Hartree-Fock Research Articles

Effect of Exact Exchange on the Energy Landscape in Self-Consistent Field Theory.

Density functional approximations can reduce the spin symmetry breaking observed for self-consistent field (SCF) solutions compared to Hartree-Fock theory, but the amount of exact Hartree-Fock (HF) exchange appears to be a key determinant in broken Ŝ2 symmetry. To elucidate the precise role of exact exchange, we investigate the energy landscape of unrestricted Hartree-Fock and Kohn-Sham density functional theory for benzene and square cyclobutadiene, which provide paradigmatic examples of closed-shell and open-shell electronic structures, respectively. We find that increasing the amount of exact exchange leads to more local SCF minima, which can be characterized as combinatorial arrangements of unpaired electrons in the carbon π system. Furthermore, we studied the pathways connecting local minima to understand the relationships between different solutions. Our analysis reveals a subtle balance between one- and two-body interactions in determining SCF symmetry breaking, shedding new light on the physical driving forces for spin-symmetry-broken solutions in SCF approaches.

Distance and Orientation Dependence of Triplet-Triplet Energy Transfer Couplings Based on Nonorthogonal Multireference Wave Functions.

Triplet-triplet energy transfer (TEnT) is of particular interest in various photochemical, photobiological, and energy science processes. It involves the exchange of spin and energy of electrons between two molecular fragments. Here, quasi-diabatic self-consistent field solutions were used to obtain the diabatic states involved in TEnT. The resonant Hartree-Fock approach was used to compute the nonorthogonal matrix elements for the two-state or four-state effective Hamiltonian and the overlap matrix. From the symmetric orthogonalized Hamiltonian, electronic coupling elements between the diabatic states in the TEnT process can be obtained. Two structural models, namely, naphthalene dimer and the 2,2'-bifluorene molecule, were employed to investigate the role of distance and orientation of the molecular fragments on the energy transfer process. It is observed that the inclusion of charge transfer states is critical to obtain the correct description of TEnT couplings. We discuss the effectiveness of the two-state model and four-state model in the successful evaluation of TEnT couplings. Spin density plots and biorthogonal orbitals were utilized to verify that the correct diabatic electronic structure of the TEnT states was determined.

A Cost-Effective Computational Strategy for the Electronic Layout Characterization of a Second Generation Light-Driven Molecular Rotary Motor in Solution.

Light-driven molecular rotary motors are nanometric machines able to convert light into unidirectional motions. Several types of molecular motors have been developed to better respond to light stimuli, opening new avenues for developing smart materials ranging from nanomedicine to robotics. They have great importance in the scientific research across various disciplines, but a detailed comprehension of the underlying ultrafast photophysics immediately after photo-excitation, that is, Franck-Condon region characterization, is not fully achieved yet. For this aim, it is first required to rely on an accurate description at ab initio level of the system in this potential energy region before performing any further step, that is, dynamics. Thus, we present an extensive investigation aimed at accurately describing the electronic structure of low-lying electronic states (electronic layout) of a molecular rotor in the Franck-Condon region, belonging to the class of overcrowded alkenes: 9-(2-methyl-2,3-dihydro-1H-cyclopenta[a]naphthalen-1-ylidene)-9H-fluorene. This system was chosen since its photophysics is very interesting for a more general understanding of similar compounds used as molecular rotors, where low-lying electronic states can be found (whose energetic interplay is crucial in the dynamics) and where the presence of different substituents can tune the HOMO-LUMO gap. For this scope, we employed different theory levels within the time-dependent density functional theory framework, presenting also a careful comparison adopting very accurate post Hartree-Fock methods and characterizing also the different conformations involved in the photocycle. Effects on the electronic layout of different functionals, basis sets, environment descriptions, and the role of the dispersion correction were all analyzed in detail. In particular, a careful treatment of the solvent effects was here considered in depth, showing how the implicit solvent description can be accurate for excited states in the Franck-Condon region by testing both linear-response and state-specific formalisms. As main results, we chose two cost-effective (accurate but relatively cheap) theory levels for the ground and excited state descriptions, and we also verified how choosing these different levels of theory can influence the curvature of the potential via a frequency analysis of the normal modes of vibrations active in the Raman spectrum. This theoretical survey is a crucial step towards a feasible characterization of the early stage of excited states in solution during photoisomerization processes wherein multiple electronic states might be populated upon the light radiation, leading to a future molecular-level interpretation of time-resolved spectroscopies.

Restoring rotational symmetry of multicomponent wavefunctions with nuclear orbitals.

In this work, we present a non-orthogonal configuration interaction (NOCI) approach to address the rotational corrections in multicomponent quantum chemistry calculations where hydrogen nuclei and electrons are described with orbitals under Hartree-Fock (HF) and density functional theory (DFT) frameworks. The rotational corrections are required in systems such as diatomic (HX) and nonlinear triatomic molecules (HXY), where localized broken-symmetry nuclear orbitals have a lower energy than delocalized orbitals with the correct symmetry. By restoring rotational symmetry with the proposed NOCI approach, we demonstrate significant improvements in proton binding energy predictions at the HF level, with average rotational corrections of 0.46eV for HX and 0.23eV for HXY molecules. For computing rotational excitation energies, our results indicate that HF kinetic energy corrections are consistently accurate, while discrepancies arise in total energy predictions, primarily from an incomplete treatment of dynamical correlation effects. Rotational energy corrections in multicomponent DFT calculations, using the epc17-2 proton-electron correlation functional, lead to an overestimation of proton binding energies. This is as a result of double-counting of proton-electron correlation effects in the off-diagonal NOCI terms. As a correction, we propose a scaling scheme that effectively adjusts the proton-electron correlation contributions, bringing our results into close agreement with reference CCSD(T) data. The scaled rotational corrections, on average, increase the epc17-2 proton binding energy predictions by 0.055eV for HX and 0.025eV for HXY and yield average deviations of 1.0 cm-1 for rotational transitions.

Symmetric Diblock Copolymers Form Versatile and Switchable Ultrasmall Nanoparticles.

Block copolymers (BCPs) can form nanoparticles having different morphologies that can be used as photonic nanocrystals and are a platform for drug delivery, sensors, and catalysis. In particular, BCP nanoparticles having disk-like shape have been recently discovered. Such nanodisks can be used as the next-generation antitumor drug delivery carriers; however, the applicability of the existing nanodisks is limited due to their poor or unknown ability to respond to external stimuli. In this work, we showed that the simplest symmetric diblock copolymers in equilibrium can form nanodisks that can be reversibly switched into a multitude of various nanoparticles potentially applicable in nanophotonics, biomedicine, and hierarchical self-assembly. These structures include patchy and onion-like nanoparticles, striped ellipsoids, mixed morphology nanocolloids, and spherical micelles. The transitions between nanodisks and the aforementioned nanoparticles are sharp, direct, and can be achieved by tuning the block-block and polymer-solvent incompatibility. We demonstrated that this versatility of nanoparticle morphologies can be achieved upon reducing the nanoparticle size to approximately two lamellar periods of the BCP. Upon aggregation of such small nanocolloids, a larger assembly can be formed. In turn, these bigger particles could form many other structures including a chain-like supramolecular aggregate of nanodisks and a multilayered disk-like nanoparticle. We obtained our results by performing self-consistent field theory calculations according to an algorithm designed to produce equilibrium nanoparticle morphology. This work demonstrates that nanodisks prepared from the simplest type of BCPs are extremely tunable; therefore, symmetric diblock copolymers can become a platform for producing the next-generation stimuli-responsive nanoparticles.

Universality of Mean-Field Antiferromagnetic Order in an Anisotropic 3D Hubbard Model at Half-Filling

We study Hartree–Fock theory at half-filling for the 3D anisotropic Hubbard model on a cubic lattice with hopping parameter t in the x- and y-directions and a possibly different hopping parameter tz in the z-direction; this model interpolates between the 2D and 3D Hubbard models corresponding to the limiting cases tz=0 and tz=t, respectively. We first derive all-order asymptotic expansions for the density of states. Using these expansions and units such that t=1, we analyze how the Néel temperature and the antiferromagnetic mean field depend on the coupling parameter, U, and on the hopping parameter tz. We derive asymptotic formulas valid in the weak coupling regime, and we study in particular the transition from the three-dimensional to the two-dimensional model as tz→0. It is found that the asymptotic formulas are qualitatively different for tz=0 (the two-dimensional case) and tz>0 0$$\\end{document}]]> (the case of nonzero hopping in the z-direction). Our results show that certain universality features of the three-dimensional Hubbard model are lost in the limit tz→0 in which the three-dimensional model reduces to the two-dimensional model.

Efficient Prediction of Superlattice and Anomalous Miniband Topology from Quantum Geometry

Two-dimensional materials subject to long-wavelength modulations have emerged as novel platforms to study topological and correlated quantum phases. In this article, we develop a versatile and computationally inexpensive method to predict the topological properties of materials subjected to a superlattice potential by combining degenerate perturbation theory with the method of symmetry indicators. In the absence of electronic interactions, our analysis provides a systematic rule to find the Chern number of the superlattice-induced miniband starting from the harmonics of the applied potential and a few material-specific coefficients. Our method also applies to anomalous (interaction-generated) bands, for which we derive an efficient algorithm to determine all Chern numbers compatible with a self-consistent solution to the Hartree-Fock equations. Our approach gives a microscopic understanding of the quantum anomalous Hall insulators recently observed in rhombohedral graphene multilayers. Published by the American Physical Society 2025

Role of Hartree–Fock exchange in spontaneous proton transfer reactions

Role of Hartree–Fock exchange in spontaneous proton transfer reactions

A cross-entropy corrected hybrid multiconfiguration pair-density functional theory for complex molecular systems

Hybrid density functionals, such as B3LYP and PBE0, have achieved remarkable success by substantially improving over their parent methods, namely Hartree-Fock and the generalized gradient approximation, and generally outperforming the second-order Møller-Plesset perturbation theory (MP2) that is more expensive. Here, we extend the linear scheme of hybrid multiconfiguration pair-density functional theory (HMC-PDFT) by incorporating a cross-entropy ingredient to balance the description of static and dynamic correlation effects, leading to a consistent improvement on both exchange and correlation energies. The B3LYP-like translated on-top functional (tB4LYP) developed along this line not only surpasses the accuracy of its parent methods, the complete active space self-consistent field (CASSCF) and the original MC-PDFT functionals (tBLYP and tB3LYP), but also outperforms the widely used complete active space second-order perturbation theory (CASPT2). Remarkably, while remaining satisfactory for general purpose, tB4LYP shows superior accuracy for challenging cases like the Cr2 dissociation and the associated low-lying vibrational energies, the ethylene torsional rotation and the ethyne diabatic colinear dissociations, with the significantly lower computational cost than CASPT2.

Unravelling the electronic structure, bonding, and magnetic properties of inorganic dysprosocene analogues [Dy(E4)2]- (E = N, P, As, CH).

Organometallic sandwich complexes of Dy(III) ion are ubiquitous for designing high-temperature single-ion magnets with blocking temperatures close to the liquid nitrogen boiling point. Magnetic bistability at the molecular level makes them potential candidates for nano-scale information storage materials. In the present contribution, we have thoroughly investigated the electronic structure, bonding, covalency, and magnetic anisotropy of inorganic dysprosocene complexes with a general formula of [Dy(E4)2]- (where E = N, P, As, CH) using state-of-the-art scalar relativistic density functional theory (SR-DFT), and a multiconfigurational complete active space self-consistent field (CASSCF) method with the N-electron valence perturbation theory (NEVPT2). Geometry optimization calculations predict stabilization of the [Dy(E4)2]- complexes with a linear geometry and D4h local symmetry Dy(III) ion in [Dy(N4)2]- (1) and [Dy(P4)2]- (2) complexes, while a bent geometry has been observed for the [Dy(As4)2]- (3), [Dy(P2(CH)2)2]- (4), and [Dy(As2(CH)2)2]- (5) complexes. Energy decomposition analysis (EDA) and natural bonding orbital (NBO) calculations reveal sizable 5d-ligand covalency followed by 6s/6p and weak 4f-ligand covalency in complexes 1-5. Both the natural localized molecular orbitals (NLMOs) at the DFT level and ab initio-based ligand field theory (AILFT) at the NEVPT2 level of theory predict an increase in the Dy-ligand covalency as we move from N to As. Spin-Hamiltonian parameter analysis of complexes 1-5 reveals stabilization of the mJ |±15/2〉 as the ground state with highly axial g values (gxx ∼ gyy ∼ 0 and gzz ∼ 20) and the barrier height of 2902, 1214, 1104, 1845, and 1509 K for 1-5, respectively. The Orbach effective demagnetization barrier (Ueff) for complexes 1-5 ranges between 2416-1175 K, with a record Ueff value of 2416 K observed for 1. In addition, we have explored the role of heavy element effects on the magnetic anisotropy by turning off the spin-orbit coupling of the pnictogens (N, P, and As), and our calculations clearly predict that heavy atoms in the first coordination sphere help in increasing the barrier height for magnetic relaxation. Heavy elements like P and As significantly enhance the SOC contributions, thereby providing a platform for designing and optimizing Dy(III) complexes with tailored magnetic behaviors.

Synthesis, Characteristics and Thermally Induced Self-Assembly of Silicon-Based Thermo/Photo-Responsive Block Copolymers Prepared from Monomer Bearing Paired Side-Chain Azo Mesogens Using RAFT Process

Novel silicon-based liquid crystalline (LC) monomer (CASPA) and corresponding Poly(MMA)-b-Poly(CASPA) diblock copolymers (DBCs) containing symmetric azobenzene mesogens in the side chain were successfully synthesized by coupling reaction and reversible addition-fragmentation chain transfer (RAFT) polymerization, respectively. Block copolymerization by RAFT proceeded with well-controlled manner in anisole solution involving new CASPA monomer, Poly(MMA)-RAFT macroinitiator and AIBN initiator, yielding Poly(MMA)-b-Poly(CASPA) DBCs with excellent control over molecular weights (Mw/Mn ≤ 1.39) and compositions. Chemical structures and properties of CASPA and Poly(MMA)-b-Poly(CASPA) were extensively studied using 1H NMR, FTIR, GPC, DSC, POM, AFM and GISAXS. The CASPA exhibited enantiotropic liquid crystallinity with nematic four-brush schlieren texture, while DBCs showed monotropic liquid crystallinity with batonnet textures of smectic A phase. All DBC films exhibited photochemical trans-cis isomerization and photoinduced phase transition from mesophase to isotropic phase under UV irradiation. Morphologies of DBC thin films under thermal annealing depended on volume ratio of building LC block relative to Poly(MMA) segment, DBC-2 containing approximately equal block volume fractions self-assembled into lamellar nanostructure with domain-spacing of 34 nm, whereas DBC-3 and DBC-4 possessing higher LC contents (65 wt% and 72 wt%) formed highly ordered hexagonal cylindrical nanostructures with domain-spacing ranging from 37 to 39 nm as evidence by AFM and GI-SAXS, conforming with the self-consistent field theory.

Characterization of 3-Chloro-4-Fluoronitrobenzene Molecule with New Calculation Methods and Discussions for Advanced Applied Sciences

This current research characterizes 2-chloro-1-fluoro-4-nitrobenzene molecule via the Hartree Fock (HF) and density functional theory (DFT) quantum mechanical computational techniques using B3LYP/6-311++G(d,p) levels of computation. The molecular geometries, thermodynamic quantities at 300 K, NMR chemical shifts, corresponding vibrational spectra, UV-vis spectra, vibrational frequencies, and atomic point charge distributions are extensively investigated. The 1H and 13C NMR chemical shifts, and theoretical vibrational frequencies are compared to the experimental results. It is obtained that all calculations are in agreement with the available experimental data which has the 1H isotropic chemical shifts range from 8.306 ppm to 7.235 ppm, while the computed values range from 9.0368 ppm to 6.8397 ppm, 8.3213 ppm to 6.1242 ppm at DFT and HF GIAO levels, respectively. Besides, calculated 13C chemical shifts vary from 141.83 ppm to 186.394 ppm and from 129.743 ppm to 174.373 ppm by using DFT and HF in CH4, while these values are in the range of 117.00 ppm to 164.59 ppm, experimentally. This points out that the chosen computation sets are highly effective methods for identifying and characterizing the compound. In addition, simulations are performed to examine frontier molecular orbitals, electrostatic potential, and molecular electrostatic potential regions. Key properties such as dipole moment, chemical hardness, transition states, electronegativity, molecular softness, nucleophilic aromatic regions, electrophilicity index, and energy band gap are also analyzed to explore potential future applications such as advanced applied sciences, industry, chemistry, medical, physics, biology, pharmaceuticals, dyes, and agrochemicals of the compound. Additionally, it is noted that the compound contains significant intramolecular charge transfer (ICT) regions, lone electron pairs, electron-donating groups, π-bond conjugation, and particularly reactive electrophilic and nucleophilic aromatic sites. Accordingly, it is pointed out that the molecule has a strong potential for metallic bonding as well as various intermolecular interactions. In summary, this study provides valuable information that will benefit both basic research and technological or industrial applications by increasing the understanding of physical, chemical, structural, and reactive features of the 3-chloro-4-fluoronitrobenzene molecule.

Stabilizing complex-Langevin field-theoretic simulations for block copolymer melts.

Complex-Langevin field-theoretic simulations (CL-FTSs) provide an approximation-free method of calculating fluctuation corrections to the self-consistent field theory (SCFT) of block copolymer melts. However, the complex fields are prone to the formation of hot spots, which causes the method to fail. This problem has been attributed to an invariance under complex translations, which allows the system to drift away from the real-valued saddle-point of SCFT. Here, we apply dynamical stabilization to CL-FTSs of diblock copolymer melts, whereby the drift is suppressed by a small imaginary force on the composition field. The force needs to be sufficient to hold the system near the real saddle-point but also small enough not to significantly bias the statistics. Although larger forces are required as the fluctuations become more intense, we are able to lower the invariant polymerization indices of the CL-FTSs by several orders of magnitude before this becomes a problem. The new CL-FTS results are then used to test conventional Langevin simulations (L-FTSs), in which the instability is removed by a partial saddle-point approximation to the pressure field. As found previously, the L-FTSs agree accurately with the CL-FTSs, provided that the comparison is performed using a Morse calibration.

The Search for the Optimal Methodology for Predicting Fluorinated Cathinone Drugs NMR Chemical Shifts.

Cathinone and its synthetic derivatives belong to organic compounds with narcotic properties. Their structural diversity and massive illegal use create the need to develop new analytical methods for their identification in different matrices. NMR spectroscopy is one of the most versatile methods for identifying the structure of organic substances. However, its use could sometimes be very difficult and time-consuming due to the complexity of NMR spectra, as well as the technical limitations of measurements. In such cases, molecular modeling serves as a good supporting technique for interpreting ambiguous spectral data. Theoretical prediction of NMR spectra includes calculation of nuclear magnetic shieldings and sometimes also indirect spin-spin coupling constants (SSCC). The quality of theoretical prediction is strongly dependent on the choice of the theory level. In the current study, cathinone and its 12 fluorinated derivatives were selected for gauge-including atomic orbital (GIAO) NMR calculations using Hartree-Fock (HF) and 28 density functionals combined with 6-311++G** basis set to find the optimal level of theory for 1H, 13C, and 19F chemical shifts modeling. All calculations were performed in the gas phase, and solutions were modeled with a polarized-continuum model (PCM) and solvation model based on density (SMD). The results were critically compared with available experimental data.

Cross-Section Calculations for Electron-Impact Ionization of Pyrimidine Molecule and Its Halogenated Derivatives: 2-Chloropyrimidine, 5-Chloropyrimidine, 2-Bromopyrimidine and 5-Bromopyrimidine.

The total cross-sections for the single electron-impact ionization of pyrimidine (C4H4N2), 2-chloropyrimidine (2-C4H3ClN2), 5-chloropyrimidine (5-C4H3ClN2), 2-bromopyrimidine (2-C4H3BrN2) and 5-bromopyrimidine (5-C4H3BrN2) molecules have been calculated with the binary-encounter-Bethe model from the ionization threshold up to 5 keV. The input data for the BEB calculations concerning electronic structure of the studied targets have been obtained with quantum chemical methods including the Hartree-Fock (H-F) and the outer valence Green function (OVGF) methods. The calculated cross-section for the ionization of the pyrimidine molecules due to electron impact is compared with available experimental and theoretical data. The question of the magnitude the pyrimidine ionization cross-section is also discussed, as is the efficiency of the ionization process of studied halogenated derivatives of pyrimidine.

Extended Active Space Ab Initio Ligand Field Theory: Applications to Transition-Metal Ions.

Ligand field theory (LFT) is one of the cornerstones of coordination chemistry since it provides a conceptual framework in which a great many properties of d- and f-element compounds can be discussed. While LFT serves as a powerful qualitative guide, it is not a tool for quantitative predictions on individual compounds since it incorporates semiempirical parameters that must be fitted to experiment. One way to connect the realms of first-principles electronic structure theory that has emerged as particularly powerful over the past decade is the ab initio ligand field theory (AILFT). The original formulation of this method involved the extraction of LFT parameters by fitting the ligand field Hamiltonian to a complete active space self-consistent field (CASSCF) Hamiltonian. The extraction was shown to be unique provided that the active space consists of 5/7 metal d/f-based molecular orbitals (MOs). Subsequent improvements have involved incorporating dynamical correlation using second-order N-electron valence state perturbation theory (NEVPT2) or second-order dynamical correlation dressed complete active space (DCDCAS). However, the limitation of past approaches is that the method requires a minimal space of 5/7 metal d- or f-based molecular orbitals. This leads to a number of limitations: (1) neglect of radial or semicore correlation would arise from the effect of a second d-shell or an sp-shell in the active space, (2) a more balanced description of metal-ligand bond covalency is lacking because the bonding ligand-based counterparts of the metal d/f orbitals are not in the active space. This usually leads to an exaggerated ionicity of the M-L bonds. In this work, we present an extended active space AILFT (esAILFT) that circumvents these limitations and is, in principle, applicable to arbitrary active spaces, as long as these contain the 5/7 metal d/f-based MOs as a subset. esAILFT was implemented in a development version of the ORCA software package. In order to help with the application of the new method, various criteria for active space extension were explored for 3d, 4d, and 5d transition-metal ions with varying charge. An interpretation of the trends in the Racah B parameter for these ions is also presented as a demonstration of the capabilities of esAILFT.

Automated chain architecture screening for discovery of block copolymer assembly with graph enhanced self-consistent field theory

The diverse chain architectures of block copolymers makes them important for exploring new self-assembly, but poses significant challenges for identifying the stability windows of desired mesophases within the vast parameter space. Here, we present an automated workflow for screening chain architectures to discover new self-assembly. Utilizing graph-enhanced self-consistent field theory complemented by a scattering-based identification strategy, our approach enables the automated computation of arbitrary chain architectures and their phase behavior. This framework successfully identifies stable windows for a novel PtS phase in AB-type block copolymer melts, with two distinct chain architectures emerging from the screening process. Our findings demonstrate the utility of this method in stabilizing desired self-assembly and exploring new mesophases. The flexibility of our approach allows for straightforward extension to multi-species and multi-component systems and further integration with metaheuristic optimization techniques to enhance its potential for materials design.

Convergent Concordant Mode Approach for Molecular Vibrations: CMA-2.

The concordant mode approach (CMA) is a promising new scheme for dramatically increasing the system size and level of theory achievable in quantum chemical computations of molecular vibrational frequencies. Here, we achieve advances in the CMA hierarchy by computations targeting CCSD(T)/cc-pVTZ (coupled cluster singles and doubles with perturbative triples using a correlation-consistent polarized-valence triple-ζ basis set) benchmarks within the G2 molecular test set, executing a statistical analysis for 1501 frequencies from 111 compounds and then separately solving the refractory case of pyridine. First, MP2/cc-pVTZ (second-order Møller-Plesset perturbation theory with the same basis set) proves to be an excellent and preferred choice for generating the underlying (Level B) normal modes of the CMA scheme. Utilizing this Level B within the CMA-0A method reproduces the 1501 benchmark frequencies with a mean absolute error (MAE) of only 0.11 cm-1 and an attendant standard deviation of 0.49 cm-1. Second, a convergent CMA-2 method is constituted that allows efficient computation of higher level (Level A) frequencies to any reasonable accuracy threshold by using only Hartree-Fock (HF) and MP2 or density functional theory (DFT) data to generate ξ parameters, which select the sparse off-diagonal force field elements for explicit evaluation at Level A. When Level B = MP2/cc-pVTZ, a cutoff of ξ = 0.02 provides an average maximum absolute error per molecule of only 0.17 cm-1 by incurring merely a 33% increase in average cost over CMA-0A. This CMA-2 method also eradicates the 4 problematic CMA-0A outliers of pyridine with even less effort (ξ = 0.04, 22% increase). Finally, the newly developed CMA procedures are shown to be highly successful when applied to 1-(1H-pyrrol-3-yl)ethanol, a new test molecule with diverse types of vibration.

Excited states from GW/BSE and Hartree-Fock theory: Effects of polarizability and transition type on accuracy of excited state energies.

GW and Bethe-Salpeter equation (BSE) methods are used to calculate energies of excited states of organic molecules in the Quest-3 database [Loos et al., J. Chem. Theory Comput. 16, 1711 (2020)]. The self-energy in the GW approximation is conventionally calculated using the RPA polarizability. Inclusion of a screened electron-hole interaction in the polarizability was recently shown to improve predictions of experimental ionization energies in organic molecules [C. H. Patterson, J. Chem. Theory Comput. 20, 7479 (2024)]. Self-energies from RPA or screened time-dependent Hartree-Fock (TDHF) polarizabilities in the GW/BSE method are used to calculate 141 singlet excited states in Quest-3. Theoretical best estimate excited state energies from the CC3 coupled cluster method and aug-cc-pVTZ basis sets are used to benchmark GW/BSE and CIS calculations using the same molecular geometries and basis sets. Differences between GW/BSE or CIS excited state energies and best estimate values show that there are systematic variations in the accuracies of excited state energies classified as ππ*, nπ*, πR (Rydberg), or nR character. The origin of these variations is the accuracy of self-energies of states of nonbonding vs π bonding character. In particular, N or O lone pair states require large self-energy corrections owing to strong orbital relaxation in the localized hole state, while π states have smaller corrections. Self-energies from a screened TDHF vs RPA polarizability are typically over(under)estimated for nonbonding states, leading to under(over)estimation of energies of excited states of nπ* or nR character.

Salt-Controlled Vertical Segregation of Mixed Polymer Brushes.

Using the self-consistent field approach, we studied the salt-controlled vertical segregation of mixed polymer brushes immersed into a selective solvent. We considered brushes containing two types of chains: polyelectrolyte (charged) chains and neutral chains. The hydrophobicity of both types of chains is characterized by the Flory-Huggins parameters χC and χN, respectively. It was assumed that the hydrophobicity is varied only for the polyelectrolyte chains (χC), while other polymer chains in the brush remain hydrophilic (χN=0) and neutral. Thus, in our model, the solvent selectivity (χ=χC-χN) was varied, which can be controlled in a real experiment, for example, by changing the temperature. At low salt concentrations, the polyelectrolyte chains swell and occupy the surface of the mixed brush. At high salt concentrations, the hydrophobic polyelectrolyte chains collapse and give place to neutral chains on the surface. By changing the selectivity of the solvent and the ionic strength of the solution, the surface properties of such mixed brushes can be controlled. Based on the numerical simulations results, it is shown how the critical selectivity corresponding to the segregation transition in polyelectrolyte/neutral brushes depends on the ionic strength of the solution. It is shown that at the same ionic strength, the critical selectivity increases with an increasing degree of dissociation of charged groups, as well as with an increasing fraction of polyelectrolyte chains in the mixed brush. It has also been shown that at low ionic strengths, the critical selectivity of the solvent decreases with increasing grafting density, while at high ionic strengths, on the contrary, it increases. Within the framework of the mean field theory, a two-parameter model has been constructed that quantitatively describes these dependencies.