Excess electron localization sites in neutral water clusters

We present approximate pseudopotential quantum-mechanical calculations of the excess electron states of equilibrated neutral water clusters sampled by classical molecular dynamics simulations. The internal energy of the clusters are representative of those present at temperatures of 200 and 300 K. Correlated electronic structure calculations are used to validate the pseudopotential for this purpose. We find that the neutral clusters support localized, bound excess electron ground states in about 50% of the configurations for the smallest cluster size studied (n = 20), and in almost all configurations for larger clusters (n > 66). The state is always exterior to the molecular frame, forming typically a diffuse surface state. Both cluster size and temperature dependence of energetic and structural properties of the clusters and the electron distribution are explored. We show that the stabilization of the electron is strongly correlated with the preexisting instantaneous dipole moment of the neutral clusters, and its ground state energy is reflected in the electronic radius. The findings are consistent with electron attachment via an initial surface state. The hypothetical spectral dynamics following such attachment is also discussed.     

Density functional theory for excited states

In this article, we justify the use of standard Kohn-Sham (KS ) band structure theory for the calculation of the low-energy spectrum of metals and the density of states. For the higher spectrum, one can start the KS excited-state calculations by using as the first-order approximation the spin orbitals of the ground state. We next deal with the problem of degenerate states by using the subspace theory minimum principle. © 1995 John Wiley & Sons, Inc.     

Distinguishing between relaxation pathways by combining dissociative ionization pump probe spectroscopy and ab initio calculations: a case study of cytosine

We present a general method for tracking molecular relaxation along different pathways from an excited state down to the ground state. We follow the excited state dynamics of cytosine pumped near the S(0)-S(1) resonance using ultrafast laser pulses in the deep ultraviolet and probed with strong field near infrared pulses which ionize and dissociate the molecules. The fragment ions are detected via time of flight mass spectroscopy as a function of pump probe delay and probe pulse intensity. Our measurements reveal that different molecular fragments show different timescales, indicating that there are multiple relaxation pathways down to the ground state. We interpret our measurements with the help of ab initio electronic structure calculations of both the neutral molecule and the molecular cation for different conformations en route to relaxation back down to the ground state. Our measurements and calculations show passage through two seams of conical intersections between ground and excited states and demonstrate the ability of dissociative ionization pump probe measurements in conjunction with ab initio electronic structure calculations to track molecular relaxation through multiple pathways.     

On the (Emultiply sign in circlee)-Jahn-Teller conical intersections in the 3p(E') and 3d(E") Rydberg electronic states of triatomic hydrogen

The static and dynamic aspects of the Jahn-Teller (JT) interactions in the 3p(E') and 3d(E") Rydberg electronic states of H3 are analyzed theoretically. The static aspects are discussed based on recent ab initio quantum chemistry results, and the dynamic aspects are examined in terms of the vibronic spectra and nonradiative decay behavior of these states. The adiabatic potential-energy surfaces of these degenerate electronic states are derived from extensive ab initio calculations. The calculated adiabatic potential-energy surfaces are diabatized following our earlier study on this system in its 2p(E') ground electronic state. The nuclear dynamics on the resulting conically intersecting manifold of electronic states is studied by a time-dependent wave-packet approach. Calculations are performed both for the uncoupled and coupled state situations in order to understand the importance of nonadiabatic interactions due to the JT conical intersections in these excited Rydberg electronic states.     

The Jahn-Teller and pseudo-Jahn-Teller effects in the anion photoelectron spectroscopy of B3 cluster

The photodetachment spectroscopy of B3- anion is theoretically studied with the aid of a quantum dynamical approach. The theoretical results are compared with the available experimental photoelectron spectra of B3-. Both B3- and B3 possess D(3h) symmetry at the equilibrium configuration of their electronic ground state. Distortion of B3 along its degenerate vibrational mode nu2 splits the degeneracy of its excited C2E' electronic manifold and exhibits (E [symbol: see text] e)-Jahn-Teller (JT) activity. The components of the JT split potential energy surface form conical intersections, and they can also undergo pseudo-Jahn-Teller (PJT) crossings with the X2A1' electronic ground state of B3 via the degenerate nu2 vibrational mode. The impact of the JT and PJT interactions on the nuclear dynamics of B3 in its X2A1'-C2E' electronic states is examined here by establishing a diabatic model Hamiltonian. The parameters of the electronic part of this Hamiltonian are calculated by performing electronic structure calculations and the nuclear dynamics on it is simulated by solving quantum eigenvalue equation. The theoretical results are in good accord with the experimental data.     

Vibrational dynamics of excited electronic states of molecular iodine

Femtosecond degenerate four-wave-mixing spectroscopy following an initial pump laser pulse was used to observe the wave packet dynamics in excited electronic states of gas phase iodine. The focus of the investigation was on the ion pair states belonging to the first tier dissociating into the two ions I-(1S) + I+(3P2). By a proper choice of the wavelengths of the initial pump and degenerate four-wave-mixing pulses, we were able to observe the vibrational dynamics of the B (3)Pi(u) (+) state of molecular iodine as well as the ion pair states accessible from there by a one-photon transition. The method proves to be a valuable tool for exploring higher lying states that cannot be directly accessed from the ground state due to selection rule exclusion or unfavorable Franck-Condon overlap.     

Alignment, vibronic level splitting, and coherent coupling effects on the pump-probe polarization anisotropy

The pump-probe polarization anisotropy is computed for molecules with a nondegenerate ground state, two degenerate or nearly degenerate excited states with perpendicular transition dipoles, and no resonant excited-state absorption. Including finite pulse effects, the initial polarization anisotropy at zero pump-probe delay is predicted to be r(0) = 3/10 with coherent excitation. During pulse overlap, it is shown that the four-wave mixing classification of signal pathways as ground or excited state is not useful for pump-probe signals. Therefore, a reclassification useful for pump-probe experiments is proposed, and the coherent anisotropy is discussed in terms of a more general transition dipole and molecular axis alignment instead of experiment-dependent ground- versus excited-state pathways. Although coherent excitation enhances alignment of the transition dipole, the molecular axes are less aligned than for a single dipole transition, lowering the initial anisotropy. As the splitting between excited states increases beyond the laser bandwidth and absorption line width, the initial anisotropy increases from 3/10 to 4/10. Asymmetric vibrational coordinates that lift the degeneracy control the electronic energy gap and off-diagonal coupling between electronic states. These vibrations dephase coherence and equilibrate the populations of the (nearly) degenerate states, causing the anisotropy to decay (possibly with oscillations) to 1/10. Small amounts of asymmetric inhomogeneity (2 cm(-1)) cause rapid (130 fs) suppression of both vibrational and electronic anisotropy beats on the excited state, but not vibrational beats on the ground electronic state. Recent measurements of conical intersection dynamics in a silicon napthalocyanine revealed anisotropic quantum beats that had to be assigned to asymmetric vibrations on the ground electronic state only [Farrow, D. A.; J. Chem. Phys. 2008, 128, 144510]. Small environmental asymmetries likely explain the observed absence of excited-state asymmetric vibrations in those experiments.     

A theoretical study of the effect of the molecular structure on the electronic transition probabilities after the 14Cβ-decay

Using wavefunctions from ab initio and INDO calculations, the probability that a molecule, of which a carbon atom undergoes β-decay, passes from its ground state to the ground state of the daughter ion, is studied. It is shown that the electronic part of this transition probability is almost independent of the molecular structure and of elec- tronic factors like aromaticity. Furthermore, the effect of β-decay on the electronic structure of a molecule turns out to be restricted to the direct environment of the decaying carbon atom.     

Exploring the Jahn-Teller and pseudo-Jahn-Teller conical intersections in the ethane radical cation

We report a theoretical account on the static and dynamic aspects of the Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) interactions in the ground and first excited electronic states of the ethane radical cation. The findings are compared with the experimental photoionization spectrum of ethane. The present theoretical approach is based on a model diabatic Hamiltonian and with the parameters derived from ab initio calculations. The optimized geometry of ethane in its electronic ground state (1A1g) revealed an equilibrium staggered conformation belonging to the D3d symmetry point group. At the vertical configuration, the ethane radical cation belongs to this symmetry point group. The ground and low-lying electronic states of this radical cation are of 2Eg, 2A1g, 2Eu, and 2A2u symmetries. Elementary symmetry selection rule suggests that the degenerate electronic states of the radical cation are prone to the JT distortion when perturbed along the degenerate vibrational modes of eg symmetry. The 2A1g state is estimated to be approximately 0.345 eV above the 2Eg state and approximately 2.405 eV below the 2Eu state at the vertical configuration. The symmetry selection rule also suggests PJT crossings of the 2A1g and the 2Eg electronic states of the radical cation along the vibrational modes of eg symmetry and such crossings appear to be energetically favorable also. The irregular vibrational progressions, with numerous shoulders and small peaks, observed below 12.55 eV in the experimental recording are manifestations of the dynamic (E x e)-JT effect. Our findings revealed that the PJT activity of the degenerate vibrational modes is particularly strong in the 2Eg-2A1g electronic manifold which leads to a broad and diffuse structure of the observed photoelectron band.     

An algebraic operator approach to electronic structure

In this paper, we introduce an algebraic approach to electronic structure calculations. Our approach constructs a Jordan algebra based on the second-quantized electronic Hamiltonian. From the structure factor of this algebra, we show that we can calculate the energy of the ground electronic state of the Hamiltonian operator. We apply our method to several generalized Hubbard models and show that we can usually obtain a significant fraction of the correlation energy for low-to-moderate values of the electronic repulsion parameter while still retaining the O(L(3)) scaling of the Hartree-Fock algorithm. This surprising result, along with several other observations, suggests that our algebraic approach represents a new paradigm for electronic structure calculations which opens up many new directions for research.     

Ground-state spectral features of molecular clusters RDX excited at THz frequencies

Calculations are presented of ground state resonance structure at THz frequencies for molecular clusters of the high explosive RDX using density functional theory (DFT). The spectral features of this resonance structure are due to coupling of resonance modes for ground state excitation. In particular, the coupling among ground state resonance modes provides a reasonable molecular level interpretation of spectral features associated with THz excitation of molecular clusters. THz excitation is associated with frequencies that are characteristically perturbative to molecular electronic states, in contrast to frequencies that can induce appreciable electronic state transition. Owing to this characteristic of THz excitation, one is able to make a direct association between local oscillations about ground-state minima of molecules, either isolated or comprising a cluster, and THz excitation spectra. The DFT software GAUSSIAN was used for the calculations of ground state resonance structure presented here.     

Photoreaction in BLUF receptors: proton-coupled electron transfer in the flavin-Gln-Tyr system

Photoinduced electron transfer from tyrosine to the flavin chromophore is involved in activation of BLUF (sensor of blue light using FAD) photoreceptors. We studied the electron transfer (ET) coupled with proton-transfer (PT) reactions, by means of XMCQDPT2//CASSCF calculations on a molecular cluster model. By defining a minimum active space in the CASSCF calculations, we could compute the entire photoreaction pathway. We find that the crossing of the locally excited and ET states is located along the flavin bond-stretching coordinate. The ET state is stabilized by a proton transfer from the electron donor to the electron acceptor. We mapped two different PT pathways from tyrosine to flavin via the conserved glutamine. These reactions generate a tautomeric form of glutamine. Along the PT coordinates, we find geometries where the ET and the electronic ground states degenerate. At the state crossing structures, either formation of the ground state biradical intermediate or a relaxation back to the Franck-Condon minimum takes places. The computed relaxation pathways reveal that the hydrogen bonds involving glutamine in the chromophore-binding pocket control BLUF photoefficiency.     

The Jahn-Teller effect in the triply degenerate electronic state of methane radical cation

A quantum dynamics study is performed to examine the complex nuclear motion underlying the first photoelectron band of methane. The broad and highly overlapping structures of the latter are found to originate from transitions to the ground electronic state, X(2)T(2), of the methane radical cation. Ab initio calculations have also been carried out to establish the potential energy surfaces for the triply degenerate electronic manifold of CH(4)(+). A suitable diabatic vibronic Hamiltonian has been devised and the nonadiabatic effects due to Jahn-Teller conical intersections on the vibronic dynamics investigated in detail. The theoretical results show fair accord with experiment.     

Time-dependent configuration-interaction calculations of laser-pulse-driven many-electron dynamics: controlled dipole switching in lithium cyanide

We report simulations of laser-driven many-electron dynamics by means of the time-dependent configuration interaction singles (doubles) approach. The method accounts for the correlation of ground and excited states, is capable of describing explicitly time-dependent, nonlinear phenomena, and is systematically improvable. Lithium cyanide serves as a molecular test system in which the charge distribution and hence the dipole moment are shown to be switchable, in a controlled fashion, by (a series of) laser pulses which induce selective, state-to-state electronic transitions. One focus of our time-dependent calculations is the question of how fast the transition from the ionic ground state to a specific excited state that is embedded in a multitude of other states can be made, without creating an electronic wave packet.     

Intramolecular energy transfer dynamics in 24-mode pyrazine by partitioning technique: A time-dependent approach

We investigate the intramolecular energy transfer dynamics of the S(2) excited electronic state of pyrazine due to radiationless transitions to energetically lower-lying singlet electronic states using a new time-dependent method. The femtosecond decay of S(2) to the S(1) excited state and the picosecond decay of S(2) to the ground electronic state S(0) are studied within an efficient methodology for computing the intramolecular dynamics in multidimensional configurational spaces. Our method is based on partitioning the full configuration space into the (small) subspace of interest Q and the rest, the subspace P. The exact equations of motion for the states in Q, under the influence of P, are derived in the time domain in form of a system of integrodifferential equations. Their numerical solution is readily obtained when the Q space consists of just a few states. Otherwise, the integrodifferential equations for the states in Q are transformed into a (larger) system of ordinary differential equations, which can be solved by a single diagonalization of a general complex matrix. The former approach is applied to study the pyrazine picosecond S(2)→S(0) dynamics and the latter is applied to the study of the ultrafast pyrazine S(2)→S(1) decay dynamics.     

Concerted hydrogen exchange tunneling in formic acid dimer

The effect of conformational relaxation on the quantum dynamics of the hydrogen exchange tunneling is studied in the D2h subspace of formic acid dimer. The fully coupled quantum dynamics in up to six dimensions are derived for potential energy hypersurfaces interpolated directly from hybrid density functional calculations with and without geometry relaxation. For a calculated electronic barrier height of 35.0 kJ/mol the vibrational ground state shows a tunneling splitting of 0.0013 cm(-1). The results support the vibrational assignment of Madeja and Havenith [J. Chem. Phys. 2002, 117, 7162-7168]. Fully coupled ro-vibrational calculations demonstrate the compatibility of experimentally observed inertia defects with in-plane hydrogen exchange tunneling dynamics in formic acid dimer.     

Potential energy landscape of the photoinduced multiple proton-transfer process in the green fluorescent protein: classical molecular dynamics and multiconfigurational electronic structure calculations

The green fluorescent protein proton wire operating upon photoexcitation of the internally caged chromophore is investigated by means of classical molecular dynamics and multiconfigurational electronic structure calculations. The structure of the proton wire is studied for the solvated protein, showing that the wire is likely to be found in a configuration ready to operate as soon as the chromophore is photoexcited, and leading to a total of three proton translocations in the vicinity of the chromophore. Multiconfigurational CASSCF and CASPT2 calculations provide a detailed overview of the energy landscape of the proton wire for the ground electronic state S0, the photoactive 1pi pi* state, and the charge-transfer 1pi sigma* state. The results allow discussion of the operation of the wire in terms of the sequence of proton-transfer events and the participation of each electronic state.     

Ab initio nonadiabatic dynamics study of ultrafast radiationless decay over conical intersections illustrated on the Na3F cluster

We present a theoretical approach for the ultrafast nonadiabatic dynamics based on the ab initio molecular dynamics carried out "on the fly" in the framework of the configuration interaction method combined with Tully's surface hopping algorithm for nonadiabatic transitions. This approach combined with our Wigner distribution approach allows us to perform accurate simulations of femtosecond pump-probe spectra in the systems where radiationless transitions among electronic states take place. In this paper we illustrate this by theoretical simulation of ultrafast processes and nonradiative relaxation in the Na(3)F cluster, involving three excited states and the ground electronic state. Furthermore, we show that our accurate simulation of the photoionization pump-probe spectrum is in full agreement with the experimental signal. Based on the nonadiabatic dynamics at high level of accuracy and taking into account all degrees of freedom, the nonradiative lifetime for the 1 (1)B(1) excited state of Na(3)F has been determined to be approximately 900 fs.     

Electronic degeneracies in symmetric (Jahn-Teller) and nonsymmetric aliphatic radical cations: global topology of sigma-bonded molecules

The ground-state potential surfaces of five aliphatic radical cations are investigated using a spin-pairing model. It is shown that the ground-state surface of an n-atomic system supports several stationary points (minima and transition states, including second-order ones). In addition, there are numerous nuclear configurations at which the ground state is electronically degenerate. The electronic degeneracies due to interactions between atoms bound to the same atom are either 2-fold (conical intersections) or 3-fold degenerate but not of a higher dimension. Each 3-fold degeneracy is accompanied by an even number of conical intersections (four or two). A systematic procedure for locating all of these nuclear configurations (that are in fact 3n - 8 or 3n - 11 dimensional hypersurfaces) is described. The model allows for the qualitative determination of the structure and charge distribution of the system at all of the stationary points and electronic degeneracies. Quantum chemical calculations confirm the predictions of the model, which is used to direct and facilitate the calculations.