Towards hybrid density functional calculations of molecular crystals via fragment-based methods.

We introduce and employ two QM:QM schemes (a quantum mechanical method embedded into another quantum mechanical method) and report their performance for the X23 set of molecular crystals. We furthermore present the theory to calculate the stress tensors necessary for the computation of optimized cell volumes of molecular crystals and compare all results to those obtained with various density functionals and more approximate methods. Our QM:QM calculations with PBE0:PBE+D3, PBE0:PBE+MBD, and B3LYP:BLYP+D3 yield at a reduced computational cost lattice energy errors close to the ones of the parent hybrid density functional method, whereas for cell volumes, the errors of the QM:QM scheme methods are in between the generalized gradient approximation and hybrid functionals.

Development of Embedded and Performance of Density Functional Methods for Molecular Crystals.

We report an alternative quantum mechanical:quantum mechanical (QM:QM) method to the currently used periodic density functional calculations including dispersion and investigate its performance with respect to main structural and energetic properties of the X23 set of molecular crystals. By setting the goal of reproducing reference periodic BLYP+D3 values and by embedding BLYP+D3 into DFTB, we obtain results similar to those of periodic BLYP+D3-typically within 1-2% in lattice energies and ∼0.4% in cell volumes. The accuracy of this QM:QM method in comparison to DFTB+D and DFT+D for the X23 set of molecular crystals is discussed.

Dipole and quadrupole polarizabilities of the water molecule as a function of geometry.

Dipolar, dipole-quadrupole and quadrupole-quadrupole static polarizabilities of the water molecule have been determined by ab initio calculations at coupled cluster level of theory with single, double and perturbative triple excitations CCSD(T) with an aug-cc-pVTZ basis set using a finite field and field-gradient method. The geometry dependence of polarizability tensor components has been explored and modeled by power series expansion in bond length and angle variations up to sum of powers equal to 4. The results provide a very detailed description of the static polarizability of water up to quadrupolar rank which can be used for the test and development of novel accurate polarizable interaction potentials for modeling aqueous solutions. © 2016 Wiley Periodicals, Inc.

Geometry-dependent distributed polarizability models for the water molecule.

Geometry-dependent distributed polarizability models have been constructed by fits to ab initio calculations at the coupled cluster level of theory with up to noniterative triple excitations in an augmented triple-zeta quality basis set for the water molecule in the field of a point charge. The investigated models include (i) charge-flow polarizabilities between chemically bonded atoms, (ii) isotropic or anisotropic dipolar polarizabilities on oxygen atom or on all atoms, and (iii) combinations of models (i) and (ii). For each model, the polarizability parameters have been optimized to reproduce the induction energy of a water molecule polarized by a point charge successively occupying a grid of points surrounding the molecule. The quality of the models is ascertained by examining their ability to reproduce these induction energies as well as the molecular dipolar and quadrupolar polarizabilities. The geometry dependence of the distributed polarizability models has been explored by changing bond lengths and HOH angle to generate 125 molecular structures (reduced to 75 symmetry-unique ones). For each considered model, the distributed polarizability components have been fitted as a function of the geometry by a Taylor expansion in monomer coordinate displacements up to the sum of powers equal to 4.

Revealing water's secrets: deuterium depleted water.

BACKGROUND: The anomalous properties of water have been of great interest for generations of scientists. However the impact of small amount of deuterium content which is always present in water has never been explored before. For the first time the fundamental properties of deuterium depleted (light) water at 4°C and 20°C are here presented. RESULTS: The obtained results show the important role of the deuterium in the properties of bulk water. At 4°C the lowest value of the kinematic viscosity (1.46 mm2/s) has been found for 96.5 ppm D/H ratio. The significant deviation in surface tension values has been observed in deuterium depleted water samples at the both temperature regimes. The experimental data provides direct evidence that density, surface tension and viscosity anomalies of water are caused by the presence of variable concentration of deuterium which leads to the formation of water clusters of different size and quantity. CONCLUSIONS: The investigated properties of light water reveal the origin of the water anomalies. The new theoretical model of cluster formation with account of isotope effect is proposed.

Linear and nonlinear optical properties of triphenylamine-functionalized C60: insights from theory and experiment.

In the present study we report on the linear and nonlinear optical properties of C(60)-triphenylamine (TPhA) hybrids. The synthesized materials were prepared following the 1,3-dipolar cycloaddition of azomethine ylides onto the skeleton of C(60) forming the TPhA-based monoadduct, equatorial bis-adduct and dumbell C(60). Complementary spectroscopic techniques, such as NMR, MALDI-TOF-MS, and ATR-IR are applied for the structural characterization of the hybrid materials, while intermolecular electronic interactions are investigated by UV-Vis measurements. In all considered cases, C(60) serves as an acceptor while the triphenylamine unit is chosen as a donor. In order to investigate the dependence of the second-order hyperpolarizability on the architecture of the systems (D-A, A-D-A) we use a Z-scan technique employing 532 nm, 35 ps laser pulses. We have found that the total second-order hyperpolarizability of the C(60)-TphA-C(60) system is several times larger than that of TPhA-C(60). The results of experimental measurements are supported by quantum-chemical calculations.

Isomerization of free base porphyrin. Theoretical study.

The photoinduced isomerization reaction of free base porphyrin molecule has been calculated using the DFT-B3LYP method combined with the 6-31G(d,p) basis set. To prove the accuracy of results, the 6-311G+(2p,2d) basis set was used. Two types of isomerization mechanisms were studied. It was found that the stepwise pathway of isomerization is the most appropriate. The geometric parameters of isomers and the transition structures of different multiplets are discussed. Zero-point energy and vibrational frequencies analysis are given.

Physical properties and spectra of IO, IO- and HOI studied by ab initio methods.

Structure and properties of the IO, IO- and HOI species, which are of potential importance for the ozone destruction catalytic cycle in the troposphere, have been calculated together with the EPR, NMR and UV-visible spectra by ab initio methodology with account of spin-orbit coupling (SOC) effects. Multi-configuration self-consistent field calculations with linear and quadratic response techniques and the multi-reference configuration interaction method have been employed. Photodissociation of these species, crucial for the catalytic ozone-destruction cycle, is critically reviewed and analyzed. Calculations predict that the singlet-triplet (S-T) transition to the lowest triplet state (X1 A' --> 3A'') should be responsible for the weak long-wavelength tail absorption (approximately 450-560 nm) and photodissociation of the HOI molecule. The second, more intense, band around 400 nm is produced by two overlapping S-S and S-T transitions. In order to check this assignment of the HOI photodissociation the isoelectronic IO- anion and IO radical have been studied by the same methods. Comparison with the EPR spectrum of the IO radical indicates that the methods are reliable which gives credit to the accuracy of the HOI spectral interpretation. NMR spectra of HOI and IO- molecules and some other properties are calculated for the first time.