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

The infrared vibration‐rotation spectrum of the ν3 band of C35Cl4 was measured by using a tunable diode laser. Heavily congested Q‐branch spectrum was analyzed on the basis of the Stark effect, which extract transitions of tetrahedral species among various isotopic species. Low‐J lines were measured in a supercooled molecular jet, which supressed hot bands and high‐J rotational lines. The following constants of the ν3 band were determined: ν0=798.7489(4) cm−1, B3−B0=−0.39(5) MHz, α220=−3.33(6) MHz, and α224=−0.185(9) MHz. The Fermi resonance between the ν3 and ν1+ν4 states was analyzed with the aid of an empirical anharmonic force field.

Diode laser spectroscopy of the ν3 band of carbon tetrachloride (C35Cl4): Stark modulation and cold jet infrared absorption spectrum

The rotational spectral lines of the H2NS and D2NS radicals in the X2B1 electronic ground state are observed with a source-modulation microwave spectrometer and a Fourier-transform microwave spectrometer. Molecular constants including hyperfine interaction constants are determined for H2NS and D2NS by a least-squares method. By using the obtained centrifugal distortion constants and inertial defects, the harmonic force field is evaluated, and the frequency of the ν4 vibrational mode (out of plane) is calculated to be 325 cm−1. This value is much smaller than that of the related molecule H2CS, indicating a floppy motion along the out-of-plane mode. The zero-point vibrational average structure was determined as follows; rz(N–H)=1.000(5) A, rz(N–S)=1.6398(13) A, and ∠z(HNH)=118.9(7)°, where the numbers in parentheses represent three times the standard deviation.

Microwave spectrum and molecular structure of the H2NS radical

We measured atmospheric opacity at 220 GHz at the summit of Mt. Fuji (alt. 3776 m) about one year in order to explore a feasibility of submillimeter-wave astronomical observations. For this purpose, a 220 GHz radiometer system enclosed in a radome (51×51×62 cm3) has been developed. The 220 GHz opacity was lower than 0.06 for a significant fraction (∼ 45 %) of time from November 1994 to March 1995. Diurnal variation of the opacity at the summit of Mt. Fuji is so small that continuous observation at submillimeter-wave is possible through day and night. Yearly variation of the opacity is studied from water vapor pressure data measured at the weather station for the past 3 years. To prevent accumulation of ice and snow on the Gore-Tex membrane in the radome, the outer membrane was supplied with a thermal flux of 0.63 kW m−2 and the adjacent metal radome surfaces were supplied with a flux of ∼0.9 kW m−2. We evaluate from the 220 GHz transmission data that this heat flux is sufficient to keep the membrane on the radome free of ice and snow during 83 % of the time in 5 winter months. The summit of Mt. Fuji appears to be a promising site for submillimeter-wave observations.

Measurements of 220 GHz atmospheric opacity on Mt. Fuji with a radiometer/radome system

Vibrational structure in C 1s photoelectron spectra of ethylene on the Si(1 0 0)(2 × 1) surface

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Satoshi Yamamoto

Papers

Diode laser spectroscopy of the ν3 band of carbon tetrachloride (C35Cl4): Stark modulation and cold jet infrared absorption spectrum

Microwave spectrum and molecular structure of the H2NS radical

Measurements of 220 GHz atmospheric opacity on Mt. Fuji with a radiometer/radome system

Vibrational structure in C 1s photoelectron spectra of ethylene on the Si(1 0 0)(2 × 1) surface

Molecular structure and large-amplitude bending vibration of carbon suboxide as studied by electron-diffraction and spectroscopic data