Non-covalent interactions govern the properties of liquids, crystal packing forces, protein structure, and drug-ligand binding. We have developed very efficient theoretical methods and computer software to compute non-covalent interactions, including analysis in terms of electrostatics, polarization, and London dispersion forces. This software has been implemented in the open-source Psi4 quantum chemistry program, which leverages recent theoretical advances for substantial speedups. We used a fragment-based partitioning of symmetry-adapted perturbation theory (F-SAPT) to examine how non-covalent interactions can stabilize transition states in organocatalysis, specifically, in the Houk-List reaction mechanism for the proline-catalyzed aldol reaction between benzaldehyde and cyclohexanone. We have used F-SAPT to directly compute interactions between pairs of functional groups to determine the most important contacts in the binding of inhibitors to Factor Xa, a protein in the blood coagulation cascade. Surprisingly, substituent effects in that system are not due to one or two specific good contacts, but instead to subtle changes in dipole-dipole interactions throughout the binding pocket.

Phosphine-borane adducts are a well-known moiety in synthetic and coordination chemistry. These complexes form a dative bond in which the Lewis basic phosphorus atom donates electron density into an empty p-orbital of the Lewis acidic boron atom. However, donation of the phosphorus lone pair is not the only stabilizing interaction, as hyperconjugation and electrostatic interaction also play important roles in bonding. This presentation describes a detailed density functional theory level (B3LYP) study completed to determine the impact electron-donating and withdrawing substituents have on phosphine-borane bonds through the investigation of a series of para-substituted PAr3-BH3 and PH3-BAr3 phosphine-borane adducts. Natural bond orbital (NBO) partitioning was used to calculate the distribution of electron density between the phosphine and borane fragments. Extended transition state and natural orbitals for chemical valence (ETS-NOCV) analysis was used to isolate contributions to the overall electronic interaction of the phosphine-borane adducts. Molecular orbital composition and charge donation was calculated using AOMix. The resulting data was correlated with Hammett σ constants.

A “first principles” potential energy function with flexible monomers is developed for carbon dioxide (CO₂) gas phase systems. This function is constructed through a fit to the electronic energies of CO₂ monomers and dimers at the CCSD(T)-F12b/aug-cc-pVTZ level and trimers at the CCSD(T)/aug-cc-pVDZ level. Thousands of CO₂ configurations were used to train the potential function, which was then used to optimize the structures of CO₂ clusters ranging in size from 3 – 13 molecules. The anharmonic vibrational frequencies of the minimum energy structures were obtained using the vibrational self-consistent-field (VSCF) and vibrational configuration-interaction (VCI) methods.

We present some of our contributions to the advancement of explicitly time-dependent coupled-cluster theory towards the efficient computation of linear absorption spectra of molecular systems. Our newly developed explicitly time-dependent equation-of-motion coupled-cluster (TD-EOM-CC) formalism based on the propagation of the CC dipole function possesses great potential for the efficient computation of linear absorption spectra over arbitrarily wide energy windows. This approach introduces no approximations and requires only half of the computational effort of TD-CC methods based on the propagation of the wave function. The TD-EOM-CC formalism is particularly useful when dealing with molecular systems with high density-of-states, or if the spectral region of interest spans a large energy range (> 10 eV). We perform illustrative calculations of the UV-Vis, NEXAFS, and electronic circular dichroism (ECD) spectra of several molecular species relevant to the study of atmospheric chemistry and astrochemistry. Furthermore, we exploit extrapolation techniques based on Pade approximants, these techniques are shown to significantly reduce the computational effort required to simulate NEXAFS spectra.

Developed during the last decade by Ruscic and collaborators at Argonne National Laboratory, Active Thermochemical Tables (ATcT) represents an entirely new and revolutionary way to approach the subject of thermochemistry. Traditional thermochemistry has advanced piecemeal and willy nilly, through individual spectroscopic, kinetic, calorimetric, etc. measurements often supplemented by `recommendation of `standard’’ (in the non-thermodynamic sense) through the actions of various critical review committees (NIST-JANAF, CODATA, etc.). In contrast, ATcT is a holistic approach that views each molecule as connected – in principle – to all other species through within constructs called thermochemical networks. In ATcT, thermochemical parameters such as bond energies, ionization potentials, and enthalpies of formation are solved for self-consistently using all available relevant information. Many bond energies once known to, say, a few kcal mol-1 have now been established with a precision that is at least an order of magnitude better than before, which clearly has enormous practical consequences for modelling studies. This talk reviews the surprisingly interesting topic of “where do these heats of formation come from?” and outlines the basic ideas in ATcT. Due to the high interconnectedness of many chemical species through the thermochemical network paradigm, it transpires that knowing any individual property (say, an ionization potential) can potentially impact properties of any number of different species. Hence, there is virtue to constantly improving our knowledge of fundamental molecular properties that goes well beyond just “putting another decimal place on it”. ATcT actively seeks accurate measurements and calculations for key quantities, which will be illustrated by a recent study of the photoelectron spectrum of hydrogen peroxide.

Psi4 is an open-source and freely available electronic structure package written in C++ for speed and Python for ease of interaction. It includes most common methods including density functional theory, many-body-perturbation theory, coupled-cluster theory, complete active space SCF, and configuration interaction. Enhancements to the top-level driver enable automatic basis-set extrapolations for optimizations, easily-specified composite methods, and n-body counterpoise-correction wrappers. Through transparent interfacing with external libraries, Psi4 gains capabilities in dispersion correction, PCM solvation, density-matrix renormalization group, effective fragment potentials (EFP), and relativistic corrections. After extensive infrastructure changes made over the past year, Psi4 is easier to obtain, extend, and interface. While retaining the ability to run as an executable with the simple text Psithon syntax, Psi4 gains a new mechanism of importing as a module into python `python -c "import psi4"` and thus ease of calling quantum chemistry operations from a Python script or Jupyter notebook. How to interact with Psi4 for research, education, and development will be discussed.

The quartic force field (QFF) is a minimal potential energy surface that defines the potential portion of the internuclear Hamiltonian as a fourth-order Taylor series expansion. By utilizing a composite approach based on the CCSD(T) method with complete basis set extrapolation, core correlation, and scalar relativistic considerations, exceptional accuracies can be achieved. Vibrational frequencies comparing to as good as 1 cm^-1 can be produced and rotational constants within a few dozen MHz are also possible. Furthermore, combinations of QFFs for different electronic states, charges, or reaction minima can produce exceptionally accurate electronic spectra, photoionization energies, and reaction schema. Recent successes will be discussed including the photoelectron spectrum of CP^- and C₂P^-, energetics for the formation/destruction of the ArHAr⁺ proton-bound complex, and the rotational constants of HPSi. Furthermore, predictions for such species related to as-of-yet unobserved properties will also reported.