Symmetry-adapted Perturbation Theory Research Articles

Origin of the intermolecular forces that produce donor–acceptor stacks in π-conjugated charge-transfer complexes

The attraction between π-conjugated planar electron donor and acceptor molecules that form many stable charge-transfer (CT) complexes has been explained by quantum chemical CT interactions, although the fundamental origin remains unclear. Here, we demonstrate the mechanism of CT complex formation by potential energy map analysis for TTF–CA and BTBT–TCNQ, using energy decomposition of intermolecular interaction by symmetry-adapted perturbation theory (SAPT) combined with coupled cluster calculation. We find that the source of attraction between donor and acceptor molecules is ascribed primarily to the dispersion force and also to the electrostatic force. In contrast, the contribution of CT interactions to the attractive forces is minimal. We demonstrate that the highly directional feature of the exchange repulsion force, coupled with the attractive dispersion and electrostatic forces, is crucial in determining the intermolecular arrangements of actual CT crystals. These findings are key for understanding the unique structural and electronic properties of π-conjugated CT complexes.

2D Nitrogen-Doped Graphene Materials for Noble Gas Separation.

Noble gases, notably xenon, play a pivotal role in diverse high-tech applications. However, manufacturing xenon is an inherently challenging task, due to its unique properties and trace abundance in the Earth's atmosphere. Consequently, there is a pressing need for the development of efficient methods for the separation of noble gases. Using mild fluorographene chemistry, nitrogen-doped graphene (GNs) materials are synthesized with abundant aromatic regions and extensive nitrogen doping within the vacancies and holes of the aromatic lattice. Due to the organized interlayer "nanochannels", nitrogen functional groups, and defects within the two-dimensional (2D)structures, GNs exhibits effective selectivity for Xe over Kr at low pressure. This enhanced selectivity is attributed to the stronger binding affinity of Xe to GN compared to Kr. The adsorption is governed by London dispersion forces, as revealed by theoretical calculations using symmetry-adapted perturbation theory (SAPT). Investigation of other GNs differing in nitrogen content, surface area, and pore sizes underscores the significance of nitrogen functional groups, defects, and interlayer nanochannels over thesurface area in achieving superior selectivity. This work offers a new perspective on the design and fabrication of functionalized graphene derivatives, exhibiting superior noble gas storage and separation activity exploitable in gas production technologies.

Dispersion leading potential energy surface of N2·NbN12-: Anion photoelectron spectroscopy and theoretical studies.

In order to understand the dispersion interactions between molecules and to provide information about the potential energy surface of geometry evolutions, NbN12- and N2·NbN12- complexes were investigated by using photoelectron spectroscopy and abinitio calculations. The experimental adiabatic detachment energy (ADE) and vertical detachment energy (VDE) of NbN12- were both measured to be 2.129 ± 0.030eV. The experimental ADE and VDE of N2·NbN12- were measured to be 2.17 ± 0.05 and 2.23 ± 0.05eV, respectively, which are slightly higher than those of NbN12-. The structures of NbN12-/0 were confirmed to be hexacoordinated octahedrons. The investigation of N2·NbN12- structures shows that it is stable for N2 to bind to the face or vertex site of octahedron NbN12-; the face-side-on structure has the lowest energy. The calculations based on symmetry-adapted perturbation theory suggest that the dispersion term is predominant and leads to the stability of N2·NbN12- complexes.

Dissociation Energies via Embedding Techniques.

Due to the large number of interactions, evaluating interaction energies for large or periodic systems results in time-consuming calculations. Prime examples are liquids, adsorbates, and molecular crystals. Thus, there is a high demand for a cheap but still accurate method to determine interaction energies and gradients. One approach to counteract the computational cost is to fragment a large cluster into smaller subsystems, sometimes called many-body expansion, with the fragments being molecules or parts thereof. These subsystems can then be embedded into larger entities, representing the bigger system. In this work, we test several subsystem approaches and explore their limits and behaviors, determined by calculations of trimer interaction energies. The methods presented here encompass mechanical embedding, point charges, polarizable embedding, polarizable density embedding, and density embedding. We evaluate nonembedded fragmentation, QM/MM (quantum mechanics/molecular mechanics), and QM/QM (quantum mechanics/quantum mechanics) embedding theories. Finally, we make use of symmetry-adapted perturbation theory utilizing density functional theory for the monomers to interpret the results. Depending on the strength of the interaction, different embedding methods and schemes prove favorable to accurately describe a system. The embedding approaches presented here are able to decrease the interaction energy errors with respect to full system calculations by a factor of up to 20 in comparison to simple/unembedded approaches, leading to errors below 0.1 kJ/mol.

Electrostatically embedded symmetry-adapted perturbation theory.

Symmetry-adapted perturbation theory (SAPT) is an abinitio approach that directly computes noncovalent interaction energies in terms of electrostatics, exchange repulsion, induction/polarization, and London dispersion components. Due to its high computational scaling, routine applications of even the lowest order of SAPT are typically limited to a few hundred atoms. To address this limitation, we report here the addition of electrostatic embedding to the SAPT (EE-SAPT) and ISAPT (EE-ISAPT) methods. We illustrate the embedding scheme using water trimer as a prototype example. Then, we show that EE-SAPT/EE-ISAPT can be applied for efficiently and accurately computing noncovalent interactions in large systems, including solvated dimers and protein-ligand systems. In the latter application, particular care must be taken to properly handle the quantum mechanics/molecular mechanics boundary when it cuts covalent bonds. We investigate various schemes for handling charges near this boundary and demonstrate which are most effective in the context of charge-embedded SAPT.

More quantum chemistry with fewer qubits

Quantum computation is one of the most promising new paradigms for the simulation of physical systems composed of electrons and atomic nuclei, with applications in chemistry, solid-state physics, materials science, and molecular biology. This requires a truncated representation of the electronic structure Hamiltonian using a finite number of orbitals. While it is, in principle, obvious how to improve on the representation by including more orbitals, this is usually unfeasible in practice (e.g., because of the limited number of qubits available) and severely compromises the accuracy of the obtained results. Here, we propose a quantum algorithm that improves on the representation of the physical problem by virtue of second-order perturbation theory. In particular, our quantum algorithm evaluates the second-order energy correction through a series of time-evolution steps under the unperturbed Hamiltonian. An important application is to go beyond the active-space approximation, allowing to include corrections of virtual orbitals, known as multireference perturbation theory. Here, we exploit the fact that the unperturbed Hamiltonian is diagonal for virtual orbitals and show that the number of qubits is independent of the number of virtual orbitals. This gives rise to more accurate energy estimates without increasing the number of qubits. Moreover, we demonstrate numerically for realistic chemical systems that the total runtime has highly favorable scaling in the number of virtual orbitals compared to previous studies. Numerical calculations confirm the necessity of the multireference perturbation theory energy corrections to reach accurate ground-state energy estimates. Our perturbation theory quantum algorithm can also be applied to symmetry-adapted perturbation theory. As such, we demonstrate that perturbation theory can help to reduce the quantum hardware requirements for quantum chemistry. Published by the American Physical Society 2024

End Group Effects on Anion Binding in Tetraglycine Peptide: A Computational Study.

The importance of anions in various processes has led to a search for molecules that can effectively recognize and interact with these anions. This study explores how the tetraglycine (Gly)4 peptide in its zwitterionic, neutral, and terminally capped forms acts as a receptor for H2PO4- and HSO4- anions within the framework of supramolecular host-guest chemistry. Using molecular dynamics (MD) simulations, we obtained the conformations of the receptor-anion complexes. Density functional theory (DFT), quantifies the complexes' interaction energies in both gas and solvent phases. Proton transfer within the zwitterionic complex with H2PO4- anion alters peptide charge distribution, affecting its conformation and binding site arrangement, as analysed by quantum mechanics/molecular mechanics (QM/MM) methods. Symmetry-adapted perturbation theory (SAPT) and noncovalent interactions analysis highlight the role of electrostatic interactions in these receptor-anion complexes. It emphasizes the key interactions such as N-H···O and O-H···O=C between the peptide backbone and anions and elucidates the molecular recognition mechanism driven by crucial noncovalent interactions. The termination of the peptide's end groups modulates anion binding sites from the backbone to the charged N-terminal, resulting in distinct binding sites. Our findings provide insights for designing peptides tailored to function as anion receptors in diverse supramolecular chemistry applications.

Non-covalent interactions and charge transfer in the CO activation by low-valent group 14 complexes.

The CO activation by low-valent group 14 catalysts encompasses the rupture of varied covalent bonds in a single transition state through a concerted pathway. The bond between the central main group atom and the hydride in the complex is elongated to trigger the formation of the C-H bond with CO accompanied by the concomitant formation of the E-O bond between the complex and CO to lead the corresponding formate product. Prior studies have established that besides the apolar nature of CO , its initial interaction with the complex is primarily governed by electrostatic interactions. Notably, other stabilizing interactions and the transfer of charge between catalysts and CO during the initial phases of the reaction have been ignored. In this study, we have quantified the non-covalent interactions and charge transfer that facilitate the activation of CO by group 14 main group complex. Our findings indicate that electrostatic interactions predominantly stabilize the complex and CO in the reactant region. However, induction energy becomes the main stabilizing force as the reaction progresses towards the transition state, surpassing electrostatics. Induction contributes about 50% to the stabilization at the transition state, followed by electrostatics (40%) and dispersion interactions (10%). Atomic charges calculated with the minimal basis iterative stockholder (MBIS) method reveal larger charge transfer for the back-side reaction path in which CO approaches the catalysts in contrast to the front-side approach. Notably, it was discovered that a minor initial bending of CO to approximately initiates the charge transfer process for all systems. Furthermore, our investigation of group 14 elements demonstrates a systematic reduction in both activation energies and charge transfer to CO while descending in group 14. All studied reactions were characterized along the reaction coordinate obtained with the intrinsic reaction coordinate (IRC) methodology at the M06-2X/6-31g(d,p) level of theory. Gibbs free energy in toluene was computed using electronic energies at the DLPNO-CCSD(T)/cc-pVTZ-SSD(E) level of theory. Vibrational and translational entropy corrections were applied to provide a more accurate description of the obtained Gibbs free energies. To better characterize the changes in the reaction coordinate for all reactions, the reaction force analysis (RFA) has been employed. It enables the partition of the reaction coordinate into the reactant, transition state, and product regions where different stages of the mechanism occur. A detailed characterization of the main non-covalent driving forces in the initial stages of the activation of CO by low-valent group 14 complexes was performed using symmetry-adapted perturbation theory (SAPT). The SAPT0-CT/def2-SVP method was employed for these computations. Charge transfer descriptors based on atomic population using the MBIS scheme were also obtained to complement the SAPT analyses.

Anisole-Water and Anisole-Ammonia Complexes in Ground and Excited (S1) States: A Multiconfigurational Symmetry-Adapted Perturbation Theory (SAPT) Study.

Binary complexes of anisole have long been considered paradigm systems for studying microsolvation in both the ground and electronically excited states. We report a symmetry-adapted perturbation theory (SAPT) analysis of intermolecular interactions in anisole-water and anisole-ammonia complexes within the framework of the multireference SAPT(CAS) method. Upon the S1 ← S0 electronic transition, the hydrogen bond in the anisole-water dimer is weakened, which SAPT(CAS) shows to be determined by changes in the electrostatic energy. As a result, the water complex becomes less stable in the relaxed S1 state despite decreased Pauli repulsion. Stronger binding of the anisole-ammonia complex following electronic excitation is more nuanced and results from counteracting shifts in the repulsive (exchange) and attractive (electrostatic, induction and dispersion) forces. In particular, we show that the formation of additional binding N-H···π contacts in the relaxed S1 geometry is possible due to reduced Pauli repulsion in the excited state. The SAPT(CAS) interaction energies have been validated against the coupled cluster (CC) results and experimentally determined shifts of the S1 ← S0 anisole band. While for the hydrogen-bonded anisole-water dimer SAPT(CAS) and CC shifts are in excellent agreement, for ammonia SAPT(CAS) is only qualitatively correct.

Exploring Ion Polarizabilities and Their Correlation with van der Waals Radii: A Theoretical Investigation.

Polarizability (α) is a fundamental property which measures the tendency of the electron cloud of an atom, ion, or molecule to be distorted by electric field. Polarizability contributes to important physical properties such as molecular interactions or dielectric constants; thus, it is essential to have accurate polarizabilities in molecular simulations. However, it remains a challenge to develop polarizable force fields (FFs) for ions in computational chemistry. In particular, a comprehensive set of polarizabilities for ions has not been derived. Herein, we derived a systematic set of polarizabilities for atoms and ions across the periodic table based on high-level quantum mechanics calculations. These values have excellent agreement with experimental data. Furthermore, we examined the relationship between the obtained polarizabilities and the van der Waals (VDW) radii (RVDW) that we previously determined (J. Chem. Theory Comput., 2023, 19, 2064). Two relationships, RVDW ∝ α1/7 and RVDW ∝ α1/3, proposed in previous studies were examined in the present work. Our results indicated the former relationship, which was derived based on the quantum harmonic oscillator model, prevails for atoms and cations, but neither relationship provides a satisfactory fit for anions. This is consistent with the tight-binding nature of the electrons in atoms and cations, while it is more challenging to quantify the polarizabilities of anions because of their more dispersed electron clouds. Moreover, we compared different approaches to determine the dispersion coefficients, including the London equation, Slater-Kirkwood equation, symmetry-adapted perturbation theory (SAPT) calculations, and time-dependent density functional theory method, along with the approach based on VDW constants. Our results indicated that although different approaches predict deviated magnitudes for the dispersion coefficients, their predictions are highly correlated, implying that each of these approaches can be used to evaluate dispersion interactions after proper scaling. Finally, we have developed a parametrization strategy for the 12-6-4 model based on the obtained insights. We specifically compared the performance of the 12-6-4 model with SAPT and SobEDA analyses to model interactions involving Na+/Mg2+ and various ligands containing He, Ne, Ar, H2O, NH3, [H2PO4]-, and [HPO4]2-. Our results demonstrate that the 12-6-4 parameters effectively reproduce both the total interaction energy and the individual energy components (electrostatics, exchange-repulsion, dispersion, and induction), highlighting the physical robustness of the 12-6-4 model and the effectiveness of our parametrization approach. This study has significant implications for advancing the development of next-generation ion models and polarizable FFs.

Interaction of fullerenes C60 with pristine and substituted buckybowls: A theoretical study

Non-covalent interactions between experimentally available buckybowls (coronene, corannulene, sumanene, triazasumanene, pentachlorocorannulene, decachlorocorannulene) and fullerenes C60 were systematically studied by using several theoretical methods. Peculiarities of these interactions were determined using electrostatic potential maps, independent gradient model, and symmetry adapted perturbation theory (SAPT0). SAPT0 calculations confirmed that dispersion (contribute 63–70 % in attraction) and electrostatic interactions (23–28 %) play the major role for C60 binding, whereas induction forces contribute to Eint only moderately (5–7%) for all structures studied herein. Cl-substituted corannulenes were calculated to be the most favorable structures for C60 binding. Ab initio molecular dynamics (AIMD) simulations confirmed stability of the studied complexes at different temperatures. Our investigations established the high potential of the studied buckybowls for usage in molecular tweezers.

Quantification of Anisotropy in Exchange and Dispersion Interactions: A Simple Model for Physics-Based Force Fields.

In some compounds, exchange repulsion is orientation dependent. However, in contrast to quantum chemical methods that treat exchange explicitly, empirical models assume exchange to be spherically symmetric, yielding an average description only. Here we quantify the anisotropy of exchange and dispersion energy for hydrogen halides and water by probing these compounds with a helium atom using the symmetry-adapted perturbation theory (SAPT). The exchange interaction is reduced by up to 33% due to the σ-hole in hydrogen iodide, depending on the location of the probe. We demonstrate how this anisotropy can be modeled in empirical force fields either using an angle-dependent potential or by introducing virtual sites, reducing the error in the empirical model by a factor of 5 compared to isotropic atoms. Lone-pairs on water, positioned close to perpendicular to the plane of the molecule, on a line with the oxygen atom, and, surprisingly, σ-holes on water both modulate the exchange interaction strongly. Both lone-pairs and σ-holes can be modeled by virtual sites, leading to an 80% reduced error.

Optimization of damping function parameters for -D3 and -D4 dispersion models for Hartree-Fock based symmetry-adapted perturbation theory.

Symmetry-adapted perturbation theory (SAPT) directly computes intermolecular interaction energy in terms of electrostatics, exchange-repulsion, induction/polarization, and London dispersion components. In SAPT based on Hartree-Fock ("SAPT0") or based on density functional theory, the most time-consuming step is the computation of the dispersion terms. Previous work has explored the replacement of these expensive dispersion terms with simple damped asymptotic models. We recently examined [Schriber et al. J. Chem. Phys. 154, 234107 (2021)] the accuracy of SAPT0 when replacing its dispersion term with Grimme's popular -D3 correction, reducing the computational cost scaling from O(N5) to O(N3). That work optimized damping function parameters for SAPT0-D3/jun-cc-pVDZ using estimates of the coupled-cluster complete basis set limit [CCSD(T)/CBS] on a 8299 dimer dataset. Here, we explore the accuracy of SAPT0-D3 with additional basis sets, along with an analogous model using -D4. Damping parameters are rather insensitive to basis sets, and the resulting SAPT0-D models are more accurate on average for total interaction energies than SAPT0. Our results are surprising in several respects: (1) improvement of -D4 over -D3 is negligible for these systems, even charged systems where -D4 should, in principle, be more accurate; (2) addition of Axilrod-Teller-Muto terms for three-body dispersion does not improve error statistics for this test set; and (3) SAPT0-D is even more accurate on average for total interaction energies than the much more computationally costly density functional theory based SAPT [SAPT(DFT)] in an aug-cc-pVDZ basis. However, SAPT0 and SAPT0-D3/D4 interaction energies benefit from significant error cancellation between exchange and dispersion terms.

Range Separation of the Interaction Potential in Intermolecular and Intramolecular Symmetry-Adapted Perturbation Theory.

Symmetry-adapted perturbation theory (SAPT) is a popular and versatile tool to compute and decompose noncovalent interaction energies between molecules. The intramolecular SAPT (ISAPT) variant provides a similar energy decomposition between two nonbonded fragments of the same molecule, covalently connected by a third fragment. In this work, we explore an alternative approach where the noncovalent interaction is singled out by a range separation of the Coulomb potential. We investigate two common splittings of the 1/r potential into long-range and short-range parts based on the Gaussian and error functions, and approximate either the entire intermolecular/interfragment interaction or only its attractive terms by the long-range contribution. These range separation schemes are tested for a number of intermolecular and intramolecular complexes. We find that the energy corrections from range-separated SAPT or ISAPT are in reasonable agreement with complete SAPT/ISAPT data. This result should be contrasted with the inability of the long-range multipole expansion to describe crucial short-range charge penetration and exchange effects; it shows that the long-range interaction potential does not just recover the asymptotic interaction energy but also provides a useful account of short-range terms. The best consistency is attained for the error-function separation applied to all interaction terms, both attractive and repulsive. This study is the first step toward a fragmentation-free decomposition of intramolecular nonbonded energy.

A comparative study of the carbon-based nanostructures for the delivery and detection of drug metronidazole

The interactions between the metronidazole molecule and three different carbon-based nanostructures, fullerene (C60), carbon nanotube (CNT), and carbon nanosheet (CNS), were examined using density functional theory (DFT) calculations. The results indicate that C60 attaches metronidazole more stronger than CNT and CNS, and the adsorption energies of metronidazole with C60, CNT, and CNS are decreased after considering the solvent effects. The variation in energy gaps follows the order of CNS > C60 > CNT, indicating that CNS has better sensing performance for metronidazole compared to fullerene and CNT. Both CNT and CNS show a reasonable recovery time at an achievable operating temperature. According to the quantum theory of atoms in molecules (QTAIM) and symmetry-adapted perturbation theory (SAPT), the dispersion component is the primary factor in stabilization, followed by electrostatic interaction. Simulated infrared (IR) and ultraviolet-visible (UV–Vis) spectra provide a basis reference for further structural characterization. This work indicates that carbon-based nanomaterials have potential applications in the delivery and detection of metronidazole.

The carbonyl-sulfur chalcogen bonding interaction: Rotational spectroscopic study of the 2,2,4,4-tetrafluoro-1,3-dithietane···formaldehyde complex

The rotational spectrum of a binary molecular cluster consisting of 2,2,4,4-tetrafluoro-1,3-dithietane (C2S2F4) and formaldehyde (H2CO) was studied by means of high-resolution Fourier transform microwave spectroscopy in conjunction with quantum chemical calculations. One of the three isomers predicted at the B3LYP-D3(BJ)/def2-TZVP level of theory was successfully detected in the supersonic expansion. Theoretical analyses using the non-covalent interactions and natural bond orbital methods reveal that the observed isomer is primarily stabilized by one C=O⋯S chalcogen bond and two C−H⋯F hydrogen bonds. The distance between the oxygen atom of H2CO and the nearest sulfur atom of C2S2F4 within the observed isomer is 2.9260(1) Å and the angle ∠O⋯S−C is 161.83(1)°. The analysis utilizing the symmetry-adapted perturbation theory approach demonstrates that electrostatic interactions play a significant role in stabilizing the studied complex, with the contribution of dispersion interactions being comparable to that of electrostatic ones.

Regioselectivity and physical nature of the interactions between (methyl)guanine with HCl and CH3OH

A comprehensive study of the hydrogen bonding interactions between guanine (G) and methyl guanine derivatives (MGs) in the presence of HCl and MeOH is carried out at B3LYP, B3LYP-D3 and M062X/6–311 + + G(d.p) levels using molecular electrostatic potential, natural bond orbital, and symmetry adapted perturbation theory. Making use of these state-of-the-art techniques, this study attempts to elucidate the chemical bonding, regioselectivity, and physical nature of the interactions responsible for the stability of (M)G…L (L = HCl, MeOH) acid–base complexes. Our calculations reveal that 1-G, 3-MG, and 5-MG interact more strongly with MeOH than HCl due to the positive hydrogen bond cooperativity. Furthermore, the carbonyl site on G is found to be the most reactive site, while methyl substitution increases the basicity of the nucleobase, thus yieding more stable complexes. The strongest H-bond interaction in G-complexes is found when HCl and MeOH attack the carbonyl group in anti-position. Finally, energy decomposition analyses through the symmetry-adapted perturbation theory reveal that most complexes are mainly stabilized via electrostatic interactions. The energy difference between complex isomers shows a competition between 3-HCl-G (MG) and 4-HCl-G (MG) at ∆G level where thermal, BSSE and entropy terms are included.

Stabilization of the Protein Structure by the Many-Body Cooperative Effect in the NH/π Hydrogen-bonding Tryptophan Triad.

The indole ring of tryptophan can form NH/π hydrogen bonds, acting both as a hydrogen donor at the NH group in the pyrrole subring and as a hydrogen acceptor at the benzene subring. In the structural core of the trimeric stable protein Pholiota squarrosa lectin (PhoSL), three indoles are symmetrically arranged and form NH/π hydrogen bonds among each other. Here, we conducted quantum chemical calculations on this indole triad by using various methods and basis sets. The analyses revealed cooperativity in triad formation, with the many-body effect contributing approximately -2 kcal mol-1, which significantly stabilizes this protein. Symmetry-adapted perturbation theory ascribed this effect to the induced polarization. The electrostatic potential and atomic charges indeed revealed a charge redistribution through the NH/π hydrogen bond, which was favorable for triad formation.

Unveiling the energetic complexity of noncovalent interactions in halogenated dimers

AbstractThe understanding of noncovalent interactions is crucial in explaining critical phenomena such as self‐assembly, chemical reactivity, and crystallization. This work examines the energetic diversity of conformations and local minima for several halogenated dimers, represented as R‐X (R = H, F, CH3, CF3; X = Cl, Br, I). Thousands of configurations are randomly generated and refined through geometric optimizations to yield a diverse set of molecular conformers. Frequency calculations were performed for all optimized conformers to confirm that they are local minima. The noncovalent interactions in optimized dimers of halogen‐containing molecules were analyzed with atom in molecules (AIM) method and symmetry‐adapted perturbation theory (SAPT). Additionally, a protocol for generating machine learning models to recover accurate predictions of the physically meaningful SAPT energy components with minor computational cost is presented. These results deepen our understanding of the intricate energy balance and dedicated equilibrium of different noncovalent interactions in halogenated dimers.

Quantum Mechanics Characterization of Non-Covalent Interaction in Nucleotide Fragments.

Accurate calculation of non-covalent interaction energies in nucleotides is crucial for understanding the driving forces governing nucleic acid structure and function, as well as developing advanced molecular mechanics forcefields or machine learning potentials tailored to nucleic acids. Here, we dissect the nucleotides' structure into three main constituents: nucleobases (A, G, C, T, and U), sugar moieties (ribose and deoxyribose), and phosphate group. The interactions among these fragments and between fragments and water were analyzed. Different quantum mechanical methods were compared for their accuracy in capturing the interaction energy. The non-covalent interaction energy was decomposed into electrostatics, exchange-repulsion, dispersion, and induction using two ab initio methods: Symmetry-Adapted Perturbation Theory (SAPT) and Absolutely Localized Molecular Orbitals (ALMO). These calculations provide a benchmark for different QM methods, in addition to providing a valuable understanding of the roles of various intermolecular forces in hydrogen bonding and aromatic stacking. With SAPT, a higher theory level and/or larger basis set did not necessarily give more accuracy. It is hard to know which combination would be best for a given system. In contrast, ALMO EDA2 did not show dependence on theory level or basis set; additionally, it is faster.