David W. Schwenke

We propose a new multi-reference Davidson correction and show that it gives a much better prediction of the role of quadruple excitations compared to existing corrections. However, due to the neglect of triple excitations, our most promising correction is a hybrid scheme whereby the average of the new correction and a conventional correction is used. This hybrid correction performs well in a wide variety of cases, including the avoided crossing of two roots of the same symmetry.

We have carried out first principles calculations of transport properties of the title atom–atom systems using accurate ab initio electronic structure methods and quantum scattering. We go beyond the Born–Oppenheimer approximation and show how one can properly include the spin and orbital angular momentum of atoms in the calculations. We give the explicit transformation between coupled LS atomic states, where L is the total electron angular momentum and S is the total spin angular momentum, and the diatomic Hund’s case (a) basis. We include both Coulomb spin–orbit interaction as well as the effect of the magnetic-moments of the electrons via the Breit interaction. The relations between the long-range forces of different symmetry electronic states are given for S + P and P + P asymptotes.

To facilitate atmospheric and spectroscopic studies of carbon disulfide, or CS₂, in both planetary and exoplanetary atmospheres, we adopt the “Best Theory + Reliable High-resolution Experiment” algorithm to generate semiempirical IR line lists for the 20 most abundant CS₂ isotopologues, denoted as Ames-296K. The IR lists are computed using the Ames-1 potential energy surface, refined using the experimental transition set and an ab initio dipole moment surface fitted from CCSD(T)/aug-cc-pV(T/Q/5+d)Z dipoles extrapolated to a one-particle basis set limit. The IR lists cover the range of 0–10,000 cm⁻¹, with an S _296K cutoff at 10⁻³¹ cm⁻¹/molecule·cm⁻² (abundance included). A “natural” IR line list at 296 K includes about 10 million lines of the 20 isotopologues, with their intensities scaled by the corresponding abundances. The zero-point energy, partition functions, and abundances are reported for each isotopologue. The energy levels in the global effective Hamiltonian model for ¹²C³²S₂ are adopted to improve the line position accuracy. This new IR list for the main isotopologue is denoted as A+I.296K. Reliable HITRAN2020 line positions are also utilized to improve the accuracy of the ³²S¹²C³⁴S, ³²S¹²C³³S, and ³²S¹³C³²S isotopologue line lists. The final composite line list is validated against Pacific Northwest National Laboratory experimental cross sections, showing excellent agreement. The agreement supports the quality of the composite line list and the power of synergy between experiment and theory. The new data are proposed for use in updating and expanding the CS₂ data in HITRAN and other high-resolution IR databases. Supplementary files are available in Zenodo and AHED.

Highly accurate IR line lists are ready for 13 CO₂ isotopologues and 7 SO₂ isotopologues. Ames-296K IR lists carry 0.01 - 0.03 cm⁻¹ accuracy up to 13,000 cm⁻¹ for CO₂ and 5500 cm⁻¹ for SO₂, and 90–95% intensity agreement for most observable bands. Good for atmospheric modeling on Venus and Exoplanets.

It is well known that the convergence of correlation energies in atomic and molecular calculations is relatively slow and that calculations aimed at high accuracy must explicitly make corrections for this. In this work we consider 1e ⁻ basis set extrapolation as a means of obtaining high accuracy. The correlation consistent basis sets of Dunning et al. have provided a convenient platform for extrapolation, with the independent variable being X = D, T, Q, 5, … . There has been much debate in the literature about the functional form to use for the extrapolation, with contention between the ‘theoretically justified’ (X + 1)⁻³ form and empirical forms based on exponentials or variable powers. We will dissect the theoretical justification of the (X + 1)⁻³ form by considering MP2 calculations on He and Ne as a function of the maximum angular momentum (⪙) in the basis using basis sets having converged radial extent. Calculations with ⪙ up to 14 were carried out for Ne. It is shown that while the asymptotic form of (⪙ + 1)⁻³ is clearly reached, higher order terms are very important in the range of ⪙ normally used in molecular calculations. We also use similar analysis techniques for an open shell atom and a small molecule. The functional form for the dependence of molecular properties with ⪙ is complex and it is safer to extrapolate fitting parameters than energies.

Ammonia (NH₃) was the first polyatomic molecule observed in the ISM. Its importance in interstellar molecules is only second to CO because its rovibrational spectroscopic signature can be used very effectively at deducing the conditions of the interstellar environment such as temperature and density, and because it is found in so many different interstellar objects in a wide temperature range. However, experimental determination of NH₃ IR spectra is extremely difficult due to the large-amplutide inversion vibration, and the existing HITRAN2008 database for NH₃ is limited in temperature, coverage, completeness, and accuracy. With rapid progress in theoretical chemistry and computational resources, now we are able to generate a highly reliable/accurate IR line list of NH₃ (and its isotopologues) for astronomical studies. Exact quantum rovibrational computations on an empirically refined potential energy surface (with nonadiabatic corrections included) have achieved accuracies of 0.02-0.05 cm⁻¹ (for line position) and better than 85-95% (for line intensity) for both NH₃ and ¹⁵NH₃ spectra. The unique feature of our work is that our predictions are essentially as accurate as reproducing existing measurements, suitable for synthetic simulation of various astrophysical environments or objects. The reliabilty and accuracy of our predictions for missing bands and higher energies computed on HSL-2 (Fig. 1) have been proved by the most recent high-resolution experiments and extended up to 7000 cm⁻¹. See Huang et al. 2008, Huang et al. 2011, & Sung et al. 2012 for more details.

An isotopic-independent, highly accurate potential energy surface (PES) has been determined for CO2 by refining a purely ab initio PES with selected, purely experimentally determined rovibrational energy levels. The purely ab initio PES is denoted Ames-0, while the refined PES is denoted Ames-1. Detailed tests are performed to demonstrate the spectroscopic accuracy of the Ames-1 PES. It is shown that Ames-1 yields σrms (root-mean-squares error) = 0.0156 cm−1 for 6873 J = 0–117 12C16O2 experimental energy levels, even though less than 500 12C16O2 energy levels were included in the refinement procedure. It is also demonstrated that, without any additional refinement, Ames-1 yields very good agreement for isotopologues. Specifically, for the 12C16O2 and 13C16O2 isotopologues, spectroscopic constants Gv computed from Ames-1 are within ±0.01 and 0.02 cm−1 of reliable experimentally derived values, while for the 16O12C18O, 16O12C17O, 16O13C18O, 16O13C17O, 12C18O2, 17O12C18O, 12C17O2, 13C18O2, 13C17O2, 17O13C18O, and 14C16O2 isotopologues, the differences are between ±0.10 and 0.15 cm−1. To our knowledge, this is the first time a polyatomic PES has been refined using such high J values, and this has led to new challenges in the refinement procedure. An initial high quality, purely ab initio dipole moment surface (DMS) is constructed and used to generate a 296 K line list. For most bands, experimental IR intensities are well reproduced for 12C16O2 using Ames-1 and the DMS. For more than 80% of the bands, the experimental intensities are reproduced with σrms(ΔI) < 20% or σrms(ΔI/δobs) < 5. A few exceptions are analyzed and discussed. Directions for future improvements are discussed, though it is concluded that the current Ames-1 and the DMS should be useful in analyzing and assigning high-resolution laboratory or astronomical spectra.