On the Use of Quartic Force Fields in Variational Calculations

The use of quartic force fields (QFFs) has been shown to be one of the most effective ways to efficiently compute vibrational frequencies for small molecules. In this paper we outline and discuss how the simple-internal or bond-length bond-angle (BLBA) coordinates can be transformed into Morse-cosine(-sine) coordinates which produce potential energy surfaces from QFFs that possess proper limiting behavior and can effectively describe the vibrational (or rovibrational) energy levels of an arbitrary molecular system. We investigate parameter scaling in the Morse coordinate, symmetry considerations, and examples of transformed QFFs making use of the MULTIMODE, TROVE, and VTET variational vibrational methods. Cases are referenced where variational computations coupled with transformed QFFs produce accuracies compared to experiment for fundamental frequencies on the order of 5 cm(exp 1) and often as good as 1 cm(exp 1)

Pathways to Detection of Strongly-Bound Inorganic Species: The Vibrational and Rotational Spectral Data of AlH2OH, HMgOH, AlH2NH2, and HMgNH2

Small, inorganic hydrides are likely hiding in plain sight, waiting to be detected toward various astronomical objects. AlH2OH can form in the gas phase via a downhill pathway, and the present, high-level quantum chemical study shows that this molecule exhibits bright infrared features for anharmonic fundamentals in regions above and below that associated with polycyclic aromatic hydrocarbons. AlH2OH along with HMgOH, HMgNH2, and AlH2NH2 are also polar with AlH2OH having a 1.22 D dipole moment. AlH2OH and likely HMgOH have nearly unhindered motion of the hydroxyl group but are still strongly bonded. This could assist in gas phase synthesis, where aluminum oxide and magnesium oxide minerals likely begin their formation stages with AlH2OH and HMgOH. This work provides the spectral data necessary to classify these molecules such that observations as to the buildup of nanoclusters from small molecules can possibly be confirmed

Benchmark Structures and Conformational Landscapes of Amino Acids in the Gas Phase: A Joint Venture of Machine Learning, Quantum Chemistry, and Rotational Spectroscopy

The accurate characterization of prototypical bricks of life can strongly benefit from the integration of high resolution spectroscopy and quantum mechanical computations. We have selected a number of representative amino acids (glycine, alanine, serine, cysteine, threonine, aspartic acid and asparagine) to validate a new computational setup rooted in quantum-chemical computations of increasing accuracy guided by machine learning tools. Together with low-lying energy minima, the barriers ruling their interconversion are evaluated in order to unravel possible fast relaxation paths. Vibrational and thermal effects are also included in order to estimate relative free energies at the temperature of interest in the experiment. The spectroscopic parameters of all the most stable conformers predicted by this computational strategy, which do not have low-energy relaxation paths available, closely match those of the species detected in microwave experiments. Together with their intrinsic interest, these accurate results represent ideal benchmarks for more approximate methods

USING ULTRAFAST VIBRATIONAL SPECTROSCOPY FOR A COMPREHENSIVE UNDERSTANDING OF STRUCTURAL AND ROTATIONAL MOTIONS FOR WATER TO PROTIC IONIC LIQUIDS

In this work, two-dimensional infrared (2D-IR) spectroscopy investigates the timescale of solvent fluctuations for proton and hydride transfers. To elucidate hydride transfer dynamics, the BH stretch of \ce{BH4-} is probed in various solvents from \ce{H2O} to ionic liquids (ILs). For proton transfer dynamics, a vibrational probe (\ce{SCN-}) explores the three-dimensional hydrogen bonding environment of a protic ionic liquid (PIL). \ce{BH4-} is first investigated in increasing NaOH concentrations to develop a molecular understanding of suppressing the hydrogen evolution reaction. As the concentration increases, the timescale of frequency fluctuations decrease. Born Oppenheimer molecular dynamics (BOMD) simulations suggest that a crowding effect of ions around \ce{BH4-} inhibits the rearrangement of dihydrogen bonds between \ce{BH4-} and \ce{H2O}. To completely suppress the hydrogen evolution reaction, ILs with \ce{BH4-} as the anion are investigated. The linear and 2D-IR spectra of the antisymmetric BH stretch of \ce{BH4-} are complicated due to Fermi resonances. The narrow linear and 2D-IR linewidths of \ce{BH4-} in an IL allow a comprehensive assignment of all diagonal peaks and crosspeaks. Confirmed with a model Hamiltonian, two anharmonicities for the antisymmetric BH stretch of \ce{BH4-} are characterized. Polarization- and temperature-dependent 2D-IR is employed to investigate the hydrogen bonding network of the PIL ethyl-ammonium nitrate (EAN). \ce{SCN-} experiences two hydrogen bonding subensembles in EAN as two separate vibrational relaxation times are resolved. Furthermore, the polarization-weighted frequency fluctuation correlation function can be separated into two components: structural spectral diffusion (SSD) and reorientation-induced spectral diffusion (RISD). For \ce{SCN-} in EAN, the timescales of frequency fluctuations are in the rotational limit as the SSD is unresolved. Temperature-dependent 2D-IR extracts the enthalpy and entropy of activation for frequency fluctuations. For \ce{SCN-} in EAN, the enthalpy of activation for rotational motions are similar as to \ce{SCN-} in \ce{H2O}, and this suggests that the breaking and forming of hydrogen bonds around \ce{SCN-} undergoes a similar mechanism in EAN as in \ce{H2O}

Development of highly efficient and accurate real-space integration methods for Hartree-Fock and hybrid density functional calculations

The central focus of molecular electronic structure theory is to find approximate solutions to the electronic Schrödinger equation for molecules, and as such represents an essential part of any theoretical (in silico) study of chemical processes. However, a steep increase of the computational cost with increasing system size often prevents the application of accurate approximations to the molecules of interest. The main focus of the present work is the efficient evaluation of Fock-exchange contributions, which typically represents the computational bottleneck in Hartree-Fock (HF) and hybrid density functional theory (DFT) calculations. This bottleneck is addressed by means of seminumerical integration, i.e., one electronic coordinate within the 4-center-2-electron integral tensor is represented analytically and one numerically. In this way, an asymptotically linear scaling method for computing the exchange matrix (denoted as sn-LinK) is developed, enabling fast and accurate ab-initio calculations on large molecules, comprising hundreds or even thousands of atoms, even in combination with large atomic orbital basis sets. The novel sn-LinK method comprises improvements to the numerical integration grids, a rigorous, batch-wise integral screening scheme, the optimal utilization of modern, highly parallel compute architectures (e.g., graphics processing units; GPUs), and an efficient combination of single- and double-precision arithmetic. In total, these optimizations enable over two orders of magnitude faster evaluation of Fock-exchange contributions. Consequently, this greatly improved performance allows to perform previously unfeasible computations, which is also demonstrated at the example of an ab initio molecular dynamics simulation (AIMD) study on the hydrogen bond strengths within double-stranded DNA. In addition to Fock-exchange, the other two computational bottlenecks in hybrid-DFT applications – the evaluation of the Coulomb potential and the numerical integration of the semilocal exchange-correlation functional – are also addressed. Finally, more efficient methods to evaluate more accurate post-HF/DFT methods, namely the random-phase approximation (RPA) and the second-order approximate coupled cluster (CC2) method, are also put forward. In this way, the highly efficient methods introduced in this thesis cover some of the most substantial computational bottlenecks in electronic-structure theory – the evaluation of the Coulomb- and the exchange-interactions, the integration of the semilocal exchange-correlation functional, and the computation of post-Hartree-Fock correlation energies. Consequently, computational chemistry studies on large molecules (>100 atoms) are accelerated by multiple orders of magnitude, allowing for much more accurate and thorough in-silico studies than ever before

Development of explicitly correlated and many-body diagrammatic techniques for the investigation of electron-hole correlation in nanomaterials

The focus of this work is to develop theoretical methods that will accurately describe electron-electron and electron-hole correlation in nanoparticles using many-body diagrammatic techniques. Diagrammatic representation is a more complex representation of quantum mechanics, however, it becomes a more advantageous representation in its application to this work due to its ease of use. Diagrammatic techniques are essential to the ve methods presented here as they prove to be pivotal in theoretical development as well as providing useful information in extracting and visualizing fundamental physics to make useful approximations to the methods. In the projected congruent transformed Hamiltonian method with partial innite order summation of diagrams (PCTH-PIOS), diagrammatic summation approach was used. In the geminal projected conguration interaction (GPCI) method, diagrammatic factorization techniques were used. In the geminal screened electron-hole interaction kernel (GSIK) method, we conclude that only linked diagrams contribute to the exciton binding energy. The approximation is made to only include rst order diagrams which captures the essential physics of the electron-hole interaction. In the composite control-variate stratied sampling (CCSS) method the calculation of the vertices of the diagrams using stratied sampling. Lastly we investigate the eect of electromagnetic (EM) eld on the generation of 2e-2h states from 1e-1h states. In this work, time independent diagrams are calculated once and used for the rest of the calculation. Diagrammatic techniques are essential to the theoretical development of the methods in this work for understanding the optical and electronic properties of nanoparticles