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We present an efficient approach for estimating the construction of molecular solar thermal systems in the context of hybrid solar thermal devices. Photochromic molecules can store thermal energy by structural changes, and the storage time depends on the chosen molecular system and temperature. The simulation framework can predict thermal energy storage rates, conversion percentages, and temperature increases. The model is based on both microscopic and macroscopic properties, which all have an impact on the energy storage efficiency. The microscopic parameters include the molecular storage energies, the quantum yield for the photoisomerization, the absorption properties for the photochromic molecule, and the lifetime of the metastable molecule, hence the possible storage time. The macroscopic parameters of the hybrid device include engineering specifications like the device dimensions, device material, the solar concentration factor, the flow rate of the fluid, and the start concentration of the photochromic molecule.

The transition from fossil fuels to renewable energy sources requires technologies that are capable of both producing and storing energy. Attractive candidates that combine these two features are molecular photoswitches used for molecular solar thermal (MOST) energy storage. One of the most promising MOST systems is the norbornadiene/quadricyclane (NBD/QC) couple. We present the first density functional theory benchmark study of a NBD/QC derivative in vacuum with respect to novel gas-phase experiments. It was found that the M06-2X/def2-TZVP and M06-2X/6-311+G(d) level of theories produced the most accurate results with respect to the maximum absorption wavelength and the energy density.

The effect of adding electron donating and withdrawing groups on the dihydroazulene (DHA)/vinylheptafulvene (VHF) photochromic system has been investigated using Raman spectroscopy in CS2 solutions. The photoswitching between DHA and VHF is often characterised with UV-Vis spectroscopy. However, Raman spectroscopy can also be used for this purpose and give structural insight, as the light induced ring-opening from DHA to VHF causes changes in the CN-stretching frequencies. The CN-stretching frequencies in DHA and VHF are isolated and optimal for the identification of DHA and VHF. The DHA system is also investigated in the solid state.

Single-molecule transistor experiments offer the ultimate probe of a molecule’s redox chemistry. With a combined quantum mechanics/molecular mechanics computational method and an electrostatic model of the junction, the Coulomb blockade diamonds can be reconstructed. We present computations on 1,4-diamino-benzene (OPVN) in a molecular transport junction, and focus on the chemical features such as electrostatic stabilization, importance of binding group, and binding distance. Thus the explicit construction of the Coulomb blockade diamonds offers a method for direct comparison between different molecular junctions and thus also with experiments.

Utilizing a comprehensive test set of 205 clusters of atmospheric relevance, we investigate how different DFT functionals (M06-2X, PW91, ω B97X-D) and basis sets (6-311++G(3df,3pd), 6-31++G(d,p), 6-31+G(d)) affect the thermal contribution to the Gibbs free energy and single point energy. Reducing the basis set used in the geometry and frequency calculation from 6-311++G(3df,3pd) → 6-31++G(d,p) implies a significant speed-up in computational time and only leads to small errors in the thermal contribution to the Gibbs free energy and subsequent coupled cluster single point energy calculation.

In this study, we present the changes in molecular solar thermal energy storage properties of the 3-amino-substituted-dihydroazulene/vinylheptafulvene photo- and thermochromic system induced by physisorption of the system onto different nanoparticles. The calculations are carried out with a combined quantum mechanical/molecular mechanical method where the molecules are described by density functional theory. The nanoparticles are represented using molecular mechanics. We find that both the energy storage capability and back reaction barrier for the system can be influenced by the nanoparticles. The influence depends on the type of nanoparticle, the orientation of the molecular system, and the molecule-cluster separation.

We utilize a density functional theory (DFT) based quantum mechanical/molecular mechanical (QM/MM) approach for the investigation of charge transfer (CT) excitation energies in molecule–metal systems. We perform calculations on a pyridine– Ag 20 system treating the most distant, with respect to pyridine, silver atoms using molecular mechanics by assigning atomic polarizabilities to each of these silver atoms. We investigate how additional silver layers affect the CT excitation energy and conclude that the CT excitation energy levels off while enlarging the silver cluster.

We present a combined quantum mechanics/molecular mechanics and quantum statistical investigation of the interactions between a Molecule (SO2) and an aerosol particle including rate constants for the uptake process. A coupled cluster/molecular mechanics method including explicit polarization is used along with a quantum statistical method for calculating sticking coefficients. The importance of the polarization of the classical subsystem (the aerosol particle), the size of the classical subsystem and the size of one-electron basis sets are studied. The interaction energy is divided into van der Waals, electrostatic and polarization contributions. Relevant binding sites for the evaluation of the sticking coefficient are identified. These are classified into three groups according to the strength of the molecule-aerosol particle interaction energy. The identification of binding sites provides relevant information used in the quantum statistical method and thereby knowledge of the magnitude of the sticking coefficients for the different binding sites along with the total rates for the uptake processes between the aerosol particle and the SO2 molecule. (Less)

The theory of a hybrid quantum mechanics/molecular mechanics (QM/MM) approach for gauge-origin independent calculations of the molecular magnetizability using Hartree-Fock or Density Functional Theory is presented. The method is applied to liquid water using configurations generated from classical Molecular Dynamics simulation to calculate the statistical averaged magnetizability. Based on a comparison with experimental data, treating only one water molecule quantum mechanically appears to be insufficient, while a quantum mechanical treatment of also the first solvation shell leads to good agreement between theory and experiment. This indicates that the gas-to-liquid phase shift for the molecular magnetizability is to a large extent of non-electrostatic nature. (c) 2007 Elsevier B.V. All rights reserved. (Less)

Infrared and Raman spectra of three drug molecules, aspirin, caffeine and ibuprofen, in gas phase and in aqueous solution have been simulated using hybrid density functional theory. The long range solvent effect is modelled by the polarizable continuum model, while the short range hydrogen bonding effects are taken care of by the super-molecular approach with explicit inclusion of water molecules. The calculated spectra are found to compare well with available experimental results. The agreement obtained make grounds for proposing theoretical modeling as a tool for characterizing changes in the bonding environments of drug molecules in terms of particular variations in their IR and Raman spectra.

Hydrogen bonding energies of several drug molecules have been calculated using hybrid density functional theory with inclusion of basis set superposition error corrections. The calculated total hydrogen bonding energy of each drug molecule has been compared with the result of a conceptually simple additive model, from which the summation of hydrogen bonding energies of individual polar groups present in the drug molecule are considered. It is shown that the validity of the additive model is strongly conditional, and to some extent predictable: In cases where the hydrogen bonding group is isolated the addition model can be of relevance, while in cases where the hydrogen bonding groups are interconnected through π-conjugation rings or chains of the drug molecules it introduces substantial errors. It is suggested that such strong cooperative effects of hydrogen bonds should always be taken into account for evaluation of the hydrogen bonding energies of drug molecules.

We present a theoretical study of the vertical electronic excitation energies of the alanine zwitterion in an aqueous environment. The study is based on calculations using the combined coupled cluster/molecular mechanics (CC/MM) method. The CC/MM results (using different water potentials) are compared to calculations based on density functional theory (DFT) using the B3LYP functional. Special attention is payed to the lowest n O → π CO ∗ transition and higher lying states including the π CO → π CO ∗ . We find that the spectra obtained using CC/MM are in good agreement with experiments. On the other hand, less satisfactory results are found for the spectrum calculated using DFT.

In this Letter, structure and properties of gas-phase dihydrates of nitric acid (NAD) have been studied using density functional theory. Three equilibrium NAD structures, including two new isomers have been determined and their structural data, vibrational transitions, infrared-spectra, dipole moments, and thermodynamics properties have been investigated. The results obtained were used to explore the formation pathways for gas-phase di and trihydrates (NAT) of the nitric acid – key components of the ice crystals formed in polar stratospheric clouds. The present study leads us to conclude that nitric acid crystals under stratospheric conditions are formed mainly from β-NAD and β-NAT.

In this letter we document the possibility of the existence of a second molecular configuration for nitric acid trihydrates. Density functional theory (DFT) methods have been used for studying the nitric acid trihydrates α and β-NAT conformations, their spectroscopic and thermodynamics properties and dipole moments have been calculated. This study describes the gas–solid phase transition of the NAT and provides two possible pathways for the molecular structure transformation between α and β-NAT.

We present calculations of ground state and excitation energies for ethene between metal electrodes. The model determines the effects of the electrodes by incorporating a heterogeneous and structured solvation model with the quantum mechanical description of the molecular system. The effects of the electrodes are of electrostatic origin and an external bias can be applied enabling a close connection to the actual physical process related to the measurements of charge transport in molecules located between electrodes.

We present an ab initio study of the n --> pi(*) electronic excitation energy together with the corresponding oscillator strength and rotatory strength tensor for two optically active cyclopropanone related structures: S,S-dimethylcyclopropanone and R,R-t-butylcyclopropanone. We discuss and compare the available experimental data for the n --> pi(*) electronic excitation energy and rotatory strength with the theoretical calculations. (C) 2004 Elsevier B.V. All rights reserved.

In this paper we employ a recently developed method which combines coupled cluster and molecular mechanics (CC/MM) theories to study the effects of solvation on some molecular properties of liquid water. We focus on the effect of varying the parameters used to define the classically treated molecules, i.e. partial charges and point dipole polarizabilities. Furthermore, we define a set of ‘polarization charges’ which include the effects of a polarizable environment (the MM system) and correctly reproduces the ground state energy of the total system. However, we find that for these polarization charges that specific ground and excited state molecular properties are not reproduced correctly. Specially, the polarizability is very difficult to model using a simple point charge model with charges confined to the MM atoms. Furthermore, we study some excited state properties such as electronic excitation energies and corresponding dipole transition moments of liquid water. Finally, we explore the effect of introducing few or many classically treated molecules in the CC/MM calculations.

We present the first study of dipole and quadrupole moments of liquid water calculated using coupled cluster/molecular mechanics (CC/MM) methods. CC/MM methods are used to calculate the total dipole moment of the water dimer and the results are compared to the corresponding ab initio quantum mechanical calculations. For liquid water we find that the introduction of polarization effects are very important for an accurate determination of dipole and quadrupole moments. Furthermore, we find that neglecting the correlation effects in the quantum mechanical part of the system leads to an overestimation of the interaction between the two sub-systems.

A recently presented dielectric continuum model is extended to a metal coated with two dielectric layers. The potential energies, binding energies and wave functions of the image potential states are presented as a function of the electron affinities and dielectric constants of the dielectric media. The double dielectric continuum model (DDCM) is compared with the single dielectric continuum model (SDCM). The heterogeneous reaction field solvation model enables the computation of the dielectric constant of a molecular layer adsorbed on the surface of a perfect conductor. The dielectric constant is presented as a function of the orientation of the molecule relative to the surface of the perfect conductor. The computed dielectric constants are used for computing the binding energies and wave functions of the lowest lying electron states of a Cu(1 1 1)-surface covered with a monolayer or bilayer of benzene. The estimated binding energies are compared with the ones measured experimentally by Velic et al. [J. Chem. Phys. 109 (1998) 9155].

PM3(tm) semi-empirical geometry optimization was applied to a variety of corrins. Notably, we present here the first quantum-mechanical equilibrium structures of cobalamins. Adenosylcobalamin (AdoB12) and methylcobalamin (MeB12) in their native and 5,6-dimethylbenzimidazole (DMB)-off, imidazole-on forms, and cob(II)alamin were subjected to full geometry optimization. The study presents three levels of in vacuo B12 models: the cobalamins themselves, the corrins, and the cobaloximes. We compare PM3(tm) data for corrins with other kinds of theory and experiment. The cobaloximes were less good B12 models due to their charge and the flexibility of the bis(dimethylglyoximato) framework. We found that the nucleotide loop and the amide chains had minor effect on the corrin geometry. This suggests that corrins are suitable as B12 models and that cobalamins are not particularly strained in comparison to the simpler corrin analogues. We have evaluated the differential preference of the two coenzymes for the two axial ligands imidazole and 5,6-dimethylbenzimidazole. AdoB12 prefers the imidazole ligand relative to MeB12 by 27 kcal mol−1. Finally, calculated electron affinities indicate that DMB serves as a delocalization reservoir, increasing electron affinity by 59 kcal mol−1 relative to ammonia. Hence, apo-enzymatic control of the CoN(ax) bond length may be a means to govern the homolysis–heterolysis selectivity, rendering delocalization of the SOMO electron possible during homolysis.

A method based on the combined use of classical Maxwell field theory and modern quantum chemistry reaction field theory is outlined for determining the spherical cavity radius of pure liquids in the Onsager reaction field model. For each electronic structure model the method provides a unique value for the radius of the spherical cavity.

We investigate basis set effects for the direct calculations of coupling elements for electron transfer reactions. We discuss the advantages of direct methods versus indirect methods. Appropriate description of the coupling elements for electron transfer reactions at large intermolecular separation requires flexible basis sets.

A dynamical model for chemical reactions in solution is described. A reacting system and a larger number of solvent molecules are enclosed within a spherical cavity. The molecular reaction dynamics is followed by solving the classical equations of motion of the molecular system within the cavity. The cavity is immersed into a dielectric phonon medium. The rate constant is estimated to be about 3.0 X 10 −30 cm 3 /s at room temperature.

We have calculated two-photon electronic transitions for sulfuric acid ( H 2 SO 4 ) in the ultraviolet region using coupled cluster quadratic response theories combined with correlation consistent basis sets. We have investigated how the intensity of two-photon electronic transitions in H 2 SO 4 depends on the wavelength of the photons absorbed. We find that the intensity is more or less constant in the wavelength region in which the density of solar flux is of any importance. We have calculated the J-values for the photodissociation of H 2 SO 4 with absorption of two photons and compared these with previously calculated J-values for one-photon absorption. We find that two-photon electronic transitions of H 2 SO 4 are not a significant mechanism in the photodissociation of H 2 SO 4 .

A model for calculating X-ray photoelectron spectral functions in physisorbed systems is proposed and applied to molecular oxygen (O 2 ) on graphite. The polarization interaction is found to be selective with respect to the final state and with respect to the orientation of the adsorbate leading to small but distinct changes in the vibronic coupling constants and spectral functions. The characteristics of the X-ray photoelectron spectrum of free oxygen are investigated at several levels of approximation.

We explore red and blue shifts of low-lying electronic excitation bands accompanying solvation by means of a self-consistent reaction field (SCRF) theory where the chromophore is treated quantum mechanically. A previously devised multi-configuration SCRF (MCSCRF) method has been enhanced to encompass restricted active space MCSCF wave functions, D 2h point group symmetries and higher multipole expansions of the charge and the reaction fields. Applications focus on the series of azabenzenes which show large solvation blue shifts for π→π* transitions but small solvation red shifts for n→π* transitions. It is found that the magnitude of the reaction field contributions to the shifts are of about the same order of magnitude as the polarization contri- butions and that the multipole expansion of the polarizing field cannot be truncated to include only a dipole. The solvation shifts are found to be sensitive to the parameters of the model, such as cavity radius and dielectric constant, but fairly insensitive to the parametrization of the solute wave functions. The role of symmetry broken, local descriptions of the excitations is discussed.

We employ a self-consistent reaction field method to explore the validity of the equivalent core approximation in conjunction with a Born-Haber analysis of liquids and solutions. Sample calculations show that the error from the equivalent core approximation is negligible compared to solvation binding energy shifts but that it may not be negligible in comparison with the reorganization contributions to solvation energies derived from a Born-Haber cycle.