A general all-atom force field for atomistic simulation of common organic molecules, inorganic small molecules, and polymers was developed using state-of-the-art ab initio and empirical parametrization techniques. The valence parameters and atomic partial charges were derived by fitting to ab initio data, and the van der Waals (vdW) parameters were derived by conducting MD simulations of molecular liquids and fitting the simulated cohesive energies and equilibrium densities to experimental data. The combined parametrization procedure significantly improves the quality of a general force field. Validation studies based on large number of isolated molecules, molecular liquids and molecular crystals, representing 28 molecular classes, show that the present force field enables accurate and simultaneous prediction of structural, conformational, vibrational, and thermophysical properties for a broad range of molecules in isolation and in condensed phases. Detailed results of the parametrization and validation f...

COMPASS: An ab Initio Force-Field Optimized for Condensed-Phase ApplicationsOverview with Details on Alkane and Benzene Compounds

The Cologne Database for Molecular Spectroscopy, CDMS: a useful tool for astronomers and spectroscopists

Understanding the formation of stars in galaxies is central to much of modern astrophysics. However, a quantitative prediction of the star formation rate and the initial distribution of stellar masses remains elusive. For several decades it has been thought that the star formation process is primarily controlled by the interplay between gravity and magnetostatic support, modulated by neutral-ion drift (known as ambipolar diffusion in astrophysics). Recently, however, both observational and numerical work has begun to suggest that supersonic turbulent flows rather than static magnetic fields control star formation. To some extent, this represents a return to ideas popular before the importance of magnetic fields to the interstellar gas was fully appreciated. This review gives a historical overview of the successes and problems of both the classical dynamical theory and the standard theory of magnetostatic support, from both observational and theoretical perspectives. The outline of a new theory relying on control by driven supersonic turbulence is then presented. Numerical models demonstrate that, although supersonic turbulence can provide global support, it nevertheless produces density enhancements that allow local collapse. Inefficient, isolated star formation is a hallmark of turbulent support, while efficient, clustered star formation occurs in its absence. The consequences of this theory are then explored for both local star formation and galactic-scale star formation. It suggests that individual star-forming cores are likely not quasistatic objects, but dynamically collapsing. Accretion onto these objects varies depending on the properties of the surrounding turbulent flow; numerical models agree with observations showing decreasing rates. The initial mass distribution of stars may also be determined by the turbulent flow. Molecular clouds appear to be transient objects forming and dissolving in the larger-scale turbulent flow, or else quickly collapsing into regions of violent star formation. Global star formation in galaxies appears to be controlled by the same balance between gravity and turbulence as small-scale star formation, although modulated by cooling and differential rotation. The dominant driving mechanism in star-forming regions of galaxies appears to be supernovae, while elsewhere coupling of rotation to the gas through magnetic fields or gravity may be important.

Control of star formation by supersonic turbulence

Of the over 150 different molecular species detected in the interstellar and circumstellar media, approximately 50 contain 6 or more atoms. These molecules, labeled complex by astronomers if not by chemists, all contain the element carbon and so can be called organic. In the interstellar medium, complex molecules are detected in the denser sources only. Although, with one exception, complex molecules have only been detected in the gas phase, there is strong evidence that they can be formed in ice mantles on interstellar grains. The nature of the gaseous complex species depends dramatically on the source where they are found: in cold, dense regions they tend to be unsaturated (hydrogen-poor) and exotic, whereas in young stellar objects, they tend to be quite saturated (hydrogen-rich) and terrestrial in nature. Based on both their spectra and chemistry, complex molecules are excellent probes of the physical conditions and history of the sources where they reside. Because they are detected in young stellar objects, complex molecules are expected to be common ingredients for new planetary systems. In this review, we discuss both the observation and chemistry of complex molecules in assorted interstellar regions in the Milky Way.

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Complex Organic Interstellar Molecules

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Control of Star Formation by Supersonic Turbulence

Infrared diode laser spectroscopy of the ν2 band of BF3

Fourier transform millimeter-wave spectroscopy of CS() and SO() in highly excited vibrational states

The rotational spectrum of the deuterated species of thioformaldehyde, HDCS, has been measured in the laboratory by microwave spectroscopy. Fifty-two a-type R-branch transitions are observed, and the rotational constants and centrifugal distortion constants are determined accurately. On the basis of the laboratory results, two unidentified lines detected during the line survey toward a cold dark cloud TMC-1 are assigned to HDCS. The [HDCS]/[H2CS] ratio is determined to be 0.020 ± 0.005, indicating relatively high deuterium fractionation in H2CS. The ratio is discussed in terms of formation processes of H2CS in dark clouds.

Laboratory Microwave Spectroscopy of HDCS and Its Astronomical Detection toward TMC-1

High-resolution vibration–rotation spectra of stable and unstable molecular species have been studied at low rotational and vibrational temperatures. Infrared absorption in a pulsed supersonic free jet has been measured by a frequency-tunable diode laser in a single- or multi-pass configuration. The modulated infrared absorption is detected by a lock-in amplifier or a two-channel boxcar integrator. The ν3 band of SiCl4 at 624 cm–1 is studied as one of the series of work on heavy non-polar inorganic compounds. Five spectroscopic constants are determined accurately for the ν3 band of 28Si35Cl4. Weakly bound molecular complexes are studied in the middle infrared region as unstable molecular species. Well resolved ro-vibrational spectra are observed in (N2O)2, C2H2·Ar, (C2H2)2, (C2H2)3, BF3·X (X = Ne, Ar, Kr), C6H6·X (X = Ar, Kr), and (NO)2 with Doppler-limited resolution. Infrared–microwave double resonance in a free jet is discussed for microwave spectroscopy of van der Waals complexes in vibrationally excited states.

Free-jet infrared absorption spectroscopy of stable and unstable molecular species

The microwave rotational spectrum of the H2NO radical in the X 2B1 ground vibronic state was observed by using a source‐modulation spectrometer and a 1 m long free‐space cell. The H2NO radical was generated in the cell by a reaction of hydroxylamine (NH2OH) with the products of 2450 MHz discharge in CF4. The spectral lines of H2NO were observed for the a‐type R‐branch transitions of N=1←0 to N=6←5, most of which showed resolved hyperfine structures due to the hydrogen and nitrogen nuclei. The 19 molecular constants including the hyperfine coupling constants of both the nuclei were precisely determined by the least‐squares fit of 129 observed lines. The observed hyperfine structure shows that the H2NO radical in the ground electronic state is essentially planar with C2v symmetry. The nitrogen hyperfine coupling constants determined indicate that an unpaired electron occupies the pπ orbital perpendicular to the molecular plane, and the electronic wave function in the ground electronic state belongs to 2B1 ...

Satoshi Yamamoto

Papers

Infrared diode laser spectroscopy of the ν2 band of BF3

Fourier transform millimeter-wave spectroscopy of CS() and SO() in highly excited vibrational states

Laboratory Microwave Spectroscopy of HDCS and Its Astronomical Detection toward TMC-1

Free-jet infrared absorption spectroscopy of stable and unstable molecular species

The microwave spectrum of the dihydronitrosyl radical, h2no (2b1)