Previously adaptions of canonical approaches were applied to algebraic forms of the classic Morse, Lennard-Jones, and Kratzer potentials. Using the classic Morse, Lennard-Jones, or Kratzer potential as reference, inverse canonical transformations allow the accurate generation of Born-Oppenheimer potentials for H₂⁺ ion, neutral covalently bound H₂, van der Waals bound Ar₂, and the hydrogen bonded 1-dimensional dissociative coordinate in water dimer. This methodology is now extending to multidimensional potential energy surfaces, and as a proof-of-concept, it is applied to the 3-dimensional water molecule potential surface. Canonical transformations previously developed for diatomic molecules are used to construct accurate approximations to the 3-dimensional potential surface of the water molecule from judiciously chosen 1-dimensional planar slices that are shown to have the same canonical shape as the classical Lennard-Jones potential curve. Spline interpolation is then used to piece together the 1-dimensional canonical potential curves, to obtain the full 3-dimensional potential surface of water molecule with a relative error less than 0.008.

The ZnTe system exhibits very interesting optoelectronic properties. It is a promising candidate for the development of detectors of Terahertz (THz) radiation, as well as a growing number of applications, particularly in the area of radiology. In this work, we report a theoretical study of the ground state and various excited states of ZnTe and its ions ZnTe⁺ and ZnTe⁻. The potential energy curves are calculated using CASSCF method, as implemented in Molpro. These curves serve to determine the different spectroscopic constants such as the internuclear distance (R_e), the harmonic vibration frequency (ω_e), the rotation constant (B_e) and the dissociation energy (D_e). The results obtained are in good agreement with the available experimental data.

Annellation Theory and ZINDO/S semiempirical calculations have been used for the calculation of the locations of maximum absorbance (LMA) of the Ultraviolet-Visible (UV-Vis) of 31 C₃₄H₁₆ PAHs (molecular mass 424 Da) with unknown protocols of synthesis. The presence of benzo[a]pyrene bay-like regions and dibenzo[a,l]pyrene fjord-like regions in several of the structures that could be linked to an enhancement of the biological behavior and carcinogenic activity stresses the importance of C₃₄H₁₆ PAHs in fields like molecular biology and cancer research. In addition, the occurrence of large PAHs in oil asphaltenes exemplifies the importance of these calculations for the characterization of complex systems. The C₃₄H₁₆ PAH group is the largest molecular mass group of organic compounds analyzed so far following the Annellation Theory and ZINDO/S methodology. Future analysis using the same approach will provide evidence regarding the LMA of other high molecular mass PAHs.

The experimental photoassociation spectrum of B ← X transition of XeI over a broad internuclear distance range have been simulated quantum mechanically using a new spectral simulation technique including an improved potential model of the X state. The photoassociation spectrum generated from the simulated upper and lower potentials reproduces all spectral details of the experimental spectrum. Spectroscopic constants obtained are consistent with but unique compared to previously reported results. The V-R coupled energy structures of XeI molecules are also verified by the simulation results.

The underlying mechanism of how MALDI matrices work is poorly understood. Experimental literature suggests that the triplet excited state (T₁) of the matrix plays a significant role in its ability to transfer charge to the analyte effectively. The heavy atom substitution effect predicts that the addition of a heavy atom to an otherwise "dead" matrix, such as 2,4-dihydroxybenzoic acid, would increase the rate of Intersystem Crossing (ISC) to the T₁ state via spin-orbit coupling. This effect was observed experimentally as there was a visible decay in singlet lifetime and an increase in triplet lifetime, as well as a better matrix performance when compared to its original, unsubstituted partner. To provide insight into the photophysical properties of 2,4-dihydroxybenzoic acid and its halogenated isomers, calculations were performed using Gaussian09. Geometry optimizations, frequencies, and IR spectra of all isomers were calculated using Density Functional Theory (DFT) with B3LYP functional and the 6-31G+(d,p) basis set. UV-Vis and fluorescence spectra were generated using Time-Dependent DFT (TDDFT). The following values for the singlet ground state (S₀), triplet excited state (T₁), and singlet excited state (S₁) were tabulated and compared: optimization energies, HOMO-LUMO energies and orbital contours, and bond distances. In addition, the energy values for Proton Affinity (PA) and Gas Phase Acidity (GPA) were determined.

Scalar relativistic equation-of-motion coupled cluster (EOMCC) calculations of core ionization/excitation energies for a set of benchmark molecules are reported. The Arnoldi algorithm as well as the core-valence-separation (CVS) scheme have been used to expedite the convergence of the wave function for the core-ionized/excited states. Scalar relativistic effects have been accounted for using the spin-free exact two-component theory in its one-electron variant (SFX2C-1e) and their importance are assessed. Preliminary calculations of ligand core excitation spectra of transition-metal containing compounds are also presented.

In this work, we present a simple approach to obtain absorption spectra from hybrid QM/QM calculations. The goal is to obtain reliable spectra for compounds that are too large to be treated entirely at a high level of theory. The approach is based on the extrapolation of the entire absorption spectrum obtained by individual subcalculations. Our program locates the main spectral features in each subcalculation, e.g. band peaks and shoulders, and fits them to Gaussian functions. Each Gaussian is then extrapolated with a formula similar to that of ONIOM (Our own N-layered Integrated molecular Orbital molecular Mechanics). However, information about individual excitations is not necessary so that difficult state-matching across subcalculations is avoided. This multi-state extrapolation thus requires relatively low implementation effort while affording maximum flexibility in the choice of methods to be combined in the hybrid approach. The test calculations show the efficacy and robustness of this methodology in reproducing the spectrum computed for the entire molecule at a high level of theory.

Theoretical studies of spectroscopy and reaction dynamics including the necessary development of potential energy surfaces rely on accurate thermochemical information. The Active Thermochemical Tables (ATcT) approach by Ruscic¹ incorporates data for a large number of chemical species from a variety of sources (both experimental and theoretical) and derives a self-consistent network capable of making extremely accurate estimates of quantities such as temperature dependent enthalpies of formation. The network provides rigorous uncertainties, and since the values don’t rely on a single measurement or calculation, the provenance of each quantity is also obtained. To expand and improve the network it is desirable to have a reliable protocol such as the HEAT approach² for calculating accurate theoretical data. Here we present and benchmark an approach based on explicitly-correlated coupled-cluster theory and vibrational perturbation theory (VPT2). Methyldioxy and Methyl Hydroperoxide are important and well-characterized species in combustion processes and begin the family of (ethyl-, propyl-based, etc) similar compounds (much less is known about the larger members). Accurate anharmonic frequencies are essential to accurately describe even the 0 K enthalpies of formation, but are especially important for finite temperature studies. Here we benchmark the spectroscopic and thermochemical accuracy of the approach, comparing with available data for the smallest systems, and comment on the outlook for larger systems that are less well-known and characterized. ¹B. Ruscic, Active Thermochemical Tables (ATcT) values based on ver. 1.118 of the Thermochemical Network (2015); available at ATcT.anl.gov ²A. Tajti, P. G. Szalay, A. G. Császár, M. Kállay, J. Gauss, E. F. Valeev, B. A. Flowers, J. Vázquez, and J. F. Stanton. JCP 121, (2004): 11599.

The Optical Rotatory Dispersion (ORD) of a chiral solute is heavily affected by solvation, but this effect does not follow the usual correlation with the solvent polarity, i.e., larger solvent polarity does not imply a larger change in the solute’s property. Therefore, a great deal of experimental and theoretical effort has been directed towards correlating the solvation effect on the ORD and the solvent properties. This discovery followed from the development of cavity ring down polarimetry (CRPD), which allows measurements of gas-phase ORD. In order to investigate this phenomenon, we chose a set of five rigid molecules to limit the effect of molecular vibrations and isolate the role of solvation. The latter was investigated with the Polarizable Continuum Model (PCM), and compared to experimental results. We used Bondi radii to build the PCM cavity, and performed extensive calculations at multiple frequencies using density functional theory (DFT) with two functionals: B3LYP and CAM-B3LYP, together with the aug-cc-pVDZ basis set. We also performed coupled cluster singles and doubles (CCSD/aug-cc-pVDZ) calculations at the wavelengths where gas-phase data are available, all of which are augmented with zero point vibrational corrections. These results are compared to experimental data and seem to indicate that PCM does not entirely account for the environmental effects on the ORD.

r0pt Figure The potential energy surface (PES) constitutes a cornerstone for theoretical studies of spectroscopy and dynamics. We fit PESs using a local interpolating moving least squares (L-IMLS) approach.M. Majumder, S. Ndengue and R. Dawes, Molecular Physics 114, 1 (2016).he L-IMLS method is interpolative and has the flexibility to fit energies or energies and gradients, where inclusion of gradient information significantly reduces the number of points required for an accurate fit. The method permits fully automated PES generation: beginning with an initial set of seed points, an automatic point selection scheme determines where new data are required and, in a series of iterations, computes new ab initio data and updates the fit until a specified accuracy is reached. We have interfaced this fitting approach to popular electronic structure codes such as Molpro and CFOUR to automatically generate ab initio 4D PESs for vdWs systems composed of two (rigid) linear fragments. We present here our freely distributed code designed to run in parallel on a computing cluster, allowing the user to specify the system (masses, interatomic equilibrium distances, symmetry, energy range of interest, etc.) through an input file. For a selection of benchmark systems, we show that PESs with fitting errors below 1 cm⁻¹ can be constructed using only a few hundred ab initio points.

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Footnotes:

M. Majumder, S. Ndengue and R. Dawes, Molecular Physics 114, 1 (2016).T

Density functional theory (DFT) is an efficient computational tool that plays an indispensable role in the design and screening of π-conjugated organic molecules with photochemical significance. However, due to intrinsic problems in DFT such as self-interaction error, the accurate prediction of energy levels is still a challenging task.A. Dreuw and M. Head-Gordon, Chem. Rev. 105, 4009 (2015).unctionals can be parameterized to correct these problems, but the parameters that make a well-behaved functional are system-dependent rather than universal in most cases. To alleviate both problems, optimally tuned range-separated hybrid functionals were introduced, in which the range-separation parameter, ω, can be adjusted to impose Koopman's theorem, ε_HOMO = −I. These functionals turned out to be good estimators for asymptotic properties like ε_HOMO and ε_LUMO.O. A. Vydrov and G. E. Scuseria, J. Chem. Phys. 125, 234109 (2006).^,L. Kronik, T. Stein, S. Refaely−Abramson, and R. Baer, J. Chem. Theory Comput. 8, 1515 (2012).n the present study, we propose a "black−box" procedure that allows an automatic construction of molecule−specific range−separated hybrid functionals following the idea of such optimal tuning. However, instead of focusing on _HOMO and _LUMO, we target more local, photochemistry−relevant energy levels such as the lowest triplet state, T_1. In practice, we minimize the difference between two E_T_1′s that are obtained from two DFT−based approaches, −SCF and linear−response TDDFT. We achieve this minimization using a non−empirical adjustment of two parameters in the range−separated hybrid functional - , and the percentage of Hartree-Fock contribution in the short−range exchange, c_HF. We apply this triplet tuning scheme to a variety of organic molecules with important photochemical applications, including laser dyes, photovoltaics, and light−emitting diodes, and achieved good agreements with the spectroscopic measurements for E_T_1 L. Kronik, T. Stein, S. Refaely-Abramson, and R. Baer, J. Chem. Theory Comput. 8, 1515 (2012).IZ. Lin and T. A. Van Voorhis, in preparation for submission to J. Chem. Theory Comput.