Ground State Geometries Research Articles

Probing London Dispersion in Proton-Bound Onium Ions: Are Alkyl-Alkyl Steric Interactions Reliably Modeled?

We report spectroscopic and spectrometric experiments that probe the London dispersion interaction between tert-butyl substituents in three series of covalently linked, protonated bis-pyridines in the gas phase. Molecular ions in the three test series, along with several reference molecules for control, were electrosprayed from solution into the gas phase and then probed by infrared multiphoton dissociation spectroscopy and trapped ion mobility spectrometry. The observed N-H stretching frequencies provided an experimental readout diagnostic of the ground-state geometry of each ion, which could be furthermore compared to a second, independent structural readout via the collision cross section. In each of the three series, the strength of a London dispersion interaction could be modulated systematically by a progressive increase in the size of substituents from H to Me to tert-Bu. Parallel to the experimental study, extensive dispersion-corrected density functional theory (DFT-D3BJ) calculations were performed with a range of exchange correlation functionals. A full analysis of the conformational space for the flexible members of the series, and an analysis of the vibrational spectra in the context of a general double-well potential, finds that DFT-D3BJ appears to significantly overbind alkyl-alkyl interactions, specifically interactions between tert-Bu groups, even failing to predict the minimum energy structures reliably in the case of molecules in which London dispersion competes with other noncovalent interactions such as hydrogen bonding.

Structure of the Se4 Isomers─An Ab Initio Study.

This study investigates the equilibrium geometries of four different Se4 isomers using the coupled cluster single and double perturbative (CCSD(T)) method, extrapolating to the complete basis sets. The ground-state geometry of the Se4 isomer with the C2v structure (2.8715 Å, 2.1750 Å, and 88.4°) is found to be very close to other theoretical values (2.910 Å, 2.224 Å, and 90.0°) for the Se═Se and Se-Se bond lengths and valence angle. Additionally, the adiabatic electron affinity, vertical electron affinity, and vertical ionization energy are calculated. The multireference configuration interaction method was used to calculate transitions from the singlet ground state to some excited counterparts, including the vertical excitation energy, oscillator strength, and main electronic configuration. The predicted wavelengths of electronic transitions to 11Bu, 11Au with C2h symmetry, and 11B1 state with the C2V symmetry could match the experimental NIR absorption band at 710-850 nm. These transitions and electronic properties may provide insights into the role of selenium in astrophysical environments, where 74Se in the solar system has been confirmed to originate from the supernova explosion process. The theoretical results offer a deeper understanding of Se4 electronic and geometric structures while also providing crucial spectroscopic data that could aid in the identification of selenium-containing molecules in extraterrestrial environments.

Probing quantum geometry through optical conductivity and magnetic circular dichroism.

Probing ground-state quantum geometry and topology through optical responses is not only of fundamental interest, but it can also offer several practical advantages. Here, using first-principles calculations on thin films of the antiferromagnetic topological insulator MnBi2Te4, we demonstrate how the generalized optical weight arising from the absorptive part of the optical conductivity can be used to probe the ground-state quantum geometry and topology. We show that three-septuple-layer MnBi2Te4 film exhibit an enhanced, almost-perfect magnetic circular dichroism for a narrow photon energy window in the infrared region. We calculate the quantum weight in this MnBi2Te4 film and show that it far exceeds the lower bound provided by the Chern number. Our results suggest that the well-known optical methods are powerful tools for probing the ground-state quantum geometry and topology.

Reconstructing real-space geometries of polyatomic molecules undergoing strong field laser-induced Coulomb explosion

Coulomb explosion is an established momentum imaging technique, where the molecules are ionized multiple times on a femtosecond time scale before breaking up into ionized fragments. By measuring the momentum of all the ions, information about the initial molecular structure is theoretically available. However, significant geometric changes due to multiple ionizations occur before the explosion, posing a challenge in retrieving the ground-state structure of molecules from the measured momentum values of the fragments. In this work, we investigate theoretically and experimentally such a connection between the ground-state geometry of a polyatomic molecule (OCS) and the detected momenta of ionic fragments from the Coulomb explosion. By relying on time-dependent density functional theory (TDDFT), we can rigorously model the ionization dynamics of the molecule in the tunneling regime. We reproduce the energy release and the Newton plot momentum patterns of an experiment in which OCS is ionized to the 6+ charge state. Our results provide insight into the behavior of molecules during strong field multiple ionization, opening a way toward precision imaging of real-space molecular geometries using tabletop lasers.

Factors Affecting the Population of Excited Charge Transfer States in Adenine/Guanine Dinucleotides: A Joint Computational and Transient Absorption Study.

There is compelling evidence that the absorption of low-energy UV radiation directly by DNA in solution generates guanine radicals with quantum yields that are strongly dependent on the secondary structure. Key players in this unexpected phenomenon are the photo-induced charge transfer (CT) states, in which an electric charge has been transferred from one nucleobase to another. The present work examines the factors affecting the population of these states during electronic relaxation. It focuses on two dinucleotides with opposite orientation: 5'-dApdG-3' (AG) and 5'-dGpdA-3' (GA). Quantum chemistry calculations determine their ground state geometry and the associated Franck-Condon states, map their relaxation pathways leading to excited state minima, and compute their absorption spectra. It has been shown that the most stable conformer is anti-syn for AG and anti-anti for GA. The ground state geometry governs both the excited states populated upon UV photon absorption and the type of excited state minima reached during their relaxation. Their fingerprints are detected in the transient absorption spectra recorded with excitation at 266 nm and a time resolution of 30 fs. Our measurements reveal that in the large majority of dinucleotides, chromophore coupling is already operative in the ground state and that the charge transfer process occurs within ~120 fs. The competition among various relaxation pathways affects the quantum yields of the CT state formation in each dinucleotide, which are estimated to be 0.18 and 0.32 for AG and GA, respectively.

Understanding melting behavior of aluminum clusters using machine learned deep neural network potential energy surfaces.

Deep neural network-based deep potentials (DP), developed by Tuo et al., have been used to compute the thermodynamic properties of free aluminum clusters with accuracy close to that of density functional theory. Although Jarrold and collaborators have reported extensive experimental measurements on the melting temperatures and heat capacities of free aluminum clusters, no reports exist for finite-temperature abinitio simulations on larger clusters (N > 55 atoms). We report the heat capacities and melting temperatures for 32 clusters in the size range of 48-342 atoms, computed using the multiple histogram technique. Extensive molecular dynamics (MD) simulations at twenty four temperatures have been performed for all the clusters. Our results are in very good agreement with the experimental melting temperatures for 19 clusters. Except for a few sizes, the interesting features in the heat capacities have been reproduced. To gain insight into the striking features reported in the experiments, we used structural and dynamical descriptors such as temperature-dependent mean squared displacements and the Lindemann index. Bimodal features observed in Al116 and the weak shoulder seen in Al52 are attributed to solid-solid structural transitions. In confirmation of the earlier reports, we observe that the behavior of the heat capacities is significantly influenced by the nature of the ground state geometries. Our findings show that the sharp drop in the melting temperature of the 56-atom cluster is a consequence of the change in the geometry of Al55. Mulliken population analysis of Al55 reveals that the charge-induced local electric field is responsible for the strong bonding between core and surface atoms, leading to the higher melting temperature. Our calculations do not support the lower melting temperature observed in experimental studies of Al69. Our results indicate that Al48 is in a liquid state above 600K and does not support the high melting temperature reported in the experiment. It turns out that the accuracy of the DP model by Tuo et al. is not reliable for MD simulations beyond 750K. We also report low-lying equilibrium geometries and thermodynamics of 11 larger clusters (N = 147-342) that have not been previously reported, and the melting temperatures of these clusters are in good agreement with the experimental ones.

Influence of terminal moiety on PCE of DSSCs: An In Silico study based on triazatruxene-benzothiadiazole dye

Our study utilized an experimentally synthesized dye as a reference molecule, employing a donor-π linker-acceptor (D-π-A) framework for organic solar cells. The molecule featured a triazatruxene group linked with alkyl branches as the donor and ethynyl benzoic acid as the acceptor, connected through a derivative of benzothiadiazole as the π linker. To improve optoelectronic and photovoltaic properties, ten theoretically designed dyes (ZA1–ZA10) are proposed, differing from the reference (R) by modifying the terminal acceptor moiety. Various quantum analyses, including frontier molecular orbitals, optical properties, reorganization energies, binding energies, transition density matrices (TDM), molecular electrostatic potential (MEP), dipole moment, and density of states were carried out at DFT/B3LYP/6-31G(d,p). Ground state geometries revealed a co-planar morphology in ZA1–ZA10, facilitating efficient charge transportation. TDM and MEP illustrated improved electronic transitions in the excited states. Computational analyses revealed superior photovoltaic properties of ZA1–ZA10. Notably, ZA5 exhibited the most significant redshift (1021 nm) in absorption, lowest bandgap (1.44 eV), smallest transition energy (1.21 eV), least binding energy (0.23 eV), and improved charge mobilities. Results from the adsorption of ZA1-ZA10 on the TiO2 layer confirmed their anchoring potential and effective injection of electrons to anatase (TiO2)9. These significant outcomes promise the potential and novelty of our designed dyes for higher power conversion efficiencies (PCE) in dye-sensitized solar cells (DSSCs).

Air-stable palladium N[sbnd]Heterocyclic carbene based CCC[sbnd]NHC pincer complexes: Synthesis, characterization, photophysical and Raman vibrational studies, and DFT studies, plus observation of an abnormal carbene pincer

A series of CCCNHC pincer Pd(II)X complexes, where X = Cl, Br, or I, were synthesized by a metalation/transmetalation reaction sequence and characterized by NMR and Raman spectroscopy, photophysical studies, X-ray crystallography, and DFT computational studies. This is the first detailed report of the CCCNHC pincer Pd complexes analogous to the previously reported Pt analogs. Photophysical measurements show that by varying the X ligand, the emission wavelength can be tuned. The chloride complex is a water and air stable blue emitter with a quantum yield of 46 % and all three complexes have photostabilities >90 %. A combined DFT and TD-DFT study has been carried out to investigate ground state and excited state geometries as well as absorption and emission processes of CCCNHC Pd complexes. Theoretical results were compared with the corresponding experimental results and showed good agreement. Raman spectroscopy (computational and experimental) was used to examine Pd-X vibrations which have been rarely reported in the literature. Several by-products of the metalation/transmetalation procedure were identified. A rare mixed normal/abnormal carbene pincer Pd(II)Cl complex was isolated and crystallographically characterized.

Temporary Anions of Benzoxazole in Charge-Transfer Cluster Photodetachment.

The photoelectron spectra of cluster anions of superoxide (O2-) solvated by one molecule of benzoxazole (BzOx) reveal two competing photodetachment mechanisms: a direct photoemission from the solvated cluster core and an indirect pathway involving temporary anion states of benzoxazole accessed via the O2-·BzOx → O2·BzOx- charge-transfer transitions. Benzoxazole is a bicyclic unsaturated organic molecule that does not form permanent anions. However, its low-lying vacant π* orbitals permit a resonant capture of the electron emitted from the O2- cluster core. The non-Hermitian theory using a complex absorbing potential predicts the existence of two BzOx- π* resonances within the experimental energy range: resonance A (π1*) at 0.891 eV and resonance B (π2*) at 1.76 eV, relative to the onset of the BzOx + e- continuum at the ground-state geometry of neutral BzOx. Within the clusters, the O2·BzOx- charge-transfer states are partially stabilized relative to the free-electron limit by interactions with the O2 molecule. These interactions depend on the electronic states of both species. The theory predicts that at the O2-·BzOx cluster geometry, the O2(X3Σg-)·BzOx-(A) and O2(a1Δg)·BzOx-(A) states lie at 0.56 and 0.47 eV (vertically) above the respective neutral states. The O2(3Σg-)·BzOx-(B) resonance is found 1.43 eV (vertically) above O2(X3Σg-)·BzOx. Intense signatures of both BzOx- resonances and the three above-mentioned charge-transfer cluster states, O2(X3Σg-)·BzOx-(A), O2(a1Δg)·BzOx-(A), and O2(3Σg-)·BzOx-(B) are observed in the 355 nm (3.495 eV) and 532 nm (2.330 eV) photoelectron spectra of the O2-·BzOx cluster anion.

Mono- and Binuclear Complexes in a Centrifuge-Less Cloud-Point Extraction System for the Spectrophotometric Determination of Zinc(II).

The hydrophobic reagent 6-hexyl-4-(2-thiazolylazo)resorcinol (HTAR) was investigated as part of a cloud-point extraction (CPE) system for the spectrophotometric determination of Zn(II). In the system, complexes with different stoichiometries, including 1:1 and 2:2 (Zn:HTAR), are formed. Their ground-state equilibrium geometries were optimized at the B3LYP/6-31G level of theory. The obtained structures were then used to calculate vertical excitation energies in order to generate theoretical UV/Vis absorption spectra. The comparison between theoretical and experimental spectra demonstrated that, under optimal conditions, a binuclear complex containing oxygen-bridging atoms is the dominant species. The absorbance was found to be linearly dependent on the concentration of Zn(II) within the range of 15.7 to 209 ng mL-1 (R2 = 0.9996). The fraction extracted (%E), logarithm of the conditional extraction constant (log Kex), and molar absorption coefficient (ε) at λmax = 553 nm were calculated to be 98.3%, 15.9, and 4.47 × 105 L mol-1 cm-1, respectively. The method developed is characterized by simplicity, convenience, profitability, sensitivity, and ecological friendliness. It has been successfully applied to the analysis of pharmaceutical and industrial samples.

Ab-initio Study of Structural, Spectroscopic and Electronic Properties of High Energy Explosive Molecules: DFT/TD-DFT Calculations

This research explores the ground state geometry and molecular properties of FOX-7 and nitroguanidine molecules, with a focus on their spectroscopic and electronic characteristics. Initially, the conformational space of each molecule was systematically scanned using molecular mechanic calculations and the most probable conformer structure was obtained for each molecule. Subsequently, geometry optimizations of molecules were conducted by using ab initio density functional theory (DFT) with Becke’s three-parameter hybrid-exchange functional, which combines the Lee–Yang–Parr correlation functional (B3LYP) method, and the standard 6-311++G(d,p) basis set. The theoretically determined geometrical parameters from optimized structure and experimental values available in the literature were compared, providing validation for the structural properties of both molecules. Furthermore, the stability and reactivity properties of both molecules are estimated in terms of HOMO-LUMO energies. Overall, this study contributes to a comprehensive understanding of the ground state geometry, molecular structure, and spectroscopic behavior of FOX-7 and nitroguanidine, paving the way for potential applications in various fields of science and technology.

Colourful 3-amino-1,8-naphthalimide alkyl-substituted fluorescent derivatives

The optical properties of four 3-amino-1,8-naphthalimide derivatives differing in the degree of substitution at the amino functionality were synthesised and studied by UV–visible absorbance and fluorescence spectroscopy in 1:1 (v/v) methanol/water and methanol. The compounds were structurally characterized by NMR, IR and HRMS. The charge transfer absorbance band was observed to shift to longer wavelength with increasing ethyl substitution. The emission band was also observed to shift to longer wavelengths with a decrease in the emission intensity with increasing ethyl substitution due to the peri effect. Irradiation of dye methanol solutions and in the solid state with a 365 nm UV lamp reveals distinct emission colours for each compound. Significantly larger Stokes shifts are observed in methanol than in mixed aqueous methanol. Quantum chemical calculations at the B3LYP/TZVP level of theory provided insight into the optimized ground state geometry, frontier molecular orbitals levels and solute-solvent dynamics revealing a stronger hydrogen bond between the 3-amino-1,8-naphthalimides and methanol in the lowest S1 excited state. The dyes were tested as fluorescent stains in MCF-7 cancer cells and observed to emit green emission in various organelles.

Quantum Chemistry Dataset with Ground- and Excited-state Properties of 450 Kilo Molecules

Due to rapid advancements in deep learning techniques, the demand for large-volume high-quality datasets grows significantly in chemical research. We developed a quantum-chemistry database that includes 443,106 small organic molecules with sizes up to 10 heavy atoms including C, N, O, and F. Ground-state geometry optimizations and frequency calculations of all compounds were performed at the B3LYP/6-31G* level with the BJD3 dispersion correction, while the excited-state single-point calculations were conducted at the ωB97X-D/6-31G* level. Totally twenty-seven molecular properties, such as geometric, thermodynamic, electronic and energetic properties, were gathered from these calculations. Meanwhile, we also established a comprehensive protocol for the construction of a high-volume quantum-chemistry dataset. Our QCDGE (Quantum Chemistry Dataset with Ground- and Excited-State Properties) dataset contains a substantial volume of data, exhibits high chemical diversity, and most importantly includes excited-state information. This dataset, along with its construction protocol, is expected to have a significant impact on the broad applications of machine learning studies across different fields of chemistry, especially in the area of excited-state research.

Investigation of antileishmanial, antioxidant activities, CT-DNA interaction and DFT study of novel cobalt(II) complexes derived from mesogenic aromatic amino acids based Schiff base ligands.

In the present work, new Co(II) complexes were synthesized from mesogenic aromatic amino acids based Schiff base ligands, HL1 [Methyl 2-((2-hydroxy-4-(tetradecyloxy)benzylidene)amino)-3-phenylpropanoate] and HL2 [Methyl 2-((2-hydroxy-4-(tetradecyloxy)benzylidene)amino)-3-(1H-indol-2-yl)propanoate]. The compounds were thoroughly characterised using different elemental, thermogravimetric and spectroscopic studies. The in-vitro antileishmanial efficacy of the compounds against Leishmania donovani was evaluated by MTT assay and the antioxidant activity was performed by Mensor's method. The cell viability percentage and IC50 values for both the antileishmanial and antioxidant studies revealed that the cobalt(II) complexes are comparable to the standard, amphotericin B and ascorbic acid, respectively, signifying the potential applications of the biogenic compounds. The CT-DNA interaction experiments study using photophysical techniques indicated that the cobalt(II) complexes exhibited pronounced interactions as compared to the parent ligand. The parent ligands were found to possess mesogenicity as evidenced from the polarizing optical microscope (POM) and differential scanning calorimetry (DSC). The optical band gap of the compounds, as estimated from the Tauc plot of the UV-Vis spectra, lies within the domain of optoelectronic material properties, which was further supported through Density Functional Theory (DFT) study. Moreover, DFT methods have been used to explore the ground state geometry and DFT based reactivity descriptorsof the two synthesised ligands, HL1 and HL2 along with their corresponding Co(II) complexes, Co(L1)2 and Co(L2)2. Reactivity descriptors obtained from Conceptual Density Functional Theory (CDFT) analysis reveal that Co(L1)2 is the most stable and Co(L2)2 is the most electrophilic.

A differentiable quantum phase estimation algorithm

Abstract The simulation of electronic properties is a pivotal issue in modern electronic structure theory, driving significant efforts over the past decades to develop protocols for computing energy derivatives. In this work, we address this problem by developing a strategy to integrate the quantum phase estimation algorithm within a fully differentiable framework. This is accomplished by devising a smooth estimator able to tackle arbitrary initial states. We provide analytical expressions to characterize the statistics and algorithmic cost of this estimator. Furthermore, we provide numerical evidence that the estimation accuracy is retained when an arbitrary state is considered and that it exceeds the one of standard majority rule. 
We explicitly use this procedure to estimate chemically relevant quantities, demonstrating our approach through ground-state and triplet excited state geometry optimization with simulations involving up to 19 qubits. This work paves the way for new quantum algorithms that combine interference methods and quantum differentiable programming.

The Photophysics and Photochemistry of Phenylalanine, Tyrosine, and Tryptophan: A CASSCF/CASPT2 Study.

Among all amino acids, the three aromatic systems including phenylalanine, tyrosine, and tryptophan are known to manifest UV light absorption at the higher wavelengths. Their fluorescent properties are important for protein detection and determination of the spatial structure. Based on ab initio quantum chemical calculations, the absorption spectra in the 0-12 eV UV range of these aromatic amino acids were interpreted, and the photochemistry of the lowest-lying excited states was studied. For that, molecular ground-state geometries were determined using both the coupled-cluster single and double (CCSD) and complete active space self-consistent field (CASSCF) methods. The vertical electronic transition energies, associated oscillator strengths, and electric dipole moments of the lowest excited states were computed at the CASPT2//CASSCF level. The decay mechanisms of the lowest-energy excited states were investigated by performing minimum energy path (MEPs) computations. The results showed that the CCSD and CASSCF computations yielded similar structures. The lowest-energy S1 state in phenylalanine and tyrosine, and S1 and S2 in tryptophan are mainly described by the π → π* excitation localized in the aromatic ring. Upon light absorption, the main photoresponse of the molecules is driven by the benzene, phenol, and indole chromophoric units.

Synthesis, biological properties, drug-likeness, In silico ADME studies and theoretical studies on 1,3,2λ5-diazaphosphinane derivatives: A DFT investigation

Under various reaction conditions, Tetraphosphorus decasulphide P4S10 and Lawesson's reagent LR interacted with 2-cyano-N-[(2-hydroxyphenyl) methylidene] acetohydrazide (1) to synthesize a new 1,3,2λ5-diazaphosphinane derivatives 2–4. The three novel compounds 2−4 have been synthesized and characterized by elemental and spectral analyses (UV–Vis, FT-IR, MS, and 1H–NMR) throughout the current inquiry. The ground state energies, harmonic vibrational wavenumbers, and ground state geometries of the investigated compounds 2−4 were optimized by applying the density functional theory (DFT) at the B3LYP level, utilizing 6–311++G (d, p) basis sets. The first principal study has been investigated for the electronic properties, molecular electrostatic potentials, and quantum chemical identities. There was a noticeable shift in intramolecular charge from the occupied to the empty molecular orbitals. Time-dependent density functional theory (TD-DFT) and the CAM-B3LYP/6–311++G (d, p) function was used to conduct the UV–Vis analysis. A few bacterial and fungal strains were used to test the antimicrobial activities. The stability of the molecule resulting from hyper conjugative interactions that cause its nonlinear optical activity and charge delocalization was investigated using a natural bond orbital approach. The Mullikan atomic charge and molecular electrostatic potential are also anticipated. The value of the HOMO-LUMO energy gap indicates the possibility of charge transfer within the molecule. Compound 4 is the most reactive, with an energy difference between the border orbital of ΔEgap = 2.97 eV, according to analysis of the global descriptors. In addition, compared to the other 2 and 3 compounds under study, compound 4 is the softest, least stable, and has the largest electronic exchange capacity. Examined the structure-activity relationship using DFT calculations (SAR) and compared the findings with real-world antimicrobial outcomes for compounds 2–4. Also, Drug-likeness and In silico ADME studies were performed.

Multi-faceted exploration of amide-amine tethered 1,2,3-triazole conjugates as antimicrobial agents using synthetic, in vitro and in silico approach

In the present manuscript, diversely substituted amine-amide linked 1,2,3-triazoles were synthesized and investigated as anti-microbial agents. Their molecular structures were established using different spectroscopy techniques including FTIR, 1H NMR, 13C NMR. Finally, the composition was confirmed by employing high resolution mass spectrometry technique. The global reactivity parameters for all the synthetics were assessed by optimizing their ground state geometry using density functional theory (DFT). The anti-microbial efficacy of synthesized compounds was examined against a variety of microbial strains including gram positive, gram negative and fungal strains. The results of in vitro assay revealed moderate to excellent inhibition of anti-microbial growth by the developed synthetics. Among all, compound 7r displayed highest inhibition of S. aureus, B. subtilis and E. coli growth with an MIC value of 0.0595 µmol/mL (Norfloxacin= 0.0783 µmol/mL). As far as inhibition of fungal growth is concerned, the compound 7o was found to have excellent activity against C. albicans and R. oryzae with an MIC value of 0.0295 µmol/mL (Fluconazole = 0.0408 µmol/mL). The experimental findings of in vitro experiments were validated using molecular docking and dynamics studies. The drug like features including oral bioavailability of the developed synthetics were assessed using ADME, which pay a new way towards drug development process.

Impact of the chemical insertion of the dimethylamino group on the electronic and optical properties of the 4-(methoxyphenyl acetonitrile) monomer (MPA): a DFT theoretical investigation.

Density functional theory (DFT) calculations on the ground and the first excited state are performed on the modified and unmodified 4-(methoxyphenyl acetonitrile) monomer (referred to as MPA). The modified monomer named MFA is obtained by Knoevenagel condensation of MPA with dimethylformamide dimethyl acetal (DMF-DMA). DFT computations show that the chemical grafting of the dimethylamino group onto the MPA unit induces a great change in the geometric, electronic, and optical properties. Going from MPA to MFA monomer, a great change in the frontier orbitals of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the ground and the first excited state is observed. Consequently, a reduction in the energy gap HOMO-LUMO and an enhancement in the absorption and emission properties are observed under the chemical modification. The observed modifications in the electronics and optical properties are the result of the charge transfer appearing between the cyano (C≡N) acceptor group and the dimethylamino (DMF-DMA)-grafted group donor ring. Quantum chemical calculations were performed in the ground and the first excited state using the density functional theory (DFT), and it extends the time-dependent density functional theory (TD-DFT), implemented in the Gaussian 09 software package. The ground state is obtained by optimization of the studied molecular geometries by employing the DFT/M062X/6-31G(d,p) level of theory. The first excited state is obtained by re-optimization of the ground state geometries using the TD-DFT/M062X/6-31G(d,p) level of theory. The contour plots of the frontier orbitals and the molecular electrostatic potential (MEP) maps are obtained from the ground and the first excited state, optimized geometries, and drawn using Gaussview software.

An automated protocol to construct flexibility parameters for classical forcefields: applications to metal-organic frameworks.

In this work, forcefield flexibility parameters were constructed and validated for more than 100 metal-organic frameworks (MOFs). We used atom typing to identify bond types, angle types, and dihedral types associated with bond stretches, angle bends, dihedral torsions, and other flexibility interactions. Our work used Manz's angle-bending and dihedral-torsion model potentials. For a crystal structure containing N atoms in its unit cell, the number of independent flexibility interactions is 3(N atoms - 1). Because the number of bonds, angles, and dihedrals is normally much larger than 3(N atoms - 1), these internal coordinates are redundant. To reduce (but not eliminate) this redundancy, our protocol prunes dihedral types in a way that preserves symmetry equivalency. Next, each dihedral type is classified as non-rotatable, hindered, rotatable, or linear. We introduce a smart selection method that identifies which particular torsion modes are important for each rotatable dihedral type. Then, we computed the force constants for all flexibility interactions together via LASSO regression (i.e., regularized linear least-squares fitting) of the training dataset. LASSO automatically identifies and removes unimportant forcefield interactions. For each MOF, the reference dataset was quantum-mechanically-computed in VASP via DFT with dispersion and included: (i) finite-displacement calculations along every independent atom translation mode, (ii) geometries randomly sampled via ab initio molecular dynamics (AIMD), (iii) the optimized ground-state geometry using experimental lattice parameters, and (iv) rigid torsion scans for each rotatable dihedral type. After training, the flexibility model was validated across geometries that were not part of the training dataset. For each MOF, we computed the goodness of fit (R-squared value) and the root-mean-squared error (RMSE) separately for the training and validation datasets. We compared flexibility models with and without bond-bond cross terms. Even without cross terms, the model yielded R-squared values of 0.910 (avg across all MOFs) ± 0.018 (st. dev.) for atom-in-material forces in the validation datasets. Our SAVESTEPS protocol should find widespread applications to parameterize flexible forcefields for material datasets. We performed molecular dynamics simulations using these flexibility parameters to compute heat capacities and thermal expansion coefficients for two MOFs.