Classical Monte Carlo simulations have been carried out for liquid water in the NPT ensemble at 25 °C and 1 atm using six of the simpler intermolecular potential functions for the water dimer: Bernal–Fowler (BF), SPC, ST2, TIPS2, TIP3P, and TIP4P. Comparisons are made with experimental thermodynamic and structural data including the recent neutron diffraction results of Thiessen and Narten. The computed densities and potential energies are in reasonable accord with experiment except for the original BF model, which yields an 18% overestimate of the density and poor structural results. The TIPS2 and TIP4P potentials yield oxygen–oxygen partial structure functions in good agreement with the neutron diffraction results. The accord with the experimental OH and HH partial structure functions is poorer; however, the computed results for these functions are similar for all the potential functions. Consequently, the discrepancy may be due to the correction terms needed in processing the neutron data or to an effect uniformly neglected in the computations. Comparisons are also made for self‐diffusion coefficients obtained from molecular dynamics simulations. Overall, the SPC, ST2, TIPS2, and TIP4P models give reasonable structural and thermodynamic descriptions of liquid water and they should be useful in simulations of aqueous solutions. The simplicity of the SPC, TIPS2, and TIP4P functions is also attractive from a computational standpoint.

Comparison of simple potential functions for simulating liquid water

This paper describes the contents of the 2016 edition of the HITRAN molecular spectroscopic compilation. The new edition replaces the previous HITRAN edition of 2012 and its updates during the intervening years. The HITRAN molecular absorption compilation is composed of five major components: the traditional line-by-line spectroscopic parameters required for high-resolution radiative-transfer codes, infrared absorption cross-sections for molecules not yet amenable to representation in a line-by-line form, collision-induced absorption data, aerosol indices of refraction, and general tables such as partition sums that apply globally to the data. The new HITRAN is greatly extended in terms of accuracy, spectral coverage, additional absorption phenomena, added line-shape formalisms, and validity. Moreover, molecules, isotopologues, and perturbing gases have been added that address the issues of atmospheres beyond the Earth. Of considerable note, experimental IR cross-sections for almost 300 additional molecules important in different areas of atmospheric science have been added to the database. The compilation can be accessed through www.hitran.org. Most of the HITRAN data have now been cast into an underlying relational database structure that offers many advantages over the long-standing sequential text-based structure. The new structure empowers the user in many ways. It enables the incorporation of an extended set of fundamental parameters per transition, sophisticated line-shape formalisms, easy user-defined output formats, and very convenient searching, filtering, and plotting of data. A powerful application programming interface making use of structured query language (SQL) features for higher-level applications of HITRAN is also provided.

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The HITRAN 2008 molecular spectroscopic database

We present an approach for deriving net atomic charges from ab initio quantum mechanical calculations using a least squares fit of the quantum mechanically calculated electrostatic potential to that of the partial charge model. Our computational approach is similar to those presented by Momany [J. Phys. Chem., 82, 592 (1978)], Smit, Derissen, and van Duijneveldt [Mol. Phys., 37, 521 (1979)], and Cox and Williams [J. Comput. Chem., 2, 304 (1981)], but differs in the approach to choosing the positions for evaluating the potential. In this article, we present applications to the molecules H2O, CH3OH, (CH3)2O, H2CO, NH3, (CH3O)2PO, deoxyribose, ribose, adenine, 9-CH3 adenine, thymine, 1-CH3 thymine, guanine, 9-CH3 guanine, cytosine, 1-CH3 cytosine, uracil, and 1-CH3 uracil. We also address the question of inclusion of “lone pairs,” their location and charge.

An approach to computing electrostatic charges for molecules

A combined quantum mechanical (QM) and molecular mechanical (MM) potential has been developed for the study of reactions in condensed phases. For the quantum mechanical calculations semiempirical methods of the MNDO and AM1 type are used, while the molecular mechanics part is treated with the CHARMM force field. Specific prescriptions are given for the interactions between the QM and MM portions of the system; cases in which the QM and MM methodology is applied to parts of the same molecule or to different molecules are considered. The details of the method and a range of test calculations, including comparisons with ab initio and experimental results, are given. It is found that in many cases satisfactory results are obtained. However, there are limitations to this type of approach, some of which arise from the AM1 or MNDO methods themselves and others from the present QM/MM implementation. This suggests that it is important to test the applicability of the method to each particular case prior to its use. Possible areas of improvement in the methodology are discussed.

A combined quantum mechanical and molecular mechanical potential for molecular dynamics simulations

The ability of simple potential functions to reproduce accurately the density of liquid water from −37 to 100 °C at 1 to 10 000 atm has been further explored. The result is the five-site TIP5P model, which yields significantly improved results; the average error in the density over the 100° temperature range from −37.5 to 62.5 °C at 1 atm is only 0.006 g cm−3. Classical Monte Carlo statistical mechanics calculations have been performed to optimize the parameters, especially the position of the negative charges along the lone-pair directions. Initial calculations with 216 molecules in the NPT ensemble at 1 atm focused on finding a model that reproduced the shape of the liquid density curve as a function of temperature. Calculations performed for 512 molecules with the final TIP5P model demonstrate that the density maximum near 4 °C at 1 atm is reproduced, while high-quality structural and thermodynamic results are maintained. Attainment of high precision for the low-temperature runs required sampling for more than 1 billion Monte Carlo configurations. In addition, the dielectric constant was computed from the response to an applied electric field; the result is 81.5±1.5 at 25 °C and the experimental curve is mirrored from 0–100 °C at 1 atm. The TIP5P model is also found to perform well as a function of pressure; the density of liquid water at 25 °C is reproduced with an average error of ∼2% over the range from 1 to 10 000 atm, and the shift of the temperature of maximum density to lower temperature with increasing pressure is also obtained.

/pdf/a-five-site-model-for-liquid-water-and-the-reproduction-of-4waghhehgx.pdf

A five-site model for liquid water and the reproduction of the density anomaly by rigid, nonpolarizable potential functions

Molecular beams of hydrogen bonded water dimer, generated in a supersonic nozzle, have been studied using electric resonance spectroscopy. Radiofrequency and microwave transitions have been observed in (H2 16O)2, (D2 16O)2, and (H2 18O)2. Transitions arising from both pure rotation and rotation–tunneling occur. The pure rotational transitions have been fit to a rigid rotor model to obtain structural information. Information on the relative orientation of the two monomer units is also contained in the electric dipole moment component along the A inertial axis μa, which is obtained from Stark effect measurements. The resultant structure is that of a ’’trans‐linear’’ complex with an oxygen–oxygen distance ROO of 2.98(1) A, the proton accepting water axis is 58(6) ° with respect to ROO, and the proton donating water axis at −51(6) ° with respect to ROO. This structure is consistent with a linear hydrogen bond and the proton acceptor tetrahedrally oriented to the hydrogen bond. The limits of uncertainty are wh...

The Structure of Water Dimer from Molecular Beam Electric Resonance Spectroscopy.

The electric dipole moment of OCS has been determined by measuring pure Stark transitions with molecular‐beam electric resonance methods. The value obtained is 0.71521 ± 0.00020 D, which is in disagreement with a previous measurement of 0.7124 ± 0.0002 D.

Electric Dipole Moment of Carbonyl Sulfide

The radio frequency spectrum of HF and DF is measured by the molecular‐beam electric resonance method. The measurements are in the lowest vibrational state (υ = 0) and first rotational state (J = 1). The constants obtained are: HFDFμ (D)1.826526 (7)1.818805 (5)CF (kHz)307.637 (20)158.356 (45)CH (kHz)− 71.128 (24)− 5.755 (19)SHF (kHz)28.675 (5)4.434 (9)JHF (kHz)0.529 (23)(eqQ) D354.238 (78) The electron coupled spin–spin interaction, JHF = + 530 Hz.

Hyperfine Structure Constants of HF and DF

Microwave spectrum and structure of hydrogen bonded water dimer

Molecular beam electric resonance spectroscopy has been used to make precision Stark effect measurements on the 111, 110, and 211 rotational states of H2O and the 111, 110, 211, and 313 states of D2O. These data have been analyzed using the dipole moment function of Clough, Beers, Klein, and Rothman [J. Chem. Phys. 59, 2254 (1973)] to obtain rotationless moments of 1.8546 ± 0.0004 D for H2O and 1.8545 ± 0.0004 D and D2O.

John S. Muenter

Papers

The Structure of Water Dimer from Molecular Beam Electric Resonance Spectroscopy.

Electric Dipole Moment of Carbonyl Sulfide

Hyperfine Structure Constants of HF and DF

Microwave spectrum and structure of hydrogen bonded water dimer

Electric dipole moments of low J states of H2O and D2O