MP2 Methods Research Articles

Structural Analysis of Phosphorus and Arsenic Clusters: A Comparative DFT and MP2 Study

AbstractPhosphorus and arsenic play pivotal roles in semiconductor and material science due to their versatile applications. We have employed Gaussian 16 software to optimize 20 initial phosphorus (P4) and arsenic (As4) cluster geometries, as well as 32 As2P2 cluster geometries, utilizing B3LYP and MP2 methods with the 6‐311++G(3d2f,3p2d) basis set. The geometries were rigorously validated through frequency calculations, followed by energy calculations using B3LYP/aug‐cc‐pVQZ and MP2/aug‐cc‐pVQZ. We thoroughly analyzed and justified the observed disparities between DFT and MP2 results. Notably, a compelling linear relationship was observed in the energy gap between the most stable and the second most stable geometries from P4 to As4 in both MP2 and B3LYP calculations, with MP2 results aligning closely with additional CCSD(full) calculations. Additionally, our investigation unveiled two enantiomeric geometries of As4, specifically the semirectangular shape, with slight energy variations reminiscent of parity violation observed in amino acids. These findings significantly enhance our comprehension of pnictogen cluster stability, offering insights into the semiconductor and material science.

Infrared Spectroscopy of Ethanethiol Monomers and Dimers at MP2 Level: Characterizing the Dimer Formation and Hydrogen Bond.

Ethanethiol, also known as ethyl mercaptan, is an organosulfur compound that appears as a colorless liquid with a distinctive odor. It has been detected in the interstellar medium, and its self-association has been the subject of a few known experimental studies, where the SH vibrational mode was used. However, unlike the analogous ethanol dimer, the ethanethiol dimer has not been thoroughly explored theoretically. In this study, ethanethiol and dimers were investigated using the MP2 method with various basis sets to determine the properties and stability of these structures. For the monomer, both trans and gauche structures were computed, with the gauche conformer being more stable, consistent with the available data in the literature. Local mode decomposition analysis of monomers showed that the CH2 rocking mode, associated with the CSH bending, is present only for the gauche isomer aligning with the experimental assignments. Furthermore, eight stable dimer configurations were identified and categorized into three groups: trans-trans, gauche-gauche, and trans-gauche isomers. Among these, the trans-gauche isomer was found to be the most stable. Dispersion is the dominant term for the ethanethiol dimer.

Computational Studies of Nucleophilic Substitution at Nitrogen Center: Reactions of NH2Cl with HO-, CH3O- and C2H5O.

The atomic-level mechanisms of the nucleophilic substitution reactions at the nitrogen center (SN2@N) were investigated for the reactions of chloramine (NH2Cl) with the alkoxide ions (RO-, where R=H, CH3, and C2H5) using DFT and MP2 methods. The computed potential energy profiles for the SN2@N pathways involving the back-side attack of the nucleophiles show the typical double-well potential with submerged barriers similar to the SN2 reactions at the carbon center (SN2@C). However, the pre-reaction and post-reaction complexes are, respectively, the N-H⋅⋅⋅O and N-H⋅⋅⋅Cl hydrogen-bonded intermediates, which are different from those generally seen in SN2@C reactions. The SN2@N pathways involving front-side attack of the nucleophiles have high-energy barriers. The potential energy surfaces (PESs) along the proton-transfer pathways were flat. In addition to the proton-transfer and SN2 pathways, we also observed a new path for the methoxide and ethoxide nucleophiles where a hydride-transfer from the nucleophile to chloramine resulted in the products Cl-+R'CHO+NH3, (R'=H, CH3), and was the most exoergic. A comparison of the energetics obtained used different DFT and MP2 methods with that of the benchmark coupled-cluster methods reveals that CAM-B3LYP best describes the PESs.

Molecular interaction studies of Gamma-aminobutyric acid (GABA) in an aqueous medium at various temperatures: A comprehensive analysis using volumetric, thermoacoustic and DFT methods

The interaction of Gamma-aminobutyric acid (GABA) in an aqueous medium at various concentrations (0.101–1.086) mol·kg−1 as a function of temperature is being studied using volumetric, viscosity and acoustic analysis. The calculation of apparent molar volume (VΦ), partial molar volume (V∅o), apparent molar isentropic compression (Kϕ) and partial molar isentropic compression (Kϕ0) of GABA in an aqueous medium has been done by measuring the densities and speed of the sound in the temperature range of 298.15–323.15 K. The thermo-acoustic parameters like adiabatic compressibility (βs), acoustic impedance (Z), intermolecular free length (Lf), relative association (RA), relaxation time (τ), internal pressure (πi), enthalpy (ΔH), Gibbs free energy (ΔG), and change in entropy (ΔS) are also computed. The strength of the hydrogen bond interaction of GABA in an aqueous medium and its dipole moment are calculated using single-point energy calculations. These calculations employ IEFPCM and PCM solvation models using DFT/B3LYP and MP2 methods with the 6-311G++ (d, p) basis set. The outcomes are interpreted in terms of hydrogen bond interactions that exist in the mixture.

Cooperativity and halonium transfer in the ternary NCI···CH3I···-CN halogen-bonded complex: An ab initio gas phase study.

The strength and nature of the two halogen bonds in the NCI···CH3I···-CN halogen-bonded ternary complex are studied in the gas phase via ab initio calculations. Different indicators of halogen bond strength were employed to examine the interactions including geometries, complexation energies, Natural Bond Order (NBO) Wiberg bond indices, and Atoms in Molecules (AIM)-based charge density topological properties. The results show that the halogen bond is strong and partly covalent in nature when CH3I donates the halogen bond, but weak and noncovalent in nature when CH3I accepts the halogen bond. Significant halogen bond cooperativity emerges in the ternary complex relative to the corresponding heterodimer complexes, NCI···CH3I and CH3I···-CN, respectively. For example, the CCSD(T) complexation energy of the ternary complex (-18.27kcal/mol) is about twice the sum of the complexation energies of the component dimers (-9.54kcal/mol). The halonium transfer reaction that converts the ternary complex into an equivalent one was also investigated. The electronic barrier for the halonium transfer was calculated to be 6.70kcal/mol at the CCSD(T) level. Although the MP2 level underestimates and the MP3 overestimates the barrier, their calculated MP2.5 average barrier (6.44kcal/mol) is close to that of the more robust CCSD(T) level. Insights on the halonium ion transfer reaction was obtained by examining the reaction energy and force profiles along the intrinsic reaction coordinate, IRC. The corresponding evolution of other properties such as bond lengths, Wiberg bond indices, and Mulliken charges provides specific insight on the extent of structural rearrangements and electronic redistribution throughout the entire IRC space. The MP2 method was used for geometry optimizations. Energy calculations were performed using the CCSD(T) method. The aug-cc-pVTZ basis set was employed for all atoms other than iodine for which the aug-cc-pVTZ-PP basis set was used instead.

A Constraint-Based Orbital-Optimized Excited State Method (COOX).

In this work, we present a novel method to directly calculate targeted electronic excited states within a self-consistent field calculation based on constrained density functional theory (cDFT). The constraint is constructed from the static occupied-occupied and virtual-virtual parts of the excited state difference density from (simplified) linear-response time-dependent density functional theory calculations (LR-TDDFT). Our new method shows a stable convergence behavior, provides an accurate excited state density adhering to the Aufbau principle, and can be solved within a restricted SCF for singlet excitations to avoid spin contamination. This also allows the straightforward application of post-SCF electron-correlation methods like MP2 or direct RPA methods. We present the details of our constraint-based orbital-optimized excited state method (COOX) and compare it to similar schemes. The accuracy of excitation energies will be analyzed for a benchmark of systems, while the quality of the resulting excited state densities is investigated by evaluating excited state nuclear forces and excited state structure optimizations. We also investigate the performance of the proposed COOX method for long-range charge transfer excitations and conical intersections with the ground-state.

Microwave spectrum and molecular structure calculations for η4-butadiene ruthenium tricarbonyl

The microwave spectrum of η4-butadiene ruthenium tricarbonyl was measured in the 5–15 GHz frequency range using a Flygare-Balle type pulsed beam Fourier transform microwave (FTMW) spectrometer. The rotational constants for the 102Ru isotopologue were determined to have the following values: A = 932.20099(42), B = 858.03248(47) and C = 831.35161(37) MHz. The centrifugal distortion constant dJ is 0.0862(29)kHz. 22 a-dipole and 4c-dipole transitions were measured. Extensive high-level G16 calculations were made using DFT and MP2 methods with various basis sets, some including core-potentials (ECP). The best structure was calculated with Gaussian 16 using B3LYP/def2-QZVPP, which includes a core potential (ECP). Extensive all-electron calculations were made based on the best ECP structure to predict 101Ru and 99Ru quadrupole coupling strengths. Quadrupole hyperfine structure splittings were measured for both 101Ru and 99Ru. The hyperfine structure splittings for the 101Ru nuclear quadrupole were measured, yielding the values of 1.5χaa = 98.12(17) MHz and 0.25(χbb-χcc) = 36.059(30). Measured hyperfine structure splittings for 99Ru quadrupole coupling yielded the values of 1.5χaa = 16.99(77) MHz and 0.25(χbb-χcc) = 6.23(32). These values are in reasonable agreement with some of the all-electron calculations.

Rational Design of Cyclopentadiene-Based Super- and Hyperacids Based on Aromaticity.

The design and synthesis of neutral organic superacids have been of interest recently due to their vast applications in chemistry and material sciences, for example, olefinic polymerization, isolation of highly reactive short-lived cations, etc. Cyclopentadiene behaves as a mild organic acid, producing a stable conjugate base by gaining aromaticity and conjugation after deprotonation. To stabilize conjugate bases of organic acids to show superacidities and hyperacidities, we considered aromatic phenyl substituents with cyclopentadiene (mono-, di-, and triphenyl-substituted cyclopentadiene and their cyano derivatives). The MP2, DFT (B3LYP, M06-2X), and CBS-QB3 methods were used to calculate the gas-phase proton affinities of the parent cyclopentadiene, and the DFT methods were chosen for the substituted cyclopentadiene as they yield an experimental proton affinity of cyclopentadiene of 253.66 kcal/mol (Expt. 253.6 ± 1.3 kcal/mol). The stable trisubstituted cyclopentadiene derivative shows gas-phase enthalpies of deprotonation (ΔHacid) of 245 and 239 kcal/mol with DFT B3LYP and M06-2X methods, respectively, with values in the range of hyperacidity. Some of the tautomers of cyclopentadiene derivatives show hyperacidity, with proton affinity values of 205-240 kcal/mol. Triphenyl-substituted cyclopentadiene behaves as a moderate acid but transforms into a superacid after replacing the phenyl group with nitrobenzene, which is a stronger acid than H2SO4 (ΔHacid = 298 kcal/mol). The nucleus-independent chemical shift (NICS) shows the extent of conjugation in the derivatives of cyclopentadienyl anions after deprotonation, which affects the stabilities of conjugate bases, i.e., the acidities of the protonated species. The calculated harmonic oscillator model of aromaticity and NICS indices reveal that the stability of conjugate bases as well as their acidities increase with increasing aromaticity of cyclopentadiene rings after deprotonation in all molecules.

Microwave spectroscopy and large amplitude motion of chlorosulfonic acid (ClSO2OH)

The high-resolution rotational spectrum of chlorosulfonic acid (ClSO2OH) has been studied using both broadband and cavity-based Fourier transform microwave spectrometers over the frequency range of 5–18 GHz. a-, b-, and c-type transitions have been recorded for both the 35Cl and 37Cl isotopologues. The observation of c-type lines establishes that the molecule lacks a plane of symmetry and suggests that the OH group can undergo large amplitude motion between equivalent structures. Interconversion between these structures can be achieved via internal rotation through two inequivalent barriers occurring at Cl-S-O-H torsional angles of 0 or 180degrees. As in previous work on triflic and methanesulfonic acids, two states are observed and are treated as tunneling states which are presumed to arise primarily due to motion through the lower of the two barriers. The a- and c-type transitions occur within each of these states while the b-type transitions cross between them. Rotational, centrifugal distortion, and chlorine nuclear quadrupole coupling constants, as well as the energy difference between the two tunneling states and associated coupling constants, have been determined. The experimental tunneling energies, ΔE, for the 35Cl and 37Cl isotopologues are 52.6926(16) MHz and 52.6397(46) MHz, respectively. Quantum chemical calculations were carried out using MP2 and B3LYP density functional theory (DFT) methods with an aug-cc-pVTZ basis set. The rotational constants from the optimized structures were in good agreement with the experimental values. The lowest energy barrier for OH motion was calculated to be 2.63 kcal/mol at the B3LYP/aug-cc-pVTZ level. The effects of the large amplitude motion are similar to those recently reported for triflic acid (CF3SO2OH) and methanesulfonic acid (CH3SO2OH). However, while the tunneling splittings in chlorosulfonic and triflic acids are virtually identical, they differ significantly from that of methanesulfonic acid.

Improving second-order Møller-Plesset perturbation theory for noncovalent interactions with the machine learning-corrected abinitio dispersion potential.

In this work, we utilize our recently developed machine learning (ML)-corrected abinitio dispersion (aiD) potential, known as D3-ML, which is based on the comprehensive SAPT10K dataset and relies solely on Cartesian coordinates as input, to address the dispersion deficiencies in second-order Møller-Plesset perturbation theory (MP2) by replacing its problematic dispersion and exchange-dispersion terms with D3-ML. This leads to the development of a new dispersion-corrected MP2 method, MP2+aiD(CCD), which outperforms other spin-component-scaled and dispersion-corrected MP2 methods as well as popular ML models for predicting noncovalent interactions across various datasets, including S66 × 8, NAP6 (containing 6 naphthalene dimers), L7, S12L, DNA-ellipticine, the C60 dimer, and C60[6]CPPA. In addition, MP2+aiD(CCD) exhibits comparable or even superior performance compared to the contemporary ωB97M-V functional. The limited performance of pure ML models for systems outside the training set or larger than those in the training set highlights their instability and unpredictability. Conversely, the outstanding performance and transferability of the hybrid MP2+aiD(CCD) method can be attributed to the fusion of the physical electronic structure method and a data-driven ML model, combining the strengths of both sides. This investigation firmly establishes MP2+aiD(CCD) as one of the most accurate and reliable fifth-order scaling correlated wave function methods currently available for modeling noncovalent interactions, even for large complexes. MP2+aiD(CCD) is expected to be reliably applicable in investigating real-life complexes at the hundred-atom scale.

KONFORMACJE SALBUTAMOLU W PRÓŻNI ORAZ ŚRODOWISKU WODNYM

albutamol is a short acting, selective beta-2 adrenergic receptor agonist used to treat asthma, bronchitis, as well as prevent exercise induced bronchospasms. Although it is a small molecule, it has considerable conformational freedom. It has four polar groups (-OH and -NH) that can form a variety of intramolecular hydrogen bonds that stabilize individual conformations. It is well known, that such a intramolecular interactions greatly impact molecular properties, because the bioactive conformation of a given ligand could be stabilized by intramolecular hydrogen bonds. The aim of our research is to determine the preferred conformation of salbutamol, and to characterize the intramolecular interactions that stabilize its various conformers, both in a gas phase and in water modeled as a continuous dielectric. The analysis was carried out by B3LYP-D3 and MP2 methods with the 6- 311++G(d,p) basis set. The relative energies and geometrical parameters of intramolecular H-bonds are presented for six selected salbutamol conformers. Moreover, calculated monohydrates of lowest energy salbutamol conformer allow to estimate the interaction energy with water molecule and geometrical parameters of preferred complexes

THEORETICAL STUDY FOR COMPARISON OF pKa OF A NUMBER OF SCHIFF BASES BY EMPLOYING PARAMETERS DERIVED FROM DFT AND MP2 METHOD

The ionization constants of seven compounds derived from Schiff bases have been studied theoretically. In this study, the factors affecting the ionization constant pKa of Schiff bases were found, which are van der Waals forces, energy levels, hardness, chemical potential and spherical electrophile index by conducting a theoretical study of these compounds using two methods of quantum mechanics for the purpose comparison, which is the Ab-initio basic calculation methods. Two methods were chosen, which are the density function theory (DFT) method and the MP2 Mueller-Pleist perturbation theory method using Chem program. Office 2008 the extent to which these two methods correspond to the known chemical bases was determined by discussing the theoretical variables that were calculated using the two methods referred to above and finding the relationship between the calculated physical variables with each other and determining their nature. After conducting the multiple statistical analysis process, it was found that the best variables that affect the ionization constant in the DFT method are (Bond C1-C7, Bend, VDW) and the rest of the variables were theoretically excluded because the coefficients of those variables are very few or equal to zero and accordingly these variables were adopted to obtain A relationship with better results, as it is noted that the values of the correlation coefficient in this DFT method are (0.994). As for the MP2 method, the best variables that affect the ionization constant are (Ƞ, KE, Tors) and the rest of the variables were theoretically excluded because the coefficients of these variables are very few or equal Therefore, these variables were adopted to obtain a relationship with better results, as it is noted that the values of the correlation coefficient in this MP2 method are (0.989). By comparing the theoretical values with the practical values, it was found that the results of the DFT method were closer to the process than the results of the MP2 method.

Study of the potential energy surface of reactions in a system containing <i>i</i>-propyl and <i>n</i>-propyl radicals

The energy pathways of possible decomposition and isomerization reactions of iso-propyl (i-C3H7) and n-propyl (n-C3H7) radicals have been studied by computational methods of quantum chemistry. B3LYP, M062X, MP2, and CBS-QB3 methods are used to localize stationary points on the potential energy surface of a system containing propyl radicals. A number of intermediate compounds formed during the isomerization and decomposition of propyl radicals have been identified, and information has been obtained on their structure and thermochemical parameters. Based on the results of the research, a diagram of the energy levels of the system under consideration was constructed.

Structure, dissociation pathways and thermochemistry of some nitrogen-containing astrophysical molecules: a theoretical study

The structure, relative energies, dissociation pathways and thermochemistry of acetonitrile (CH3CN or methyl cyanide), acrylonitrile (CH2CHCN or vinyl cyanide), propionitrile (CH3CH2CN or ethyl cyanide) and their stable isomers have been studied in detail using density functional B3LYP, B3PW91 and ab initio MP2, QCISD and CCSD(T) methods. Several space missions have already confirmed the abundance of these molecules in different interstellar mediums (ISMs). The existence of related radicals, atoms and molecules are also confirmed using various IR-spectrometric techniques and that may be evolved in solar irradiation in the cold dark core TMC-1 or massive star-forming regions such as Sagittarius B2 or Oion-KL. Possible ways of fragmentation of ground state CH3NC, CH2CHNC and CH3CH2NC through different dissociation channels and their link to other isomers have been considered to justify the presence of such observed radicals in the interstellar cloud. Cyanide(-C≡N), iso-cyanide (-N = C) and imine (-C = N-H) isomeric forms have been considered here and out of these, cyanide isomers are more stable species compared to its iso-nitrile and imine form. Favourable product molecules, such as CH2 radical, C2H2, C2H4 and HCN/HNC, are considered in the dissociation channels.

Translational eigenstates of He@C60 from four-dimensional abinitio potential energy surfaces interpolated using Gaussian process regression.

We investigate the endofullerene system 3He@C60 with a four-dimensional potential energy surface (PES) to include the three He translational degrees of freedom and C60 cage radius. We compare second order Møller-Plesset perturbation theory (MP2), spin component scaled-MP2, scaled opposite spin-MP2, random phase approximation (RPA)@Perdew, Burke, and Ernzerhof (PBE), and corrected Hartree-Fock-RPA to calibrate and gain confidence in the choice of electronic structure method. Due to the high cost of these calculations, the PES is interpolated using Gaussian Process Regression (GPR), owing to its effectiveness with sparse training data. The PES is split into a two-dimensional radial surface, to which corrections are applied to achieve an overall four-dimensional surface. The nuclear Hamiltonian is diagonalized to generate the in-cage translational/vibrational eigenstates. The degeneracy of the three-dimensional harmonic oscillator energies with principal quantum number n is lifted due to the anharmonicity in the radial potential. The (2l + 1)-fold degeneracy of the angular momentum states is also weakly lifted, due to the angular dependence in the potential. We calculate the fundamental frequency to range between 96 and 110 cm-1 depending on the electronic structure method used. Error bars of the eigenstate energies were calculated from the GPR and are on the order of ∼±1.5 cm-1. Wavefunctions are also compared by considering their overlap and Hellinger distance to the one-dimensional empirical potential. As with the energies, the two abinitio methods MP2 and RPA@PBE show the best agreement. While MP2 has better agreement than RPA@PBE, due to its higher computational efficiency and comparable performance, we recommend RPA as an alternative electronic structure method of choice to MP2 for these systems.

Solvation energies of the ferrous ion in water and in ammonia at various temperatures.

The solvation of metal ions is crucial to understanding relevant properties in physics, chemistry, or biology. Therefore, we present solvation enthalpies and solvation free energies of the ferrous ion in water and ammonia. Our results agree well with the experimental reports for the hydration free energy and hydration enthalpy. We obtained [Formula: see text]kJ mol[Formula: see text] for the hydration free energy and [Formula: see text]kJ mol[Formula: see text] for the hydration enthalpy of ferrous ion in water at room temperature. At ambient temperature, we obtained [Formula: see text]kJ mol[Formula: see text] as the [Formula: see text] ammoniation free energy and [Formula: see text]kJ mol[Formula: see text] for the ammoniation enthalpy. In addition, the free energy of solvation is deeply affected when the temperature increases. This pattern can be attributed to the rise of entropy when the temperature rises. Besides, the temperature does not affect the ammoniation enthalpies and the hydration enthalpy of the [Formula: see text] ion. All the geometry optimizations are performed at the MP2 methods associated with the 6-31++g(d,p) basis set of Pople. solvated phase structures of [Formula: see text] ion in water or in ammonia are performed using the PCM model. The [Formula: see text] program suite was used to perform all the calculations. The program TEMPO was also used to evaluate the temperature sensitivity of the different obtained geometries.

Theoretical investigation on isomerization and decomposition reactions of pentanol radicals-part II: linear pentanol isomers.

High-level ab initio calculations are conducted for studying the kinetics of three linear pentanol radicals generated through H-atom abstraction reactions. The species involved are optimized using the M06-2X/6-311++G(d,p) level of theory, while a relaxed scan at the M06-2X/6-31g level of theory with 10° increments is used for the hindrance potential for low-frequency torsional modes. Single-point energies for all stationary points are obtained through the QCISD(T) and MP2 methods in combination with cc-pVDZ, cc-pVTZ, and cc-pVQZ basis sets, which can be extrapolated to the complete basis set (CBS) limit. The rate constants and branching ratios for isomerization and decomposition reactions are computed over a temperature range of 250-2000 K and a pressure range of 0.01-100 atm. Isomerization reactions are dominant at low temperatures, while decomposition reactions are more dominant at high temperatures. The branching ratio of the isomerization reaction exhibits a slight decrease with increasing pressure, while the trend for decomposition reactions depends on the type of the breaking bond. Based on the calculations for five branched pentanol radicals in part I, kinetics of linear and branched pentanol radicals are compared in this work and the results reveal that, for the same kind of β-scission reaction at similar positions of linear and branched pentanol radicals, the rate constants of branched ones are faster than those of linear ones at low temperatures. The hydroxyl group adjacent to the breaking bond can increase the β-scission reaction rate constants, while the effect can be ignored when the hydroxyl group is not adjacent to the breaking bond. Moreover, compared to when the hydroxyl group is located in the middle of the carbon chain, its positioning at the chain's end yields a more noticeable impact on the products and rate constants of C-O bond and O-H bond β-scission reactions. Besides, when incorporating calculated rate constants into the CRECK model, the updated mechanism shows a better performance for ignition delay times of 1-pentanol in the NTC range but exhibits lower reactivity at higher temperatures. The simulation of speciation profiles also shows better agreement with the experimental data obtained using a flow reactor.

Systematic analysis of electronic barrier heights and widths for concerted proton transfer in cyclic hydrogen bonded clusters: (HF)n, (HCl)n and (H2O)n where n = 3, 4, 5.

The MP2 and CCSD(T) methods are paired with correlation consistent basis sets as large as aug-cc-pVQZ to optimize the structures of the cyclic minima for (HF)n, (HCl)n and (H2O)n where n = 3-5, as well as the corresponding transition states (TSs) for concerted proton transfer (CPT). MP2 and CCSD(T) harmonic vibrational frequencies confirm the nature of each minimum and TS. Both conventional and explicitly correlated CCSD(T) computations are employed to assess the electronic dissociation energies and barrier heights for CPT near the complete basis (CBS) limit for all 9 clusters. Results for (HF)n are consistent with prior studies identifying Cnh and Dnh point group symmetry for the minima and TSs, respectively. Our computations also confirm that CPT proceeds through Cs TS structures for the C1 minima of (H2O)3 and (H2O)5, whereas the process goes through a TS with D2d symmetry for the S4 global minimum of (H2O)4. This work corroborates earlier findings that the minima for (HCl)3, (HCl)4 and (HCl)5 have C3h, S4 and C1 point group symmetry, respectively, and that the Cnh structures are not minima for n = 4 and 5. Moreover, our computations show the TSs for CPT in (HCl)3, (HCl)4 and (HCl)5 have D3h, D2d, and C2 point group symmetry, respectively. At the CCSD(T) CBS limit, (HF)4 and (HF)5 have the smallest electronic barrier heights for CPT (≈15 kcal mol-1 for both), followed by the HF trimer (≈21 kcal mol-1). The barriers are appreciably higher for the other clusters (around 27 kcal mol-1 for (H2O)4 and (HCl)3; roughly 30 kcal mol-1 for (H2O)3, (H2O)5 and (HCl)4; up to 38 kcal mol-1 for (HCl)5). At the CBS limit, MP2 significantly underestimates the CCSD(T) barrier heights (e.g., by ca. 2, 4 and 7 kcal mol-1 for the pentamers of HF, H2O and HCl, respectively), whereas CCSD overestimates these barriers by roughly the same magnitude. Scaling the barrier heights and dissociation energies by the number of fragments in the cluster reveals strong linear relationships between the two quantities and with the magnitudes of the imaginary vibrational frequency for the TSs.

Electron correlation effects on uranium isotope fractionation in U(VI)-U(VI) and U(IV)-U(VI) equilibrium isotopic exchange systems.

Uranium isotope fractionation has been extensively investigated in the fields of nuclear engineering and geochemical studies, yet the underlying mechanisms remain unclear. This study assessed isotope fractionations in U(VI)-U(VI) and U(IV)-U(VI) systems by employing various relativistic electron correlation methods to explore the effect of electron correlation and to realize accurate calculations of isotope fractionation coefficients (ε). The nuclear volume term, ln Knv, the major term in ε, was estimated using the exact two-component relativistic Hamiltonian in conjunction with either HF, DFT(B3LYP), MP2, CCSD, CCSD(T), FSCCSD, CASPT2, or RASPT2 approaches for small molecular models with high symmetry. In contrast, chemical species studied in prior experimental work had moderate sizes and were asymmetrical. In such cases, electron correlation calculations other than DFT calculations were not possible and so only the HF and B3LYP approaches were employed. For closed-shell U(VI)-U(VI) systems, the MP2, CCSD and CCSD(T) methods yielded similar ln Knv values that were intermediate between those for the HF and B3LYP methods. Comparisons with experimental results for U(VI)-U(VI) systems showed that the B3LYP calculations gave results closer to the experimental data than the HF calculations. Because of the open-shell structure of U(IV), multireference methods involving the FSCCSD, CASPT2 and RASPT2 techniques were used for U(IV)-U(VI) systems, but these calculations exhibited instability. The average-of-configuration HF method showed better agreement with the experimental ε values for U(IV)-U(VI) systems than the B3LYP method. Overall, electron correlation improved the description of ε for the U(VI)-U(VI) systems but challenges remain with regard to open-shell U(IV) calculations because an energy accuracy of 10-6-10-7Eh is required for ln Knv calculations.

СРАВНЕНИЕ МЕТОДОВ MP2 И DFT ДЛЯ РАСЧЕТА СТРУКТУРЫ И ИК-СПЕКТРОВ НА ПРИМЕРЕ СОЕДИНЕНИЙ МАГНИЯ С АМИНОКИСЛОТАМИ

Structural models of magnesium compounds with valine and glycine have been constructed. MP2 and DFT (B3LYP functional) methods in the 6-31G(d) basis were used for the construction. The frequencies of normal oscillations in the IR spectrum of the models have been calculated. Comparison of the calculated IR spectra with each other and with the spectra of the synthesised compounds is shown. Conclusions about the structure of the compounds are drawn. The obtained data on coordination of magnesium compounds with amino acids can help to establish the structure of their complexes.