentation, i.e., the diagonal BO correction (DBOC) and the geometric phase (GP), which are divergent at the CI. At the same time, there are cusps in the adiabatic PESs. Thus usually it is regarded that there is numerical difficulty in a quantum dynamics calculation for treating CI in the adiabatic representation. A popular numerical method in nuclear quantum dynamics calculations is the Sinc discrete variable representation (DVR) method. We examine the numerical accuracy of the Sinc DVR method for solving the Schrödinger equation of a two dimensional model of two electronic states with a CI in both the adiabatic and diabatic representation. The results suggest that the Sinc DVR method is capable of giving reliable results in the adiabatic representation with usual density of the grid points, without special treatment of the divergence of the DBOC and the GP. The numerical uncertainty is not worse than that after the introduction of an arbitrary vector potential for accounting the GP, whose accurate form usually is not easy to obtain.

Data on inelastic processes in low-energy collisions of barium atoms and ions with hydrogen atoms and anions

ABSTRACT Inelastic rate coefficients for 686 partial processes in low-energy Ba + H, Ba+ + H−, Ba++ H and Ba2+ + H− collisions are calculated. These data are needed for the non-local thermodynamic equilibrium (non-LTE) modelling of Ba i and Ba ii spectra, especially in cool stellar atmospheres. The calculations of the rate coefficients are performed by means of the quantum model approach, based on the asymptotic semi-empirical method for the electronic structure calculations and on multichannel formulas for the non-adiabatic nuclear dynamical calculations. The inelastic rate coefficients for all transitions between the 17 lowest covalent states and one ionic molecular state in Ba + H and Ba+ + H− collisions, as well as the inelastic rate coefficients for all transitions between the 19 lowest covalent states and one ionic molecular state in Ba+ + H and Ba2+ + H− collisions are calculated. In Ba+ + H− collisions, the highest rate coefficients correspond to the mutual neutralization processes into the Ba(6s6p1P°), Ba(6s7s3S) and Ba(6s7s1S) final states, with the largest value of 5.93 × 10−8 cm3 s−1 at T = 6000 K for the process Ba+ + H− → Ba(6s7s3S) + H. The highest rate coefficient for excitation and de-excitation processes in Ba + H collisions corresponds to the Ba(6s7s1S) → Ba(6s7s3S) transition, with the value of 7.62 × 10−9 cm3 s−1 at T = 6000 K. In Ba2+ + H− collisions, the highest rate coefficients correspond to the neutralization processes into the Ba+( 7p2P°), Ba+( 4f 2F°), Ba+( 6d 2D) and Ba+( 7s 2S) final states. The highest neutralization rate has the value of 3.96 × 10−8 cm3 s−1 at T = 6000 K for the Ba2+ + H− → Ba+( 7p 2P°) + H process. The largest rate coefficient for excitation and de-excitation processes in Ba+ + H collisions corresponds to the Ba+(7s 2S) → Ba+( 6p 2P°) transition, with the value of 1.23 × 10−9 cm3 s−1 at T = 6000 K.

Photophysics of phenol and pentafluorophenol: The role of nonadiabaticity in the optical transition to the lowest bright 1ππ* state.

We report multimode vibronic coupling of the energetically low-lying electronic states of phenol and pentafluorophenol in this article. First principles nuclear dynamics calculations are carried out to elucidate the optical absorption spectrum of both of the molecules. This is motivated by the recent experimental measurements [S. Karmakar et al., J. Chem. Phys. 142, 184303 (2015)] on these systems. Diabatic vibronic coupling models are developed with the aid of adiabatic electronic energies calculated ab initio by the equation of motion coupled cluster quantum chemistry method. A nuclear dynamics study on the constructed electronic states is carried out by both the time-independent and time-dependent quantum mechanical methods. It is found that the nature of low-energy πσ* transition changes, and in pentafluorophenol the energy of the first two 1πσ* states, is lowered by about half an eV (vertically, relative to those in phenol), and they become energetically close to the optically bright first excited 1ππ* (S1) state. This results in strong vibronic coupling and multiple multi-state conical intersections among the ππ* and πσ* electronic states of pentafluorophenol. The impact of associated nonadiabatic effects on the vibronic structure and dynamics of the 1ππ* state is examined at length. The structured vibronic band of phenol becomes structureless in pentafluorophenol. The theoretical results are found to be in good accord with the experimental finding at both high energy resolution and low energy resolution.

Theoretical study of photodetachment spectroscopy of hydrogenated boron cluster anion H2B7- and its deuterated isotopomer.

Photodetachment spectroscopy of H2B7- and its deuterated isotopomer probing the energetically low-lying electronic states of the respective neutral cluster is theoretically investigated in this paper. The theoretical methodology is based on detailed quantum chemistry calculations of electronic state energies, construction of a vibronic coupling model in the diabatic electronic basis, and nuclear dynamics calculations from first principles using time-dependent and time-independent quantum mechanical methods. The theoretical model consists of five coupled electronic states and fifteen vibrational modes. Several reduced dimensional calculations are performed to identify the relevant vibrational modes contributing to the vibronic structure of electronic bands and the impact of non-adiabatic coupling on them. The low-energy part of the spectrum of both H2B7 and its deuterated analogue is assigned by examining the vibronic wavefunctions and the results are compared with the experimental findings. The nonadiabatic decay dynamics of the electronic excited states of the neutral clusters is examined at length.

Direct Quantum Dynamics Using Grid-Based Wave Function Propagation and Machine-Learned Potential Energy Surfaces.

We describe a method for performing nuclear quantum dynamics calculations using standard, grid-based algorithms, including the multiconfiguration time-dependent Hartree (MCTDH) method, where the potential energy surface (PES) is calculated "on-the-fly". The method of Gaussian process regression (GPR) is used to construct a global representation of the PES using values of the energy at points distributed in molecular configuration space during the course of the wavepacket propagation. We demonstrate this direct dynamics approach for both an analytical PES function describing 3-dimensional proton transfer dynamics in malonaldehyde and for 2- and 6-dimensional quantum dynamics simulations of proton transfer in salicylaldimine. In the case of salicylaldimine we also perform calculations in which the PES is constructed using Hartree-Fock calculations through an interface to an ab initio electronic structure code. In all cases, the results of the quantum dynamics simulations are in excellent agreement with previous simulations of both systems yet do not require prior fitting of a PES at any stage. Our approach (implemented in a development version of the Quantics package) opens a route to performing accurate quantum dynamics simulations via wave function propagation of many-dimensional molecular systems in a direct and efficient manner.

Newly characterized interaction stabilizes DNA structure: oligoethylene glycols stabilize G-quadruplexes CH-π interactions.

Oligoethylene glycols are used as crowding agents in experiments that aim to understand the effects of intracellular environments on DNAs. Moreover, DNAs with covalently attached oligoethylene glycols are used as cargo carriers for drug delivery systems. To investigate how oligoethylene glycols interact with DNAs, we incorporated deoxythymidine modified with oligoethylene glycols of different lengths, such as tetraethylene glycol (TEG), into DNAs that form antiparallel G-quadruplex or hairpin structures such that the modified residues were incorporated into loop regions. Thermodynamic analysis showed that because of enthalpic differences, the modified G-quadruplexes were stable and the hairpin structures were slightly unstable relative to unmodified DNA. The stability of G-quadruplexes increased with increasing length of the ethylene oxides and the number of deoxythymidines modified with ethylene glycols in the G-quadruplex. Nuclear magnetic resonance analyses and molecular dynamics calculations suggest that TEG interacts with bases in the G-quartet and loop via CH–π and lone pair–π interactions, although it was previously assumed that oligoethylene glycols do not directly interact with DNAs. The results suggest that numerous cellular co-solutes likely affect DNA function through these CH–π and lone pair–π interactions.

Direct grid-based quantum dynamics on propagated diabatic potential energy surfaces

We present a method for performing non-adiabatic, grid-based nuclear quantum dynamics calculations using diabatic potential energy surfaces (PESs) generated “on-the-fly”. Gaussian process regression is used to interpolate PESs by using electronic structure energies, calculated at points in configuration space determined by the nuclear dynamics, and diabatising the results using the propagation diabatisation method reported recently (Richings and Worth, 2015). Our new method is successfully demonstrated using a grid-based approach to model the non-adiabatic dynamics of the butatriene cation. Overall, our scheme offers a route towards accurate quantum dynamics on diabatic PESs learnt on-the-fly.

In Process Citation

In continuation to our earlier work [Sarkar et al., Mol. Phys. 2015, 113, 3073–3084], we present here the vibronic structure and nonradiative decay dynamics of energetically low-lying electronic states of CH2F2+. Interesting comparison is made with the results obtained for the deuterated isotopomer CD2F2+. The results are compared with both broad band as well as high-resolution experimental spectroscopy data available in the literature. The progression of vibrational modes in the spectra is identified, assigned and discussed in relation to the assignments available in the literature. Both time-independent and time-dependent quantum mechanical methods are used to carry out nuclear dynamics calculations. The underlying potential energy surfaces and their coupling surfaces are constructed by rigorous ab initio quantum chemistry calculations.

Dynamics of the C1s→π*excitation and decay inCO2probed by vibrationally and angularly resolved Auger spectroscopy

Participator resonant Auger decay spectra populating the $X, A, B$, and $C$ states of ${\mathrm{CO}}_{2}{}^{+}$ are recorded with angular and vibrational resolution for selected photon energies in the vicinity of the C $1s\ensuremath{\rightarrow}{\ensuremath{\pi}}^{*}$ resonance of ${\mathrm{CO}}_{2}$ using a narrow photon bandwidth and a high-resolution electron spectrometer. The measured electron spectra and the corresponding angular distribution parameters exhibit significant changes as functions of the photon energy across the resonance and with respect to the vibrational sublevels of a final ionic state. The measured spectra are interpreted by ab initio electronic structure and nuclear dynamics calculations which attribute observed variations to the effects of lifetime vibrational interference and of electronic state interference between the direct ionization amplitude and the resonant amplitudes for the excitation and decay of two overlapping resonant states of different symmetry. The present results provide deeper insight into the femtosecond relaxation dynamics of the core-excited ${\mathrm{CO}}_{2}$, which is not achievable with lower resolution, angle-averaged measurements.

Two-photon resonant excitation of interatomic coulombic decay in neon dimers

The recent availability of intense and ultrashort extreme ultraviolet sources opens up the possibility of investigating ultrafast electronic relaxation processes in matter in an unprecedented regime. In this work we report on the observation of two-photon excitation of interatomic Coulombic decay (ICD) in neon dimers using the tunable intense pulses delivered by the free electron laser FERMI. The unique characteristics of FERMI (narrow bandwidth, spectral stability, and tunability) allow one to resonantly excite specific ionization pathways and to observe a clear signature of the ICD mechanism in the ratio of the ion yield created by Coulomb explosion. The present experimental results are explained by ab initio electronic structure and nuclear dynamics calculations.

Vibronic coupling in the first four electronic states of CH2F+2

Vibronic coupling in the energetically lowest first four electronic states of CH2F+2 is studied in this paper. A model 4×4 Hamiltonian is constructed in a diabatic electronic representation employing normal coordinates of vibrational modes and standard vibronic coupling theory. Extensive ab initio quantum chemistry calculations are carried out to determine the parameters of the Hamiltonian and energetic ordering of the electronic states. The topographical features of the latter are examined at length and several conical intersections are established. Nuclear dynamics calculations on coupled electronic states are carried out from first principles by propagating wave packet. Theoretically calculated broad band vibronic structure of the four states are found to be in good accord with the experimental results.