The lowest 2A' excited state of the water-hydroxyl complex

Vertical and adiabatic excitation energies of the lowest (2)A(') excited state in the water-hydroxyl complex have been determined using coupled cluster, multireference configuration interaction, multireference perturbation theory, and density-functional methods. A significant redshift of about 0.4 eV in the vertical excitation energy of the complex compared to that of the hydroxyl radical monomer is found with the coupled cluster calculations validating previous results. Electronic excitation leads to a structure with near-equal sharing of the hydroxyl hydrogen by both oxygen atoms and a concomitantly large redshift of the adiabatic excitation energy of approximately 1 eV relative to the vertical excitation energy. The combination of redshifts ensures that the electronic transition in the complex lies well outside the equivalent excitation in the hydroxyl radical monomer. The complex is approximately five times more strongly bound in the excited state than in the ground state.     

Does chlorine peroxide absorb below 250 nm?

Low-lying singlet and triplet electronic excited states of ClOOCl are presented. Calculations of the excitation energies and oscillator strengths are reported using excited state coupled cluster response methods, as well as the complete active space self-consistent field method with the full Breit-Pauli spin-orbit operator. These calculations predict that for ClOOCl there should be a weakly absorbing triplet state lying below the lowest absorbing singlet excited state. This state is predicted to have an absorption maximum at about 385 +/- 25 nm. This lowest triplet state is calculated to be dissociative and leads to ClOO+Cl.     

Gas phase solvatochromic effects of phenol and naphthol photoacids

A quantum chemical study of spectral shifts by single molecule solvation of phenol, α-naphthol, and β-naphthol is presented. The methods employed include the equation-of-motion coupled cluster, the similarity transformed equation-of-motion coupled cluster, single excitation configuration-interaction, and time-dependent density functional theory. Based on the calculations, there is no evidence that there is significant charge-transfer between the solute and the solvent. Instead, it appears that the observed solvation redshift is due to the nature of the excited state on the solute molecule.     

Coupled cluster theory with the polarizable continuum model of solvation

The environment may significantly affect molecular properties. Thus, it is desirable to account explicitly for these effects on the wave function and its derivatives, especially when the latter are evaluated with accurate methods, such as those belonging to coupled cluster (CC) theory. In this tutorial review, we discuss how to combine CC methods with the polarizable continuum model of solvation (PCM). We describe useful approximations that include the solvent response to the correlation and excited state equations while maintaining the computational cost comparable to in vacuo calculations. Although applied to PCM, the theoretical framework presented in this review is general and can be used with any polarizable embedding model. Representative applications of the CC-PCM method to ground and excited state properties of solvated molecules are presented, and comparisons with experiment, and between the full and approximate schemes are discussed.     

Generation of potential energy curves for the X1Sigma(+)g, B1Delta(+)g, and B'1Sigma(+)g states of C2 using the effective valence shell Hamiltonian method

Calculations of the ground and excited state potential energy curves of C2 using the third-order effective valence Hamiltonian (Hv3rd) method are benchmarked against full configuration interaction and other correlated single-reference perturbative and nonperturbative theories. The large nonparallelity errors (NPEs) exhibited even by state-of-art coupled cluster calculations through perturbative triples indicate a serious deficiency of these single-reference theories. The Hv method, on the other hand, produces a much reduced NPE, rendering it a viable approximate many-body method for accurately determining global ground and excited state potential energy curvessurfaces.     

Explicitly time-dependent coupled cluster singles doubles calculations of laser-driven many-electron dynamics

We report explicitly time-dependent coupled cluster singles doubles (TD-CCSD) calculations, which simulate the laser-driven correlated many-electron dynamics in molecular systems. Small molecules, i.e., HF, H(2)O, NH(3), and CH(4), are treated mostly with polarized valence double zeta basis sets. We determine the coupled cluster ground states by imaginary time propagation for these molecules. Excited state energies are obtained from the Fourier transform of the time-dependent dipole moment after an ultrashort, broadband laser excitation. The time-dependent expectation values are calculated from the complex cluster amplitudes using the corresponding configuration interaction singles doubles wave functions. Also resonant laser excitations of these excited states are simulated, in order to explore the limits for the numerical stability of our current TD-CCSD implementation, which uses time-independent molecular orbitals to form excited configurations.     

Hybrid coupled cluster and molecular dynamics approach: application to the excitation spectrum of cytosine in the native DNA environment

Evolution of the excited state energies of cytosine base in the native DNA environment was investigated using a hybrid coupled cluster and classical molecular dynamics approach. The time averaged excitation energies obtained with the variant of the completely renormalized equation-of-motion with singles, doubles, and non-iterative triples approach that includes a bulk of the correlation effects for excited states, are compared with the analogous calculations in the gas phase. Significant blue shifts for the two lowest singlet excitation energies can be observed as a result of the interaction of the quantum system with the surrounding environment.     

Transition strengths and first-order properties of excited states from local coupled cluster CC2 response theory with density fitting

A new ab initio method for calculating transition strengths and orbital-unrelaxed first-order properties of singlet ground and excited states of extended molecular systems is presented. It is based on coupled cluster response theory at the level of the CC2 model with local approximations introduced to the doubles-excitation part of the wave function. Density fitting is employed for the calculation of the electron repulsion integrals, so that--with the exception of doubles amplitudes--only three-indexed objects do occur in the formalism. The new method was tested by performing calculations for a set of various molecules and excited states and by comparing the results with corresponding canonical (nonlocal) calculations. It turned out that for calculating transition strengths and properties of excited states the ordinary Boughton-Pulay domains are insufficient in numerous cases. To circumvent this problem a new scheme for extending domains is proposed, which is based on the solution of the coupled perturbed localization and Hartree-Fock equations. When such extended domains are used, a satisfactory agreement between canonical and local results is achieved.     

The importance of spin-orbit coupling and electron correlation in the rationalization of the ground state of the CUO molecule

We present calculations at the relativistic coupled cluster theory that predict the (1)Sigma(0) (+) ground state of CUO to lie 58.2 kJ/mol below the first excited state, (3)Phi(2). This can be contrasted with the outcome of earlier density functional theory and complete active space second order perturbation theory (CASPT2) calculations that both predicted a (3)Phi(2) ground state upon inclusion of spin-orbit coupling in the calculations. Our result gives further justification to the interpretation of the measured frequency shifts of this species in various noble gas matrices as being caused by significant interaction between the uranium and the heavier noble gas atoms.     

Photoinduced dissociation of water and transport of hydrogen between silver clusters

Theoretical electronic structure calculations are reported for the dissociation of water adsorbed on a 31-atom silver cluster, Ag31, and subsequent transfer of a H to a second Ag31 cluster leaving OH on the first cluster. Both ground and excited electronic state processes are considered for two choices of Ag cluster separation, 6.35 and 7.94 A, on the basis of preliminary calculations for a range of separation distances. The excited electronic state of interest is formed by photoemission of an electron from one Ag cluster and transient attachment of the photoemitted electron to the adsorbed water molecule. A very large energy barrier is found for the ground-state process (3.53 eV at a cluster separation of 6.35 A), while the barrier in the excited state is small (0.38 eV at a cluster separation of 6.35 A). In the excited state, partial occupancy of an OH antibonding orbital facilitates OH stretch and concomitant movement of the negatively charged OH toward the electron-hole in the metal cluster. The excited-state pathway for dissociation of water and transfer of H begins with the formation of an excited electronic state at 3.59-3.82 eV. Stretch of the OH bond occurs with little change in energy (0.38-0.54 eV up to a stretch of 1.96 A). In this region of OH stretch the molecule must return to the ground-state potential energy surface to fully dissociate and to transfer H to the other Ag cluster. Geometry optimizations are carried out using a simplex algorithm and a semigrid method. These methods allow the total energy to be calculated directly using configuration interaction theory.     

The structure of 5-cyanoindole in the ground and the lowest electronically excited singlet states, deduced from rotationally resolved electronic spectroscopy and ab initio theory

The structure and electronic properties of the electronic ground and the lowest excited singlet states of 5-cyanoindole (5CI) were determined using rotationally resolved spectroscopy of the vibrationless electronic origin of 5CI. In contrast to most other indole derivatives, the lowest excited state of 5CI is determined to be of L(a) character. The conventional approximate coupled cluster singles and doubles model (CC2) fails to describe the geometry of the excited state correctly. Nevertheless, scaling the spin components of equal and opposite spins within the CC2 model as proposed by Hellweg et al. (Phys. Chem. Chem. Phys., 2008, 10, 1159) resulted in very good geometry parameters for the excited state.     

Solvatochromic shifts of Br2 and I2 in water cages of type 512, 51262, 51263, and 51264

Despite the relatively small size of molecular bromine and iodine, the physicochemical behavior in different solvents is not yet fully understood, in particular when excited‐state properties are sought. In this work, we investigate isolated halogen molecules trapped in clathrate hydrate cages. Relativistic supermolecular calculations reveal that the environment shift to the excitation energies of the (nondegenerate) states and lie within a spread of 0.05 eV, respectively, suggesting that environment shifts can be estimated with scalar‐relativistic treatments. As even scalar‐relativistic calculations are problematic for excited‐state calculations for clathrates with growing size and basis sets, we have applied the subsystem‐based scheme frozen‐density embedding, which avoids a supermolecular treatment. This allows for the calculation of excited states for extended clusters with coupled‐cluster methods and basis sets of triple‐zeta quality with additional diffuse functions mandatory for excited‐state properties, as well as a facile treatment at scalar‐relativistic exact two‐component level of theory for the heavy atoms bromine and iodine. This simple approach yields scalar‐relativistic estimates for solvatochromic shifts introduced by the clathrate cages. © 2015 Wiley Periodicals, Inc.     

Combined experimental-theoretical study of the lower excited singlet states of paravinyl phenol, an analog of the paracoumaric acid chromophore

The low-lying excited singlet states of paravinyl phenol (pVP) are investigated experimentally and theoretically paying attention to their similarity to excited states of paracoumaric acid, the chromophore of the photoactive yellow protein (PYP). Resonance enhanced multiphoton ionization and laser induced fluorescence spectroscopic techniques are employed to obtain supersonically cooled, vibrationally resolved excitation and emission spectra related to the lowest (1)A'(V') excited state of pVP. Comprehensive analyses of the spectral structures are carried out by means of the equation-of-motion coupled cluster singles and doubles and time dependent density functional theory methods in combination with the linear vibronic coupling model and Franck-Condon calculations. The assignments of the spectral patterns are given, mostly in terms of excitations of totally symmetric modes. Weak activity of the non-totally-symmetric modes indicates low probability of photochemical processes in the Franck-Condon region of the (1)A'(V') state. The second (1)A'(V) and third (1)A" (Ryd) excited states of pVP are characterized with regard to their electronic structure, properties, and effects of geometry relaxations. The lengthening of the double bond relevant to the trans-cis isomerization of the PYP chromophore is found for the (1)A'(V) state. A possibility of photochemical processes and strong vibronic interactions in this state can be expected. The theoretical results for the (1)A"(Ryd) state predict that dissociation with respect to the O-H bond is possible.     

Relativistic DMRG calculations on the curve crossing of cesium hydride

Over the past few years, it has been shown in various studies on small molecules with only a few electrons that the density-matrix renormalization group (DMRG) method converges to results close to the full configuration-interaction limit for the total electronic energy. In order to test the capabilities of the method for molecules with complex electronic structures, we performed a study on the potential-energy curves of the ground state and the first excited state of 1sigma+ symmetry of the cesium hydride molecule. For cesium relativistic effects cannot be neglected, therefore we have used the generalized arbitrary-order Douglas-Kroll-Hess protocol up to tenth order, which allows for a complete decoupling of the Dirac Hamiltonian. Scalar-relativistic effects are thus fully incorporated in the calculations. The potential curves of the cesium hydride molecule feature an avoided crossing between the ground state and the first excited state, which is shown to be very well described by the DMRG method. Compared to multireference configuration-interaction results, the potential curves hardly differ in shape, for both the ground state and the excited state, but the total energies from the DMRG calculations are in general consistently lower. However, the DMRG energies are as accurate as corresponding coupled cluster energies at the equilibrium distance, but convergence to the full configuration-interaction limit is not achieved.     

Wavelength dependence of nitrate radical quantum yield from peroxyacetyl nitrate photolysis: experimental and theoretical studies

The photolysis wavelength dependence of the nitrate radical quantum yield for peroxyacetyl nitrate (CH(3)C(O)OONO(2), PAN) is investigated. The wavelength range used in this work is between 289 and 312 nm, which mimics the overlap of the solar flux available in the atmosphere and PAN's absorption cross section. We find the nitrate radical quantum yield from PAN photolysis to be essentially invariant; Phi(NO3)(PAN) = 0.30 +/- 0.07 (+/-2sigma) in this region. The excited states involved in PAN photolysis are also investigated using ab initio calculations. In addition to PAN, calculations on peroxy nitric acid (HOONO(2), PNA) are performed to examine general photochemical properties of the -OONO(2) chromophore. Equation of motion coupled cluster calculations (EOM-CCSD) are used to examine excited state energy gradients for the internal coordinates, oscillator strengths, and transition energies for the n --> pi* transitions responsible for the photolysis of both PNA and PAN. We find in both molecules, photodissociation of both O-O and O-N bonds occurs via excitation to predissociative electronic excited states and subsequent redistribution of that energy as opposed to directly dissociative excitations. Comparison and contrast between experimental and theoretical studies of HOONO(2) and PAN photochemistry from this and other work provide unique insight on the photochemistry of these species in the atmosphere.     

Coupled cluster calculation of electron affinities of LiF

The three lowest states of LiF⁻ have been studied using a combination of numerical Hartree-Fock orbitals and Slatertype orbitals as a basis set for coupled cluster calculations. The ground state electron affinity is 0.31 eV, while in the first excited state of the ion the extra electron is bound by 0.38 × 10⁻² eV and in the next state by 0.43 × 10⁻⁴ eV. The last value differs from Garrett's non-adiabatic estimate of 0.12 × 10⁻⁴ eV, suggesting a breakdown of the Born-Oppenheimer approximation.     

Two-dimensional H2O-Cl2 and H2O-Br2 potential surfaces: an ab initio study of ground and valence excited electronic states

All electron ab initio calculations for the interaction of H2O with Cl2 and Br2 are reported for the ground state and the lowest triplet and singlet Pi excited states as a function of both the X-X and O-X bond lengths (X = Cl or Br). For the ground state and lowest triplet state, the calculations are performed with the coupled cluster singles, doubles, and perturbative triple excitation level of correlation using an augmented triple-zeta basis set. For the 1Pi state the multireference average quadratic coupled cluster technique was employed. For several points on the potential, the calculations were repeated with the augmented quadruple-zeta basis set. The ground-state well depths were found to be 917 and 1,183 cm-1 for Cl2 and Br2, respectively, with the triple-zeta basis set, and they increased to 982 and 1,273 cm-1 for the quadruple-zeta basis set. At the geometry of the ground-state minimum, the lowest energy state corresponding to the unperturbed 1Pi states of the halogens increases in energy by 637 and 733 cm-1, respectively, relative to the ground-state dissociation limit of the H2O-X2 complex. Adding the attractive ground-state interaction energy to that of the repulsive excited state predicts a blue-shift, relative to that of the free halogen molecules, of approximately 1,600 cm-1 for H2O-Cl2 and approximately 2,000 cm-1 for H2O-Br2. These vertical blue-shifts for the dimers are greater than the shift of the band maximum upon solvation of either halogen in liquid water.     

Solvent effects on the n-->pi* electronic transition in formaldehyde: a combined coupled cluster/molecular dynamics study

We present a study of the blueshift of the n-->pi* electronic transition in formaldehyde in aqueous solution using a combined coupled cluster/molecular mechanics model including mutual polarization effects in the Hamiltonian. In addition, we report ground and excited state dipole moments. Configurations are generated from molecular dynamics simulations with two different force fields, one with and one without an explicit polarization contribution. A statistical analysis using 1200 configurations is presented. Effects of explicit polarization contributions are found to be significant. It is found that the main difference in the effects on the excitation energies arises from the fact that the two force fields result in different liquid structures, and thus a different set of configurations is generated for the coupled cluster/molecular mechanics calculations.     

Photoluminescence of oxygen-containing surface defects in germanium oxides: a theoretical study

Photoabsorption and photoluminescence properties of nonbridging oxygen -O-Ge[triple bond](NBO), -OO-Ge[triple bond] (peroxy radical), O=Ge=, and (O2)Ge= defects in germanium oxides have been investigated by high-level ab initio calculations. Geometry optimization for excited electronic states of model clusters simulating these defects was carried out at the complete-active-space self-consistent-field level, and relative energies were calculated by various methods including time-dependent density-functional theory, outer-valence Green's functions, equation-of-motion coupled cluster theory with single and double excitations, symmetry-adapted cluster configuration interaction, multireference second-order perturbation theory, and multireference configuration interaction. The results demonstrate that the considered excited states of the aforementioned defects normally exhibit large Stokes shifts and that, with few exceptions, UV photoabsorption is accompanied by red or IR photoluminescence.