Explicit reversible integrators, suitable for use in large-scale computer simulations, are derived for extended systems generating the canonical and isothermal-isobaric ensembles. The new methods are compared with the standard implicit (iterative) integrators on some illustrative example problems. In addition, modification of the proposed algorithms for multiple time step integration is outlined.

Explicit reversible integrators for extended systems dynamics

The Complete Active Space Self‐Consistent Field Method and its Applications in Electronic Structure Calculations

The capabilities of the Crystal14 program are presented, and the improvements made with respect to the previous Crystal09 version discussed. Crystal14 is an ab initio code that uses a Gaussian-type basis set: both pseudopotential and all-electron strategies are permitted; the latter is not much more expensive than the former up to the first-second transition metal rows of the periodic table. A variety of density functionals is available, including as an extreme case Hartree–Fock; hybrids of various nature (global, range-separated, double) can be used. In particular, a very efficient implementation of global hybrids, such as popular B3LYP and PBE0 prescriptions, allows for such calculations to be performed at relatively low computational cost. The program can treat on the same grounds zero-dimensional (molecules), one-dimensional (polymers), two-dimensional (slabs), as well as three-dimensional (3D; crystals) systems. No spurious 3D periodicity is required for low-dimensional systems as happens when plane-waves are used as a basis set. Symmetry is fully exploited at all steps of the calculation; this permits, for example, to investigate nanotubes of increasing radius at a nearly constant cost (better than linear scaling!) or to perform self-consistent-field (SCF) calculations on fullerenes as large as (10,10), with 6000 atoms, 84,000 atomic orbitals, and 20 SCF cycles, on a single core in one day. Three versions of the code exist, serial, parallel, and massive-parallel. In the second one, the most relevant matrices are duplicated, whereas in the third one the matrices in reciprocal space are distributed for diagonalization. All the relevant vectors are now dynamically allocated and deallocated after use, making Crystal14 much more agile than the previous version, in which they were statically allocated. The program now fits more easily in low-memory machines (as many supercomputers nowadays are). Crystal14 can be used on parallel machines up to a high number of cores (benchmarks up to 10,240 cores are documented) with good scalability, the main limitation remaining the diagonalization step. Many tensorial properties can be evaluated in a fully automated way by using a single input keyword: elastic, piezoelectric, photoelastic, dielectric, as well as first and second hyperpolarizabilies, electric field gradients, Born tensors and so forth. Many tools permit a complete analysis of the vibrational properties of crystalline compounds. The infrared and Raman intensities are now computed analytically and related spectra can be generated. Isotopic shifts are easily evaluated, frequencies of only a fragment of a large system computed and nuclear contribution to the dielectric tensor determined. New algorithms have been devised for the investigation of solid solutions and disordered systems. The topological analysis of the electron charge density, according to the Quantum Theory of Atoms in Molecules, is now incorporated in the code via the integrated merge of the Topond package. Electron correlation can be evaluated at the Moller–Plesset second-order level (namely MP2) and a set of double-hybrids are presently available via the integrated merge with the Cryscor program. © 2014 Wiley Periodicals, Inc.

/pdf/crystal14-a-program-for-the-ab-initio-investigation-of-53dvn35xcj.pdf

CRYSTAL14: A program for the ab initio investigation of crystalline solids

Crystal Engineering and Organometallic Architecture.

The impact of agostic interactions (i.e., 3-center-2-electron M-H-C bonds) on the structures and reactivity of organotransition metal compounds is reviewed.

https://www.pnas.org/content/pnas/104/17/6908.full.pdf

Agostic interactions in transition metal compounds

A detailed single‐crystal structure analysis is reported for ethylene oxide hydrate. This confirms the polyhedral host lattice formed by 46 hydrogen‐bonded water molecules in a cubic unit cell of 12.03 A with space‐group symmetry Pm3n. Disordered ethylene oxide molecules occupy the six equivalent tetrakaidecahedral cavities, and their electron‐density distribution suggests that they are hindered axial rotors. Additional guest molecules, either ethylene oxide, oxygen, or nitrogen molecules occupy the two dodecahedral cavities with high disorder and low statistical weight. The compound is therefore nonstoichiometric having the limiting formula 2M·6C2H4O·46H2O, in which the percentage of M is variable. If M is assumed to be entirely ethylene oxide, as is likely from the method of preparation, the composition of the crystals obtained from an equilibrated aqueous solution of 8 mole % ethylene oxide at 9°C is 6.4C2H4O·46H2O.

Polyhedral Clathrate Hydrates. IX. Structure of Ethylene Oxide Hydrate

The structure of the tetrahydrofuran/hydrogen sulfide double hydrate has been determined from three‐dimensional single‐crystal data. The analysis confirmed the clathrate host lattice characteristic of 17‐A cubic (Type II) gas hydrates. The hexakaidecahedral voids enclose tetrahydrofuran molecules which appear to undergo free rotation. Statistically, 46% of the pentagonal dodecahedra are occupied by hydrogen sulfide molecules.

Polyhedral Clathrate Hydrates. X. Structure of the Double Hydrate of Tetrahydrofuran and Hydrogen Sulfide

Lattice parameters for urea have been measured at seven temperatures in the range 12 to 173 K and the crystal structure has been determined at 12, 60 and 123 K from neutron diffraction data. With 342 reflections (sin 0/A <0.77 A -I) measured for an octant of reciprocal space, full-matrix least-squares refinement gave Rw(F2) = 0.030, 0.029 and 0.029 at 12, 60 and 123 K. The atomic anisotropic thermal parameters are consistent with overall rigid-body motion together with intramolecular librations of the NH2 groups. With increasing temperature, the H-atom motion out of the molecular plane becomes dampened with respect to the overall rigid-body motion. The effect appears to be due to the H-bonding which couples the molecules in the crystal. After correction for thermal motion, including anharmonic N-H stretching, there is good agreement in the bond lengths at 12, 60 and 123 K. [Crystal data: P£~21m; 12 K: a = 5.565 (1), c = 4 6 8 4 ( 1 ) A ; 30K: a =5 .565 ( 1 ) , c = 4 6 8 5 ( 1 ) A ; 60K: a=5 .570 (1 ) , c = 4 . 6 8 8 ( 1 ) A ; 90K: a = 5.576(1), c = 4 . 6 8 9 ~ ; 123K: a = 5 . 5 8 4 ( 1 ) , c = 4 .689(1)A; 150K: a=5 .590 ( 1 ) , c = 4 . 6 9 2 ( 1 ) A ; 173 K: a = 5.598 (1), c = 4.694 (1)/~.]

The crystal structure and molecular thermal motion of urea at 12, 60 and 123 K from neutron diffraction

Et3NH+Co(CO)4-: hydrogen-bonded adduct or simple ion pair? Single-crystal neutron diffraction study at 15 K

The structure of [Os([eta][sup 2]-H[sub 2])en[sub 2]CH[sub 3]CO[sub 2]]PF[sub 6] (1) has been studied by X-ray diffraction at 120 K and neutron diffraction at 165 K. Compound 1 belongs to a series of dihydrogen complexes of osmium(II) amines, exhibiting an extensive range in the strength of the H-H interaction as judged by the wide variation in observed J[sub H-D] coupling constants. The ethylenediamine ligands in I have a trans arrangement. The H[sub 2] and the acetate ligands occupy positions on opposite sides of the OsN[sub 4] plane, giving overall approximately octahedral coordination. Consistent with the relatively low J[sub H-D] value (9.0 Hz), the H-H distance in 1.134(2) [angstrom] as determined by neutron diffraction, is unusually long and approaches that of 1.357(7) [angstrom] found in the polyhydride ReH[sub 7][P(p-tolyl)[sub 3]][sub 2]. It is presumed that I may represent an intermediate stage in the oxidative addition of dihydrogen at transition-metal centers. The Os-H distances in 1.1.59(1) [angstrom] and 1.60(2) [angstrom], are shorter than those in the classical osmium polyhydride OsH[sub 4](PMe[sub 2]Ph)[sub 3] (mean Os-H 1.659(3) [angstrom]) lending support to the assertion that the Os([eta][sup 2]-H[sub 2]) interaction is relatively strong. Crystal data are provided. 46 refs., 3 figs., 2 tabs.

R. K. McMullan

Papers

Polyhedral Clathrate Hydrates. IX. Structure of Ethylene Oxide Hydrate

Polyhedral Clathrate Hydrates. X. Structure of the Double Hydrate of Tetrahydrofuran and Hydrogen Sulfide

The crystal structure and molecular thermal motion of urea at 12, 60 and 123 K from neutron diffraction

Et3NH+Co(CO)4-: hydrogen-bonded adduct or simple ion pair? Single-crystal neutron diffraction study at 15 K

Determination of the Structure of [Os(.eta.2-H2)en2CH3CO2]PF6 by X-ray and Neutron Diffraction