The QM/MM approach for wavefunctions, energies and response functions within self-consistent field and coupled cluster theories

This paper presents the coupled cluster/molecular mechanics (CC/MM) and self-consistent field/molecular mechanics (SCF/MM) approaches for wavefunctions, energies and response properties. Two physically different theories are derived, the mean-field and the direct-field interaction approaches, together with expressions for the optimization condition of both variational and non-variational wavefunctions and energies. Also derived are the linear response functions at the CC/MM and SCF/MM levels of theory, and the expressions are compared with the vacuum response functions

Dipole and quadrupole moments of liquid water calculated within the coupled cluster/molecular mechanics method

We present the first study of dipole and quadrupole moments of liquid water calculated using coupled cluster/molecular mechanics (COMM) methods. CUMM 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

Polarizability of molecular clusters as calculated by a dipole interaction model

We have developed and investigated a dipole interaction model for calculating the polarizability of molecular clusters. The model has been parametrized from the frequency-dependent molecular polarizability as obtained from quantum chemical calculations for a series of 184 aliphatic, aromatic, and heterocyclic compounds. A damping of the interatomic interaction at short distances is introduced in such a way as to retain a traceless interaction tensor and a good description of the damping over a wide range of interatomic distances. By adopting atomic polarizabilities in addition to atom-type parameters describing the damping and the frequency dependence, respectively, the model is found to reproduce the molecular frequency-dependent polarizability tensor calculated with ab initio methods. A study of the polarizability of four dimers has been carried out: the hydrogen fluoride, methane, benzene, and urea dimers. We find in general good agreement between the model and the quantum chemical results over a wide range of intermolecular distances. To demonstrate the power of the model, the polarizability has been calculated for a linear chain of urea molecules with up to 300 molecules and one- and two-dimensional clusters o

Frequency-dependent polarizability of boron nitride nanotubes: A theoretical study

In the present work, we have calculated the static and frequency-dependent polarizability tensors for a series of single-walled boron nitride nanotubes and compared them with corresponding results for carbon nanotubes. The calculations have been performed by employing a dipole-dipole interaction model based on classical electrostatics and an Unsöld dispersion formula. In comparison, we have carried out ab intio calculations at the SCF level of the static polarizability of the smaller nanotubes with the STO-3G basis set. For the frequency-dependent polarizability of C60, we found excellent agreement among the most accurate SCF calculations in the literature, the interaction model, and experimental results. In particular, the frequency dependence is modeled accurately indicating that the interaction model is a useful tool for studying the frequency dependence of materials. For the nanotubes, we observe the same trends in the interaction model and in the SCF STO-3G results when the number of atoms is increased. However, the values obtained with the interaction model are about 100 % larger than the corresponding SCF STO-3G results, due to the small size of the STO-3G basis set. We also find that the boron nitride nanotubes have smaller magnitudes of the polarizability tensor components than the corresponding components for the carbon nanotubes with the same geometry and number of atoms. Furthermore, we find that the geometry of the tube has a large influence on the anisotropy of the polarizability components, whereas the mean polarizability remains almost unaffected when the geometrical configuration is modified. Finally, we observe a relatively small frequency dependence of the polarizability tensor of BN nanotubes. I

Solvent effects on rotatory strength tensors. 1. Theory and application of the combined coupled cluster/dielectric continuum model

In this article we present the first theoretical study of solvent effects on the rotatory strength tensor. The system chosen is solvated formaldehyde for which only one tensor element is nonvanishing, and the solvent is modeled as a linear, homogeneous, and isotropic dielectric continuum. We present results using both an equilibrium and a nonequilibrium description of the solvent. Four illustrative solvents (ethyl ether, acetone, methanol, and water) are considered together with the corresponding results for formaldehyde in vacuum. We utilize the following ab initio methods: the coupled cluster model including singles and doubles (CCSD) and the coupled cluster second-order approximate singles and doubles (M). Furthermore, we compare the coupled cluster results with the corresponding uncorrelated self-consistent-field (SCF) results. In addition to the rotatory strength tensor we also present solvent effects on the low-lying electronic excitation energies and corresponding ordinary intensities using both the length and velocity gauges. We find that both correlation and solvent effects have a significant influence on the transition properties. The introduction of the solvent is, in some cases, found to result in a sign change of the rotatory strength tensor elements which clearly demonstrates the importance of a proper description of the solvent influence on this property

A coupled cluster study of the oriented circular dichroism of the n ->pi* electronic transition in cyclopropanone and natural optical active related structures

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

Modelling of dissolved oxygen concentration during storage of packaged liquid doods

A mathematical model that combines oxygen uptake from the outside environment with oxygen consumption by oxidative reactions, in a liquid packed food during storage, was developed. The model was applied to orange juice aseptically packaged in Tetra Brik Aseptic cartons, during storage of up to 5 months at 4, 8, 20, 30, 40 and 50 °C. The parameters of the model, the oxygen mass transfer coefficient and the rate constant of consumption reactions, were estimated by fitting the model to the experimental data. The value of the rate constant estimated for the system tested in this work, was three orders of magnitude greater than the value of the oxygen mass transfer coefficient. The influence of temperature on the reaction rate was well described by an Arrhenius type equation, with an activation energy of 46 kJ/mole. This model was further tested with data reported in literature and it was found that it adequately describes the dissolved oxygen concentration changes during storage

Coupled cluster calculations of the optical rotation of S-propylene oxide in gas phase and solution

We present ab initio calculations of the optical rotation of S-propylene oxide in both gas phase and solution using the coupled cluster methodology combined with a dielectric continuum description of the solvent. The coupled cluster calculations are performed using the CCS, CC2, CCSD and CC3 methods. None of the presented gas phase results are in accord with the experimental sign of the optical rotation at 355 nm. Thereby, the experimental sign change between the gas phase and the cyclohexane,solution optical rotation at 355 nm is not reproduced theoretically. The vibrational effects are considered to be significant and may be of crucial importance in order to bring accordance between the calculated and the experimentally established sign of the gas phase optical rotation at 355 nm. (C) 2004 Elsevier B.V. All rights reserved

Polarizability of molecular clusters as calculated by a dipole interaction model

We have developed and investigated a dipole interaction model for calculating the polarizability of molecular clusters. The model has been parametrized from the frequency-dependent molecular polarizability as obtained from quantum chemical calculations for a series of 184 aliphatic, aromatic, and heterocyclic compounds. A damping of the interatomic interaction at short distances is introduced in such a way as to retain a traceless interaction tensor and a good description of the damping over a wide range of interatomic distances. By adopting atomic polarizabilities in addition to atom-type parameters describing the damping and the frequency dependence, respectively, the model is found to reproduce the molecular frequency-dependent polarizability tensor calculated with ab initio methods. A study of the polarizability of four dimers has been carried out: the hydrogen fluoride, methane, benzene, and urea dimers. We find in general good agreement between the model and the quantum chemical results over a wide range of intermolecular distances. To demonstrate the power of the model, the polarizability has been calculated for a linear chain of urea molecules with up to 300 molecules and one- and two-dimensional clusters o

Characterization of PAH/DPPG layer-by-layer films by VUV spectroscopy

The spectroscopic characterization of layer-by-layer (LbL) films containing liposomes is essential not only for determining the precise film architecture but also to guide the design of drug delivery systems. In this study we provide the first report of vacuum ultraviolet spectroscopy (VUV) characterization of LbL films made with liposomes from 1.2-dipalmitoyl-sn-Glycero-3-[Phospho-rac-(1-glycerol)] (Sodium Salt) (DPPG) alternated with poly(allylamine hydrochloride) (PAH). Measurements in the 6.0–9.5 eV range allowed us to identify the electronic transitions responsible for the spectra, which were assigned to carboxyl, hydroxyl and phosphate groups in DPPG while the PAH spectra were governed by electronic transitions in the amino groups. The surface mass density of the LbL films could be determined, from which the formation of a DPPG bilayer was inferred. This rupture of the liposomes into bilayers was confirmed with atomic force microscopy measurements. In subsidiary experiments we ensured that the UV irradiation in vacuum had negligible damage in the DPPG liposomes during the course of the VUV measurements. In addition to demonstrating the usefulness of VUV spectroscopy, the results presented here may be exploited in biological applications of liposome-containing films