Gunnar Karlström

Molcas: a program package for computational chemistry.

The program system MOLCAS is a package for calculations of electronic and structural properties of molecular systems in gas, liquid, or solid phase. It contains a number of modern quantum chemical methods for studies of the electronic structure in ground and excited electronic states. A macromolecular environment can be modeled by a combination of quantum chemistry and molecular mechanics. It is further possible to describe a crystalline material using model potentials. Solvent effects can be treated using continuum models or by combining quantum chemical calculations with molecular dynamics or Monte-Carlo simulations. MOLCAS is especially adapted to treat systems with a complex electronic structure, where the simplest quantum chemical models do not work. These features together with the inclusion of relativistic effects makes it possible to treat with good accuracy systems including atoms from the entire periodic system. MOLCAS has effective methods for geometry optimization of equilibria, transition states, conical intersections, etc. This facilitates studies of excited state energy surfaces, spectroscopy, and photochemical processes.

Phase equilibria in the phosphatidylcholine-cholesterol system.

A thermodynamic and a microscopic interaction model are proposed to describe the phase equilibria in the phosphatidylcholine-cholesterol system. The model calculations allow for a solid phase with conformationally ordered acyl chains and liquid phases with conformationally ordered as well as disordered chains. The resulting phase diagram is in excellent agreement with the experimental phase diagram for dipalmitoylphosphatidylcholine bilayers with cholesterol as determined by a recent NMR and calorimetry study. It is thus demonstrated that the phase behaviour of phosphatidylcholine-cholesterol mixtures can be rationalized using only a few basic assumptions: (i) Cholesterol interacts favourably with phosphatidylcholine chains in an extended conformation, (ii) the main transition of pure phosphatidylcholine bilayers takes place in terms of translational variables as well acyl-chain conformational variables, and (iii) cholesterol disturbs the translational order in the crystalline (gel) state of phosphatidylcholine. These results suggest that the occurrence of specific phosphatidylcholine-cholesterol complexes is not implied by the experimental thermodynamic data.

Local properties of quantum chemical systems: The LoProp approach

A new method is presented, which makes it possible to partition molecular properties like multipole moments and polarizabilities, into atomic and interatomic contributions. The method requires a subdivision of the atomic basis set into occupied and virtual basis functions for each atom in the molecular system. The localization procedure is organized into a series of orthogonalizations of the original basis set, which will have as a final result a localized orthonormal basis set. The new localization procedure is demonstrated to be stable with various basis sets, and to provide physically meaningful localized properties. Transferability of the methyl properties for the alkane series and of the carbon and hydrogen properties for the benzene, naphtalene, and anthracene series is demonstrated.

Driving forces for phase separation and partitioning in aqueous two-phase systems

A set of simple analytical equations, derived from the Flory-Huggins theory, are used to identify the dominant driving forces for phase separation and solute (e.g., protein) partitioning, in the absence and presence of added electrolyte, in every general class of aqueous two-phase systems. The resulting model appears to capture the basic nature of two-phase systems and all trends observed experimentally. Case studies are used to identify fundamental differences in and the magnitudes of enthalpic and entropic contributions to partitioning in polymer-polymer (e.g., PEG-dextran), polymer-salt, and thermoseparating polymer-water (e.g., UCON-water) two-phase systems. The model therefore provides practitioners with a better understanding of partition systems, and industry with a simple, fundamental tool for selecting an appropriate two-phase system for a particular separation.

Thermal gelation of nonionic cellulose ethers and ionic surfactants in water

Abstract A novel gel-forming system which shows an anomalous temperature dependence is reported. The system consists of a nonionic cellulose ether and an ionic surfactant in water (the effect is particularly strong for ethyl (hydroxyethyl)cellulose [EHEC]). When the system is heated (typically to 40–50°C), it undergoes a transformation from a low-viscous solution to a clear and stiff gel. The unusual behaviour is reversible and an apparently unlimited number of temperature cycles can be performed without loss of the gelling capability. The rheological properties were characterized by steady-flow and dynamic viscosity measurements, and it is found that the gel has many features in common with weak polymer gels. The cellulose ether could either be relatively hydrophobic (methyl-, hydroxypropyl- and ethyl(hydroxyethyl)cellulose), or contain a small number of long alkyl groups (e.g.,‘hydrophobically modified’ hydroxyethylcellulose). The mechanism behind the formation of this novel type of gel is discussed in terms of cooperative binding of the ionic surfactants to the EHEC polymer, leading to small micelle-like aggregates. These may act as a cross-links between hydrophobic segments in different polymer chains, giving rise to extended and rigid polymer networks. Moreover, we have found that the interaction has a reverse temperature dependence — an increase in temperature leads to a higher degree of surfactant binding — which thus may explain the temperature anomaly in the gelling process.

The water dimer interaction energy: Convergence to the basis set limit at the correlated level

The water dimer interaction energy and its convergence to the basis set limit was investigated, with electron correlation effects treated at the level of second order Mo/ller-Plesset perturbation theory (MP2). ANO-type and large uncontracted basis sets were used, spreading over a wide range in size; the biggest set included 1046 functions with angular momentum up to (l=7). Core correlation effects were treated accurately by augmenting the original valence basis with extended sets of core polarization functions. The MP2 dimer interaction energy at the basis set limit was determined to −4.94±0.02 kcal/mol, with a contribution due to core correlation of −0.04 kcal/mol. Furthermore, based on some elementary considerations from intermolecular perturbation theory, a simple procedure was devised, which brings the counterpoise corrected interaction energies of moderate basis set calculations closer to the basis set limit. The interaction energies so obtained turned out be surprisingly stable with respect to extensions of the basis set.The water dimer interaction energy and its convergence to the basis set limit was investigated, with electron correlation effects treated at the level of second order Mo/ller-Plesset perturbation theory (MP2). ANO-type and large uncontracted basis sets were used, spreading over a wide range in size; the biggest set included 1046 functions with angular momentum up to (l=7). Core correlation effects were treated accurately by augmenting the original valence basis with extended sets of core polarization functions. The MP2 dimer interaction energy at the basis set limit was determined to −4.94±0.02 kcal/mol, with a contribution due to core correlation of −0.04 kcal/mol. Furthermore, based on some elementary considerations from intermolecular perturbation theory, a simple procedure was devised, which brings the counterpoise corrected interaction energies of moderate basis set calculations closer to the basis set limit. The interaction energies so obtained turned out be surprisingly stable with respect to exten...

Basis set superposition effects on properties of interacting systems. Dipole moments and polarizabilities

Basis set superposition effects which are not removed by the counterpoise correction are shown to modify the electric properties of interacting subsystems and influence indirectly the calculated interaction energies. The role of these higher-order basis set superposition effects is illustrated by the calculation of the water molecule dipole moment and polarizability in the basis set of the water dimer.

Nonionic polyhers and surfactants - some anomalies in temperature dependence and in interactions with ionic surfactants

Nonionic polymers and surfactants in aqueous systems show in a large number of properties unusual or ‘anomalous’ temperature dependences which are briefly reviewed. It is also reported that these solutes in the presence of ionic amphiphilic cosolutes show anomalous effects such as a strong synergistic ionic surfactant-electrolyte effect in the phase behaviour and the formation of stiff gels at higher temperature. With the aim of understanding the effect of ethylene oxidecontaining and related systems in general, including homogeneous solutions, microheterogeneous self-assembled solutions and macroscopic interfaces, a general discussion of possible interactions is presented. Different observations suggest that in all these systems, the solute or interface becomes less hydrophilic at higher temperature, which is in agreement with recent quantumchemical calculations. The effects of conformational equilibria on phase diagrams is considered and in particular it is shown that complex phase equilibria of nonionic polymer-ionic surfactant systems can be rationalized by a simple model.

On the demonstration of bicontinuous structures in microemulsions

Abstract The problem of the demonstration of bicontinuous structures in microemulsions is critically examined, with an emphasis on systems with low concentrations of surfactant (s) and comparable volume fractions of oil and water. While the problem of microemulsion bicontinuity has been investigated by a large number of experimental approaches, it is found that to date, only a few can provide an unambiguous clarification of this point. Multicomponent self-diffusion data appear to provide the most convenient and reliable examination of the problem, and therefore the possibilities and limitations of this approach are analysed in some detail. From experimental studies it is found that microemulsions with approximately equal volume fractions of the two solvents are in general bicontinuous, also in the most narrow sense of the word. It is also demonstrated that surfactant molecules in these microemulsions are arranged in monomolecular layers, which are characterized by a spontaneous packing into surfaces of zero, or at least low, mean curvature. The relation between surfactant packing characteristics and microemulsion structure is discussed, in particular for the typically observed sequence of curvature towards oil ⪢ zero mean curvature ⪢ curvature towards water, on change of some parameter (e.g., salinity, cosurfactant, surfactant composition, temperature). As regards an understanding of the physical mechanisms behind alterations in surfactant packing (and thus also microstructure), the problem of nonionic oligo (ethylene oxide) surfactants appears to provide the most intriguing (and controversial) problem. This problem is therefore discussed in a slightly broader context.

Ab initio molecular orbital calculations on the water-carbon dioxide system: Molecular complexes

Abstract An ab initio LCAO MO SCF calculation has been carried out on three different complexes between water and carbon dioxide. It has been found that a planar complex in which the carbon atom is bound to the oxygen atom in the water is the most stable, with a calculated energy of formation of −25.7 kJ/mol.