Van Der Waals Surface Research Articles

Development of mixed matrix membrane comprising titanium (IV) oxide dispersed with octaisobutyl polyhedral oligomeric silsesquioxane

In designing mixed matrix membranes (MMMs) to improve gas separation performance, inorganic fillers such as titanium (IV) oxide (TiO2) nanoparticles are commonly added due to their excellent intrinsic properties and high affinity towards CO2. However, the addition of TiO2 nanoparticles causes the formation of agglomerates which deteriorate the gas separation properties of the membrane due to their high surface energy and Van der Waals forces. In this study, MMMs comprising octaisobutyl polyhedral oligomeric silsesquioxane (OPOSS) incorporated with TiO2 nanoparticles were successfully developed using phase inversion technique. MMMs were synthesized at different loadings of TiO2 and OPOSS. The effectiveness of OPOSS as a dispersant was determined by using SEM, EDS and single gas permeation analysis. The optimum amount of TiO2-OPOSS in the THF/DMAc casting solution was at 4wt% TiO2 and 2wt% of OPOSS, as the agglomeration of nanoparticles did not occur based on the morphology and gas separation performance. The membrane with the highest performance was achieved by 4/2-T/OPOSS, which is at 1.8 of CO2/CH4 gas selectivity. Based on the findings, it can be concluded that the OPOSS did play an important role in enhancing the dispersion of TiO2 nanoparticles in the polymer matrix as the TiO2 agglomerates were not seen upon addition of OPOSS.

Microscopic insight into nanodiamond polymer composites: reinforcement, structural, and interaction properties.

Nanodiamond (ND)-reinforced polymer composites attract a great deal of attention; however, insufficient understanding of their reinforced behavior is greatly limiting their further design and application. In this paper, a microscopic investigation of the stretched processes of aminated ND and resin composites is undertaken to elucidate the reinforcement mechanism of ND in the polymer matrix by employing molecular dynamics and semi-empirical quantum chemistry simulations. The stretching process and reinforcement behavior are observed on the nanoscale, and the nano-pinning effect of ND on the resin matrix is clarified. Further analyses indicate that due to the increase of system volume, the interactions between the ND and resin molecules increase with stretching; this facilitates the combination of ND and resin molecules and thus recovers the breakage of the resin matrix. This recovery process is regarded as the origin of the ND reinforcement of the polymer. The strong interaction between the ND and resin molecules is quantitatively assessed by the interpenetration of the van der Waals surface and the reduced density gradient analysis of electrons using quantum chemistry calculations. The high stability of their noncovalent combination is also revealed. This investigation provides an excellent approach for revealing the origin of ND reinforcement behavior and thus facilitates further optimization of the performance of the ND-polymer composites.

QSAR study for predicting the ecotoxicity of NADES towards Aliivibrio fischeri. Exploring the use of mixing rules.

(Eco)toxicological information of natural deep eutectic solvents (NADES) is scarce, and thus, quantitative structure activity relationship (QSAR) models are an important tool for achieving the prediction of toxicity in this case. For that reason, in this manuscript, a new QSAR model for predicting the ecotoxicity of NADES towards the Aliivibrio fischeri biomodel, using mixing rules, is proposed. The main advantage of the method is that the individual components of the mixtures are molecularly modelled, and then, a mixing rule is used, which simplifies the process. For developing the model, a total of 11 descriptors for each component is used: the dissociation constant, partition coefficient, Van der Waals volume, Van der Waals surface area, topological polar surface area, solvent accessible surface area, minimum projection area, maximum projection area, minimum projection radius, maximum projection radius and molecular weight. The final obtained model includes the topological polar surface area and the dissociation constant, mechanistically interpreted as the ability of a NADES to transport through biological membranes and the severe negative effect of pH on the toxicity and biological response of Aliivibrio fischeri bacteria. The OECD Guidance Document on the Validation of (Quantitative) Structure-Activity Relationships is followed to develop the mathematical model.

A novel HIV-1 protease inhibitor, GRL-044, has potent activity against various HIV-1s with an extremely high genetic barrier to the emergence of HIV-1 drug resistance.

We designed, synthesized, and identified two novel nonpeptidic HIV-1 protease inhibitors (PIs), GRL- 037 and GRL-044, containing P2-tetrahydropyrano-tetrahydrofuran (Tp-THF), P1-benzene and P1-methoxybenzene, respectively, and P2'-isopropyl-aminobenzothiazole (Ip-Abt), based on the structure of the prototypic PI, darunavir (DRV). The 50% inhibitory concentrations (IC50s) of GRL-037 and GRL-044 against wild-type HIV-1NL4-3 were 0.042 and 0.0028-0.0033 nM with minimal cytotoxicity profiles compared to the IC50 values of four most potent FDA-approved PIs, ranging from 2.6 to 70 nM. GRL-044 was also potent against HIV-2EHO (IC50=0.0004 nM) and various PI-resistant HIV-1 variants (IC50 ranging from 0.065 to 19 nM). In the selection assays we conducted, the emergence of HIV-1 variants resistant to GRL-044 was significantly delayed compared to that against DRV. Thermal stability test using differential scanning fluorimetry employing purified HIV-1 protease (PR) and SYPRO® Orange showed that both GRL-037 and GRL-044 tightly bound to PR. A28S substitution emerged in the homologous recombination-based selection assays with GRL-044. Structural analyses showed that the larger size of GRL-044 over DRV, enabling GRL-044 to fit better to the hydrophobic cavity of protease, contributed to the greater potency of GRL- 044 against HIV-1. Structural analyses also suggested that the van der Waals surface contact of GRL-044 with A28' appears to be better compared to that of DRV because of the larger surface of Ip-Abt of GRL-044, which may be partially responsible for the emergence of A28S. The present antiviral data and structural features of GRL-044 should provide molecular insights for further design and development of potent and "resistance-repellant" novel PIs.

Statistical Analysis of the Relationship Between Antioxidant Activity and the Structure of Flavonoid Compounds

Using different methods of statistics, this paper aims to highlight the potential link between the antioxidant activity of flavonoids and the corresponding molecular descriptors. By calculating the descriptors (van der Waals surface (A), molar volume (V), partition coefficient (LogP), refractivity (R), polarizability (a), forming heat (Hformation), hydration energy (Ehidr), the dipole moment (mt)), together with antioxidant activities (RSA) calculated or taken from the literature, number of phenolic -OH groups and the presence (2) or absence (1) of C2=C3 double bond) for 29 flavonoid compounds and by intercorrelation between the studied parameters, the link between the number of phenolic groups grafted to the basic structure of flavonoids and their antioxidant activity was confirmed. Simultaneously, by using the chi-squared test and the intercorrelations matrix, a satisfactorily correlation coefficient (r2=0.5678; r=0.7536) between the structure of the flavonoids and their activity was obtained, fact that confirms the correlation of the antioxidant activity with the number of -OH phenolic groups.

Development of Predictive Linear and Non-linear QSTR Models for Aliivibrio Fischeri Toxicity of Deep Eutectic Solvents

Deep eutectic solvents (DESs) have emerged as a very important group of chemicals in recent years. Although generally considered as environment friendly or ‘green,' recent investigations reported toxic behaviors of some DESs towards various biological species. In this work, quantitative structure toxicity relationship analysis was performed on a dataset containing 72 DESs and their components to find the structural determinants responsible for higher DES mediated toxicity. Additionally, efficiencies of various machine learning tools as well as different feature selection algorithms were estimated. To understand the true predictivity of the derived models, three external validation strategies, namely ‘points out,' ‘mixtures out,' and ‘compounds out' were applied along with an ‘all out' technique, a modification of the earlier reported ‘everything out' validation. The models highlight importance of the number of nitrogen atoms, the van der Waals surface area, molar refractivity, dipole moment and molecular mass for shaping the toxicity of DESs and their components towards A. fischeri.

Quantitative Structure-Activity Relationship Study of Camptothecin Derivatives as Anticancer Drugs Using Molecular Descriptors.

A Quantitative Structure-Activity Relationship (QSAR) has been widely developed to derive a correlation between chemical structures of molecules to their known activities. In the present investigation, QSAR models have been carried out on 76 Camptothecin (CPT) derivatives as anticancer drugs to develop a robust model for the prediction of physicochemical properties. A training set of 60 structurally diverse CPT derivatives was used to construct QSAR models for the prediction of physiochemical parameters such as Van der Waals surface area (SvdW), Van der Waals Volume (VvdW), Molar Refractivity (MR) and Polarizability (α). The QSAR models were optimized using Multiple Linear Regression (MLR) analysis. A test set of 16 compounds was evaluated using the defined models. The Genetic Algorithm And Multiple Linear Regression Analysis (GA-MLR) were used to select the descriptors derived from the Dragon software to generate the correlation models that relate the structural features to the studied properties. QSAR models were used to delineate the important descriptors responsible for the properties of the CPT derivatives. The statistically significant QSAR models derived by GA-MLR analysis were validated by Leave-One-Out Cross-Validation (LOOCV) and test set validation methods. The multicollinearity and autocorrelation properties of the descriptors contributed in the models were tested by calculating the Variance Inflation Factor (VIF) and the Durbin-Watson (DW) statistics. The predictive ability of the models was found to be satisfactory. Thus, QSAR models derived from this study may be helpful for modeling and designing some new CPT derivatives and for predicting their activity.

A comparative study to evaluate the role of caged hybrid frameworks in the precise dispersion of titanium (IV) oxide for the development of gas separation membranes

In designing mixed-matrix membranes (MMMs), inorganic fillers such as titanium (IV) oxide (TiO 2 ) nanoparticles are commonly added because of their excellent intrinsic properties and high affinity toward CO 2 . However, the addition of TiO 2 nanoparticles causes formation of agglomerates because of their high surface energy and van der Waals forces. In this study, MMMs comprising caged hybrid framework, i.e., octaisobutyl polyhedral oligomeric silsesquioxane (OPOSS) incorporated with TiO 2 nanoparticles were developed by phase inversion technique. The effectiveness of OPOSS as a dispersant was determined by using qualitative and quantitative methods. Based on the findings, OPOSS successfully enhanced the dispersion of TiO 2 nanoparticles as it reduced the surface energy of TiO 2 nanoparticles. The optimum amount of TiO 2 -OPOSS in the tetrahydrofuran/dimethylacetamide casting solution was at 4 wt % TiO 2 and 2 wt % OPOSS based on qualitative and quantitative analyses. At 4/2-T/OPOSS MMM, the elemental mapping showed uniform dispersion of TiO 2 nanoparticles without formation of large agglomerates. In addition, the free-space length and d -metric value were the lowest for 4/2-T/OPOSS, which implied that this membrane has the highest degree of particles' dispersion and distribution. Hence, the optimum CO 2 /CH 4 gas selectivity performance was achieved by 4/2-T/OPOSS.

Core–shell structured graphene sphere-silver nanowire hybrid filler embedded polydimethylsiloxane nanocomposites for stretchable conductor

Three-dimensional (3D) core–shell structured graphene-silver nanowire (AgNW) hybrid fillers are prepared through facile spray drying and an optical welding process. The spray drying process enables formation of a core–shell structure with AgNWs attached onto the spherical graphene surface by van der Waals force and surface tension during evaporation. AgNW shell is optically welded for enhanced mechanical stability and interfacial resistance reduction. 3D core–shell structured graphene-AgNW hybrid fillers are partially embedded into polydimethylsiloxane (PDMS) to fabricate highly stretchable and conductive nanocomposites. The electrical conductivity of nanocomposites largely increases up to ∼116 S cm−1 and the electrical properties are well maintained under high stretchability of ∼140% strain with 100 stretching cycles despite small amount of AgNW. These enhancements are attributed to the formation of electrically conducting network by excellent dispersion property of spherical graphene core in PDMS matrix and low contact resistance of AgNW shell. We anticipate that 3D core–shell structured graphene-AgNW/PDMS nanocomposites have great potential for application in various stretchable electronic devices.

The reaction pathway analysis of phosphoric acid with the active radicals: a new insight of the fire-extinguishing mechanism of ABC dry powder.

Dry powder fire-extinguishing agent is one of Halon substitutes due to its superior fire-extinguishing performance, non-toxicity, and environmental friendliness. As one of the most widely used dry powders, ABC dry powder has attracted wide attention. Understanding its reaction mechanism is important to the design of more efficient compound dry powder based on it. When ABC dry powder was applied to the flame, ammonium dihydrogen phosphate (the main fire-extinguishing component of ABC dry powder) would rapidly decompose into phosphoric acid (H3PO4) and ammonia. Therefore, in order to figure out the chemical reaction mechanism of ABC dry powder and active radicals, the main focus of this paper is on the H3PO4. Analysis of the electrostatic potential on van der Waals surface of H3PO4 was carried out. Besides, detailed theoretical investigation has been performed on the mechanism, kinetics, and thermochemistry of the reactions of H3PO4 with H, OH, and CH3 radicals and further decomposition of H3PO4 using M06-2X/6-311G(d,p)//CCSD(T)/cc-pVTZ level of theory. Mayer bond order for all intrinsic reaction coordinate points was also calculated. Finally, it is theoretically proved that ABC dry powder extinguishes the fire mainly by chemical inhibition on H and OH radicals. Grapical Abstract .

QSAR Analysis of Multimodal Antidepressants Vortioxetine Analogs Using Physicochemical Descriptors and MLR Modeling

Vortioxetine is a multimodal antidepressant drug with combined effects on SERT as an inhibitor, 5-HT1A as agonist and 5-HT3A as an antagonist. Series of vortioxetine analogs have been reported as multi antidepressant compounds and they block serotonin transport into the neuronal cells, activate the postsynaptic 5-HT1A receptors and eliminate the low activity of 5-HT3A receptors. To explore the important properties of vortioxetine analogs involved in antidepressant activity by developing 2D QSAR models. Selections of significant descriptors were performed by Least Absolute Shrinkage and Selection Operator (LASSO) method and, the Multiple Linear Regression (MLR) method and All Subsets and GA algorithm included in QSARINS software were used for generating QSAR models. Further, the virtual screening was performed based on bioactivity and structure similarity using the PubChem database. The four descriptor model of complementary information content (CIC2), solubility (bcutp3), mass (bcutm8) and partial charge in van der Waals surface area (PEOEVSA7) of the molecules is obtained for SERT inhibition with the significant statistics of R2= 0.69, RMSEtr= 0.44, R2 ext= 0.62 and CCCext= 0.78. For 5-HT1A agonist, the two descriptor model of molecular shape (Kappm3) and van der Waals volume of the atoms (bcutv11) with R2= 0.78, RMSEtr= 0.33, R2 ext = 0.83, and CCCext= 0.87 is established. The three descriptor model of information content (IC3), solubility (bcutp9) and electronegativity (GATSe5) of the molecules with R2= 0.61, RMSEtr= 0.34, R2 ext= 0.69 and CCCext= 0.72 is obtained for 5-HT3A antagonist. The antidepressant activities of 16 virtual screened compounds were predicted using the developed models. The developed QSAR models may be useful to predict antidepressant activity for the newly synthesized vortioxetine analogs.

A Universal Quantitative Descriptor of the Dispersion Interaction Potential.

London dispersion, universally attractive forces originating from fluctuating dipoles, is omnipresent in molecules. While its understanding has recently made tremendous progress, its general appreciation is still lagging behind electrostatics. This can be explained by the simple tools available to study electrostatic interactions, such as electrostatic potential (ESP) maps and partial charges, and a lack thereof for dispersion. We herein report a universal quantitative descriptor of dispersion interaction potentials, which allows assessing dispersion visually by London dispersion potential (LDP) maps, and quantitatively using the average LDP on the van der Waals surface. We demonstrate the utility of these new tools by constructing a quantitative dispersion energy scale of the elements and common substituents, studying non-covalent interactions (NCIs), and developing modern linear free energy relationships in catalysis.

A Universal Quantitative Descriptor of the Dispersion Interaction Potential

AbstractLondon dispersion, universally attractive forces originating from fluctuating dipoles, is omnipresent in molecules. While its understanding has recently made tremendous progress, its general appreciation is still lagging behind electrostatics. This can be explained by the simple tools available to study electrostatic interactions, such as electrostatic potential (ESP) maps and partial charges, and a lack thereof for dispersion. We herein report a universal quantitative descriptor of dispersion interaction potentials, which allows assessing dispersion visually by London dispersion potential (LDP) maps, and quantitatively using the average LDP on the van der Waals surface. We demonstrate the utility of these new tools by constructing a quantitative dispersion energy scale of the elements and common substituents, studying non‐covalent interactions (NCIs), and developing modern linear free energy relationships in catalysis.

Multistability of armchair single-walled carbon nanotubes: a molecular dynamics investigation

We report by molecular dynamics (MD) that the armchair single-walled carbon nanotubes (ACNTs) has a multistability rather than bistability as the diameters are above a threshold value of about 3.5 nm. The present work demonstrates that ACNTs have four stable states when the diameters are above the threshold value. The newly found stable states are caused by surface van der Waals forces. We also find the transformation between different stable states can be realized by external loading or temperature. These imply the ACNTs have a great potential application in the future nanoscale devices.

Pressure-viscosity relation of 2,2,4-trimethylhexane predicted using all-atom TEAM force field

The pressure-dependent viscosity of 2,2,4-trimethylhexane is predicted using all-atom TEAM force field and the non-equilibrium periodic perturbation (PP) method. The viscosities predicted using the PP method do not show Newtonian plateau in the precision range of prediction for high-viscosity fluids. Using four alkane molecules as test cases, we found that our previously published force field, developed for ambient pressure conditions, is inaccurate for predicting viscosities under elevated pressures. Two approaches were attempted to enhance the pressure transferability of the force field: one involves moving the non-bond interaction site of hydrogen closer to that of attached carbon, which smooths the van der Waals (vdW) surface of molecule, and the other involves replacing the LJ-12-6 function by LJ-9-6, making the repulsion softer. Both approaches enhanced the pressure transferability; however, the former works well below 500 MPa, while the latter is valid under pressures up to 1 GPa.

Problem in the molecular area of polar probe molecules used in inverse gas chromatography

Although the inverse gas chromatography technique has been used to measure the surface acid-base properties for almost 50 years, several basic problems still hinder its further development and application. One of the problems is that for the polar probe molecules of dichloromethane, acetone, ethyl acetate, tetrahydrofuran, and ethanol, the molecular area of each probe has different values in literature. In this paper, we point out the incorrect molecular area values presented in the literature by comparing these values with those indicated in Smallwood's Handbook of Organic Solvent Properties. The correct molecular area values of 11 polar probes are determined by plotting the molecular areas of the polar probes versus the van der Waals surface areas reported in the handbook. The correct molecular areas for six common polar probes, namely, dichloromethane, trichloromethane, acetone, ethyl acetate, diethyl ether, and tetrahydrofuran, are 38.0, 44.0, 42.5, 48.0, 47.0, and 45.0 Å2, respectively. Benzene is a specific molecule with a conjugated π-bond structure, and its molecular area is estimated to be larger than that of other molecules. Therefore, benzene is unsuitable for use as a probe molecule.

Micro-scale solubility assessments and prediction models for active pharmaceutical ingredients in polymeric matrices

The number of models for assessing the solubility of active pharmaceutical ingredients (APIs) in polymeric matrices on the one hand and the extent of available associated data on the other hand has been rising steadily in the past few years. However, according to our knowledge an overview on the methods used for prediction and the respective experimental data is missing. Therefore, we compiled experimental data, the techniques used for their determination and the models used for estimating the solubility. Our focus was on polymers commonly used in spray drying and hot-melt extrusion to form amorphous solid dispersions (ASDs), namely polyvinylpyrrolidone grades (PVP), polyvinyl acetate (PVAc), vinylpyrrolidone-vinyl acetate copolymer (copovidone, COP), polyvinyl caprolactam-polyvinyl acetate-polyethylene glycol graft polymer (Soluplus®, SOL), different types of methacrylate copolymers (PMMA), polyethylene glycol grades (PEG) and hydroxypropyl-methylcellulose grades (HPMC). The literature data were further supplemented by our own results. The final data set included 37 APIs and two sugar derivatives. The majority of the prediction models was constituted by the melting point depression method, dissolution endpoint measurements, indirect solubility determination by Tg and the use of low molecular weight analogues. We observed that the API solubility depended more on the working group which conducted the experiments than on the measuring technique used. Furthermore, this compilation should assist researchers in choosing a prediction method suited for their investigations.Furthermore, a statistical assessment using recursive feature elimination was performed to identify descriptors of molecules, which are connected to the API solubility in polymeric matrices. It is capable of predicting the criterium 20% API soluble at 100 °C (Yes/No) for an unknown compound with a balanced accuracy of 71%. The identified 8 descriptors to be connected to API solubility in polymeric matrices were the number of hydrogen bonding donors, three descriptors related to the hydrophobicity of the molecule, glass transition temperature, fractional negative polar van der Waals surface area, out-of-plane potential energy and the fraction of rotatable bonds. Finally, in addition to our own model, the data set should help researchers in training their own solubility prediction models.

Dispersion of Titanium(IV) Oxide Nanoparticles in Mixed Matrix Membrane Using Octaisobutyl Polyhedral Oligomeric Silsesquioxane for Enhanced CO2/CH4 Separation Performance

Titanium(IV) oxide (TiO2) nanoparticles have been incorporated into mixed matrix membranes (MMMs) to improve gas separation performance. However, TiO2 nanoparticles tend to agglomerate due to high surface energy and van der Waals forces. This leads to precipitation which causes the formation of non-homogeneous MMM morphology. In this study, the effect of octaisobutyl polyhedral oligomeric silsesquioxane (POSS) addition on TiO2/polysulfone MMM was investigated. The aims are to enhance gas separation performance whilst preventing agglomeration of TiO2 nanoparticles. The results demonstrated that inclusion of POSS as dispersant increases MMMs’ CO2/CH4 selectivity and permeance, possibly due to less void formation and more evenly distributed pore structure. For example, synergistic addition of 5 wt% TiO2 and 5 wt% POSS increased the CO2/CH4 selectivity up to 390% compared to MMM without POSS. This is supported by elemental mapping of titanium which revealed that POSS successfully dispersed TiO2 nanoparticles and prevented aggregation. TiO2-POSS/PSf MMMs also retained their favorable thermal stability.

Examination of Trifluoroethanol Interactions with Trp-Cage in Trifluoroethanol-Water at 298 K through Molecular Dynamics Simulations and Intermolecular Nuclear Overhauser Effects.

Molecular dynamics simulations of the protein model Trp-cage in 42% trifluoroethanol (TFE)-water at 298 K have been carried out with the goal of exploring peptide hydrogen-solvent fluorine nuclear spin cross-relaxation. The TFE5 model of TFE developed in a previous work was used with the TIP5P-Ew model of water. System densities and component translational diffusion coefficients predicted by the simulations were within 20% of the experimental values. Consideration of the calculated relative amounts of TFE and water surrounding the hydrogens of Trp-cage indicated that the composition of the solvent mixture beyond ∼1.5 nm from the van der Waals surface of the peptide is close to the composition of the bulk solvent, but as observed by others, TFE accumulates preferentially near the peptide surface. In the simulations, both TFE and water molecules make contacts with the peptide surface; water molecules predominate in contacts with the peptide backbone atoms and TFE molecules generally preferentially interact with side chains. Translational diffusion of solvent molecules appears to be slowed near the surface of the peptide. Depending on the location in the structure, TFE molecules form complexes with the peptide that may persist for up to ∼7 ns. Many of the peptide spin-solvent fluorine cross-relaxation parameters (ΣHF) for which experimental values are available are reasonably well-predicted from the simulations. However, the calculated ΣHF values were too small for some hydrogens of the 6Trp indole ring and the amino acid hydrogens near this residue in the native structure, whereas ΣHF values for hydrogens on the side chains of 1Asn, 4Ile, and 7Leu are too large. In 42% TFE-water, persistent conformations of Trp-cage are found, which differ from the conformation found in water by the orientation of the 3Tyr ring.

Electrostatic Spectral Tuning Maps for Biological Chromophores.

The mechanism by which the absorption wavelength of a molecule is modified by a protein is known as spectral tuning. Spectral tuning is often achieved by electrostatic interactions that stabilize/destabilize or modify the shape of the excited and ground-state potential energy surfaces of the chromophore. We present a protocol for the construction of three-dimensional "electrostatic spectral tuning maps" that describe how vertical excitation energies in a chromophore are influenced by nearby charges. The maps are built by moving a charge on the van der Waals surface of the chromophore and calculating the change in its excitation energy. The maps are useful guides for protein engineering of color variants, for interpreting spectra of chromophores that act as probes of their environment, and as starting points for further quantum mechanical/molecular mechanical studies. The maps are semiquantitative and can approximate the magnitude of the spectral shift induced by a point charge at a given position with respect to the chromophore. We generate and discuss electrostatic spectral tuning maps for model chromophores of photoreceptor proteins, fluorescent proteins, and aromatic amino acids. Such maps may be extended to other properties such as oscillator strengths, absolute energies (stability), ionization energies, and electron affinities.