Showing papers on "Ab initio quantum chemistry methods published in 1999"
TL;DR: In this article, an approach to the development of many-body interatomic potentials for monoatomic metals with improved accuracy and reliability is presented. But the functional form of the potentials is that of the embedded-atom method, but the interesting features are as follows: (1) the database used for the development a potential includes both experimental data and a large set of energies of different alternative crystalline structures of the material generated by ab initio calculations.
Abstract: We demonstrate an approach to the development of many-body interatomic potentials for monoatomic metals with improved accuracy and reliability. The functional form of the potentials is that of the embedded-atom method, but the interesting features are as follows: (1) The database used for the development of a potential includes both experimental data and a large set of energies of different alternative crystalline structures of the material generated by ab initio calculations. We introduce a rescaling of interatomic distances in an attempt to improve the compatibility between experimental and ab initio data. (2) The optimum parametrization of the potential for the given database is obtained by alternating the fitting and testing steps. The testing step includes a comparison between the ab initio structural energies and those predicted by the potential. This strategy allows us to achieve the best accuracy of fitting within the intrinsic limitations of the potential model. Using this approach we develop reliable interatomic potentials for Al and Ni. The potentials accurately reproduce basic equilibrium properties of these metals, the elastic constants, the phonon-dispersion curves, the vacancy formation and migration energies, the stacking fault energies, and the surface energies. They also predict the right relative stability of different alternative structures with coordination numbers ranging from 12 to 4. The potentials are expected to be easily transferable to different local environments encountered in atomistic simulations of lattice defects.
1,269 citations
970 citations
TL;DR: Modifications to the Cornell et al. force field are tested in molecular dynamics simulations of mononucleosides and double helices of DNA and RNA (to assess helical and sequence specific structural properties) and lead to improved agreement with experimental data.
Abstract: We have examined some subtle parameter modifications to the Cornell et al. force field, which has proven quite successful in reproducing nucleic acid properties, but whose C2'-endo sugar pucker phase and helical repeat for B DNA appear to be somewhat underestimated. Encouragingly, the addition of a single V2 term involving the atoms C(sp3)-O-(sp3)-C(sp3)-N(sp2), which can be nicely rationalized because of the anomeric effect (lone pairs on oxygen are preferentially oriented relative to the electron withdrawing N), brings the sugar pucker phase of C2'-endo sugars to near perfect agreement with ab initio calculations (W near 162 degrees). Secondly, the use of high level ab initio calculations on entire nucleosides (in contrast to smaller model systems necessitated in 1994-95 by computer limitations) lets one improve the chi torsional potential for nucleic acids. Finally, the O(sp3)-C(sp3)- C(sp3)-O(sp3) V2 torsional potential has been empirically adjusted to reproduce the ab initio calculated relative energy of C2'-endo and C3'-endo nucleosides. These modifications are tested in molecular dynamics simulations of mononucleosides (to assess sugar pucker percentages) and double helices of DNA and RNA (to assess helical and sequence specific structural properties). In both areas, the modified force field leads to improved agreement with experimental data.
915 citations
TL;DR: In this article, two new schemes for computing molecular total atomization energies (TAEs) and/or heats of formation (ΔHf∘) of first and second-row compounds to very high accuracy are presented.
Abstract: Two new schemes for computing molecular total atomization energies (TAEs) and/or heats of formation (ΔHf∘) of first- and second-row compounds to very high accuracy are presented. The more affordable scheme, W1 (Weizmann-1) theory, yields a mean absolute error of 0.30 kcal/mol and includes only a single, molecule-independent, empirical parameter. It requires CCSD (coupled cluster with all single and double substitutions) calculations in spdf and spdfg basis sets, while CCSD(T) (i.e., CCSD with a quasiperturbative treatment of connected triple excitations) calculations are only required in spd and spdf basis sets. On workstation computers and using conventional coupled cluster algorithms, systems as large as benzene can be treated, while larger systems are feasible using direct coupled cluster methods. The more rigorous scheme, W2 (Weizmann-2) theory, contains no empirical parameters at all and yields a mean absolute error of 0.23 kcal/mol, which is lowered to 0.18 kcal/mol for molecules dominated by dynami...
911 citations
TL;DR: In this article, the structural, elastic, and vibrational properties of single-wall carbon nanotubes with different radii and chiralities were investigated using pseudopotential-density-functional theory, which allows calculations on systems with a large number of atoms per cell.
Abstract: A study based on ab initio calculations is presented on the structural, elastic, and vibrational properties of single-wall carbon nanotubes with different radii and chiralities. These properties are obtained using an implementation of pseudopotential-density-functional theory, which allows calculations on systems with a large number of atoms per cell. Different quantities are monitored versus tube radius. The validity of expectations based on graphite is explored down to small radii, where some deviations appear related to the curvature-induced rehybridization of the carbon orbitals. Young moduli are found to be very similar to graphite and do not exhibit a systematic variation with either the radius or the chirality. The Poisson ratio also retains graphitic values except for a possible slight reduction for small radii. It shows, however, chirality dependence. The behavior of characteristic phonon branches as the breathing mode, twistons, and high-frequency optic modes, is also studied, the latter displaying a small chirality dependence at the top of the band. The results are compared with the predictions of the simple zone-folding approximation. Except for the known deficiencies of the zone-folding procedure in the low-frequency vibrational regions, it offers quite accurate results, even for relatively small radii.
889 citations
TL;DR: In this paper, the authors analyzed the CH-O interaction between FnH3-nCH as proton donor and H 2O, CH3OH, and H2CO as acceptor and found that the interaction is quite weak with CH4 as donor but is enhanced by 1 kcal/mol with each F added to the donor.
Abstract: Ab initio calculations are used to analyze the CH···O interaction between FnH3-nCH as proton donor and H2O, CH3OH, and H2CO as acceptor. The interaction is quite weak with CH4 as donor but is enhanced by 1 kcal/mol with each F added to the donor. The CH···O interaction behaves very much like a conventional OH···O H-bond in most respects, including shifts in electron density that accompany the formation of the bond and the magnitudes of the various components of the interaction energy. The two sorts of H-bonds also gravitate toward a similar equilibrium geometry and are comparably sensitive to deformations from that structure. In a quantitative sense, while both CH···O and OH···O prefer a linear configuration, the former is somewhat more easily bent and is less sensitive to stretches from its equilibrium H-bond length. Whereas the OH bond has been shown to stretch and undergo a red shift in its vibrational frequency upon formation of a H-bond, the CH bond of the molecules studied here follows the opposite ...
864 citations
TL;DR: In this article, the force field parameters for amines were developed by fitting to experimental data for pure liquids and to hydrogen-bond strengths from gas-phase ab initio calculations.
Abstract: Classical force-field parameters have been developed for amines primarily by fitting to experimental data for pure liquids and to hydrogen-bond strengths from gas-phase ab initio calculations. The resultant parameters were used to calculate relative free energies of hydration for ammonia, methylamine, dimethylamine, and trimethylamine using free energy perturbation calculations in Monte Carlo simulations (MC/FEP). The results including the fact that the most favorable ΔGhyd occurs for methylamine are in excellent agreement with the experimental data, in contrast to numerous prior computational reports. The calculations reveal two opposing trends in water: increased contribution from hydrogen-bond acceptance and diminished contribution from hydrogen-bond donation with increasing methylation of the amines. The proper balance of hydrogen-bond strengths, which is achieved with the OPLS-AA force field, is essential for correct ordering of the free energies of hydration. MC simulations for the pure liquids of ...
595 citations
TL;DR: In this article, structural, electronic, and bonding properties of liquid water have been studied using the maximally localized Wannier functions, which can describe the molecular charge distribution and the polarization effects in liquid water with a degree of accuracy not previously possible.
Abstract: We study, from first principles, structural, electronic, and bonding properties of liquid water. Our system is twice as large as that used in previous ab initio simulations and our computed structural properties are in good agreement with the most recent neutron scattering experiments. Moreover, the use of a novel technique, based on the generation of maximally localized Wannier functions, allowed us to describe the molecular charge distribution and the polarization effects in liquid water with a degree of accuracy not previously possible. We find that, in the liquid phase, the water molecule dipole moment has a broad distribution around an average value of about 3.0 D. This value is 60% higher than that of the gas phase and significantly larger than most previous estimates. A considerable increase is also observed in the magnitude of the average eigenvalues of the quadrupole moment tensor. We also find that the anisotropy of the electronic charge distribution of the water molecule is reduced in the liquid. The relevance of these results for current modeling of liquid water is discussed.
561 citations
TL;DR: In this article, the density matrix renormalization group was used for quantum chemical calculations for molecules, as an alternative to traditional methods, such as configuration interaction or coupled cluster approaches.
Abstract: In this paper we describe how the density matrix renormalization group can be used for quantum chemical calculations for molecules, as an alternative to traditional methods, such as configuration interaction or coupled cluster approaches. As a demonstration of the potential of this approach, we present results for the H2O molecule in a standard gaussian basis. Results for the total energy of the system compare favorably with the best traditional quantum chemical methods.
489 citations
TL;DR: In this paper, a pseudobond approach was developed to solve the problem for ab initio QM/MM calculations, where a one-free-valence atom with an effective core potential was constructed to replace the boundary atom of the environment part and to form a pseudoobond with the boundary atoms of the active part.
Abstract: A major challenge for combined quantum mechanical and molecular mechanical methods (QM/MM) to study large molecules is how to treat the QM/MM boundary that bisects some covalent bonds. Here a pseudobond approach has been developed to solve this problem for ab initio QM/MM calculations: a one-free-valence atom with an effective core potential is constructed to replace the boundary atom of the environment part and to form a pseudobond with the boundary atom of the active part. This pseudobond, which is described only by the QM method, is designed to mimic the original bond with similar bond length and strength, and similar effects on the rest of the active part. With this pseudobond approach, some well-known deficiencies of the link atom approach have been circumvented and a well-defined potential energy surface of the whole QM/MM system has been provided. The construction of the effective core potential for the pseudobond is independent of the molecular mechanical force field and the same effective core potential is applicable to both Hartree–Fock and density functional methods. Tests on a series of molecules yield very good structural, electronic, and energetic results in comparison with the corresponding full ab initio quantum mechanical calculations.
450 citations
TL;DR: In this article, an algorithm for first-principles calculation of vibrational spectroscopy of polyatomic molecules is proposed, which combines electronic ab initio codes with the vibrational self-consistent field (VSCF) method, and with a perturbation-theoretic extension of VSCF.
Abstract: An algorithm for first-principles calculation of vibrational spectroscopy of polyatomic molecules is proposed, which combines electronic ab initio codes with the vibrational self-consistent field (VSCF) method, and with a perturbation-theoretic extension of VSCF. The integrated method directly uses points on the potential energy surface, computed from the electronic ab initio code, in the VSCF part. No fitting of an analytic potential function is involved. A key element in the approach is the approximation that only interactions between pairs of normal modes are important, while interactions of triples or more can be neglected. This assumption was found to hold well in applications. The new algorithm was applied to the fundamental vibrational excitations of H2O, Cl−(H2O), and (H2O)2, using the Moller–Plesset method for the electronic structure. The vibrational frequencies found are in very good accord with experiments. Estimates suggest that this electronic ab initio/VSCF approach should be feasible, with...
TL;DR: The fundamental underpinnings of theoretical chemistry were uncovered in a relatively short period at the beginning of the present century as mentioned in this paper, with the discovery of the nucleus in 1910 completed the identification of the constituent subparticles of atoms and molecules and was followed shortly thereafter by the Bohr treatment of electronic orbits in atoms.
Abstract: The fundamental underpinnings of theoretical chemistry were uncovered in a relatively short period at the beginning of the present century. Rutherford’s discovery of the nucleus in 1910 completed the identification of the constituent subparticles of atoms and molecules and was followed shortly thereafter by the Bohr treatment of electronic orbits in atoms, the ‘‘old quantum theory.’’ The relation between the positive nuclear charge, atomic number and position of an atom in the periodic table was uncovered by 1913. It proved difficult to extend Bohr’s orbits to a polyatomic situation and the next advance had to await the development of the wave theory of matter and the associated quantum mechanics in the early 1920s. By 1926, Heisenberg had developed matrix mechanics and Schrödinger had proposed the basic nonrelativistic wave equation governing the motion of nuclei and electrons in molecules. The latter,
TL;DR: The unusual solid-state nonplanarity seen in the X-ray structures of the [N]phenylenes (an example is shown here) is most likely the result of crystal-packing-induced deformations as mentioned in this paper.
Abstract: The unusual solid-state nonplanarity seen in the X-ray structures of the [N]phenylenes (an example is shown here) is shown to be most likely the result of crystal-packing-induced deformations. These conclusions are based on a study of published and eight new structure determinations, comparison with data on napththalenes and anthracenes obtained from the Cambridge Structural Database, and ab initio calculations.
TL;DR: In this article, a hybrid method for molecular dynamics simulations of solutes in complex solvents as represented, for example, by substrates within enzymes is presented. But it is not suitable for the simulation of proteins.
Abstract: We present a hybrid method for molecular dynamics simulations of solutes in complex solvents as represented, for example, by substrates within enzymes. The method combines a quantum mechanical (QM) description of the solute with a molecular mechanics (MM) approach for the solvent. The QM fragment of a simulation system is treated by ab initio density functional theory (DFT) based on plane-wave expansions. Long-range Coulomb interactions within the MM fragment and between the QM and the MM fragment are treated by a computationally efficient fast multipole method. For the description of covalent bonds between the two fragments, we introduce the scaled position link atom method (SPLAM), which removes the shortcomings of related procedures. The various aspects of the hybrid method are scrutinized through test calculations on liquid water, the water dimer, ethane and a small molecule related to the retinal Schiff base. In particular, the extent to which vibrational spectra obtained by DFT for the solute can be...
TL;DR: In this paper, a four-dimensional intermolecular potential energy surface for the carbon dioxide dimer has been computed using the many-body symmetry-adapted perturbation theory (SAPT) and a large 5s3p2d1f basis set including bond functions.
Abstract: A four-dimensional intermolecular potential energy surface for the carbon dioxide dimer has been computed using the many-body symmetry-adapted perturbation theory (SAPT) and a large 5s3p2d1f basis set including bond functions. The SAPT level applied is approximately equivalent to the supermolecular many-body perturbation theory at the second-order level. An accurate fit to the computed data has been obtained in a form of an angular expansion incorporating the asymptotic coefficients computed ab initio at the level consistent with the applied SAPT theory. A simpler site-site fit has also been developed to facilitate the use of the potential in molecular dynamics and Monte Carlo simulations. The quality of the new potential has been tested by computing the values of the second virial coefficient which agree very well with the experimental data over a wide range of temperatures. Our potential energy surface turns out to be substantially deeper than previous ab initio potentials. The minimum of −484 cm−1 has ...
TL;DR: In this article, the authors used ab initio methods to compute the free energies of both solid and liquid iron, and they argued that the resulting theoretical melting curve competes in accuracy with those obtained from high-pressure experiments.
Abstract: The solid inner core of the Earth and the liquid outer core consist mainly of iron1 so that knowledge of the high-pressure thermodynamic properties of iron is important for understanding the Earth's deep interior. An accurate knowledge of the melting properties of iron is particularly important, as the temperature distribution in the core is relatively uncertain2,3,4 and a reliable estimate of the melting temperature of iron at the pressure of the inner-core boundary would put a much-needed constraint on core temperatures. Here we used ab initio methods to compute the free energies of both solid and liquid iron, and we argue that the resulting theoretical melting curve competes in accuracy with those obtained from high-pressure experiments. Our results give a melting temperature of iron of ∼6,700 ± 600 K at the pressure of the inner-core boundary, consistent with some of the experimental measurements. Our entirely ab initio methods should also be applicable to many other materials and problems.
TL;DR: In this article, structural and electronic properties of pure zirconia and yttria-stabilized cubic cubic cubic ZIRconia (YSZ) have been investigated.
Abstract: Ab initio calculations have been performed on the structural and electronic properties of pure zirconia and yttria-stabilized cubic zirconia (YSZ) We use the local-density approximation to the exchange and correlation energy functional We expand the Kohn-Sham orbitals in plane waves and use norm-conserving fully separable pseudopotentials We find, in agreement with experiments that the most stable phase at zero temperature and pressure is the monoclinic baddelyte structure which transforms under pressure in the brookite orthorhombic phase We then study the properties of the YSZ cubic phase using a supercell of 96 atoms This is a defective structure where oxygen vacancies and yttrium substitutional impurities play a major role The pattern of relaxation around the defects is consistent with the most recent scattering data, as well as their relative interaction which leads to a next-nearest-neighbor attraction between vacancy and yttrium The analysis of the electronic properties show that single occupied color centers ${\mathrm{F}}^{+}$ are only marginally stable and decay into neutral, doubly occupied F centers and empty (doubly charged) vacancies Therefore, we found that the ${\mathrm{F}}^{+}$ center in YSZ is a negative Hubbard-$U$ site
TL;DR: In this article, a table of gas-phase Na+ affinities for organic and small biological molecules with a wide variety of functional groups is presented, with an accuracy of ca. 1 kcal·mol-1.
Abstract: High-pressure mass spectrometric experiments and ab initio calculations have been carried out in order to establish a series of accurate gas-phase sodium ion affinities of organic molecules with a wide variety of functional groups. Ab initio calculations have also been performed on the sodium complexes of three amino acids: serine, cysteine, and proline. A systematic critical evaluation of experimental and computational literature results shows that a significant number require revision. Based on comparisons with accurate experimental measurements, the ab initio procedure used is shown to yield sodium ion affinities with an accuracy of ca. 1 kcal·mol-1. This enables the construction of the first reliable table of gas-phase Na+ affinities for organic and small biological molecules.
TL;DR: In this paper, the chemical structure and bonding of Al4C and al4C- have been studied by photoelectron spectroscopy and ab initio calculations. But the structure and the bonding of the two molecules are not known.
Abstract: The chemical structure and bonding of Al4C and Al4C- have been studied by photoelectron spectroscopy and ab initio calculations. While Al4C is known to be a tetrahedral molecule, the data reported ...
TL;DR: In this article, a multi-state empirical valence bond model was proposed to study the microscopic nature of the hydrated proton and its transport mechanism, fitted to ab-initio results.
Abstract: In order to study the microscopic nature of the hydrated proton and its transport mechanism, we have introduced a multi-state empirical valence bond model, fitted to ab-initio results [J. Phys. Chem. B 102, 4261 (1998) and references therein]. The model makes it possible to take into account an arbitrary number N of valence states for the system proton+water and the electronic ground-state is obtained by diagonalization of a N×N interaction matrix. The resulting force field was applied to the study, at low computational cost, of the structure and dynamics of an excess proton in liquid water. The quantum character of the proton is included by means of an effective parametrization of the model using a preliminary path-integral calculation. In the light of the simulations, the mechanism of proton transfer is interpreted as the translocation of a privileged H5O2+ structure along the hydrogen bond network, with at any time a special O–H+–O bond, rather than a series of H3O++H2O→H2O+H3O+ reactions. The transloc...
TL;DR: In this article, a time-independent scattering formalism is presented to study the electronic transport of molecular wire circuits, which includes an ab initio description of the molecular electronic structure and an estimate for the injecting energy of the electron onto the molecule by varying the distance between the molecule and the attached gold clusters.
Abstract: To study the electronic transport of molecular wire circuits, we present a time-independent scattering formalism which includes an ab initio description of the molecular electronic structure. This allows us to obtain the molecule–metal coupling description at the same level of theory. The conductance of junction α, α′ xylyl dithiol and benzene-1,4-dithiol between gold electrodes is obtained and compared with available experimental data. The conductance depends dramatically on the relative position of the Fermi energy of the metal with respect to the molecular levels. We obtain an estimate for the injecting energy of the electron onto the molecule by varying the distance between the molecule and the attached gold clusters. Contrary to the standard assumption, we find that the injecting energy lies close to the molecular highest occupied molecular orbital, rather than in the middle of the gap; it is just the work function of the bulk metal. Finally, the adequacy of the widely used extended Huckel method for...
TL;DR: In this paper, a new polarizable model for water based on Thole's method was proposed, which produces energies and nearest-neighbor (H-bonded) oxygen-oxygen distances that agree well with the ab initio results for small water clusters.
Abstract: We present the parametrization of a new polarizable model for water based on Thole’s method [Chem. Phys. 59, 341 (1981)] for predicting molecular polarizabilities using smeared charges and dipoles. The potential is parametrized using first principles ab initio data for the water dimer. Initial benchmarks of the new model include the investigation of the properties of water clusters (n=2–21) and (hexagonal) ice Ih using molecular dynamics simulations. The potential produces energies and nearest-neighbor (H-bonded) oxygen–oxygen distances that agree well with the ab initio results for the small water clusters. The properties of larger clusters with 9–21 water molecules using predicted structures from Wales et al. [Chem. Phys. Lett. 286, 65 (1998)] were also studied in order to identify trends and convergence of structural and electric properties with cluster size. The simulation of ice Ih produces a lattice energy of −65.19 kJ/mol (expt. −58.9 kJ/mol) with an average dipole moment of 2.849 D. The calculated...
TL;DR: In this article, the structural and electronic properties for various models of nanotubular and layered boron solids, and compares them to well-known tubular and stacked forms of pure carbon and mixed borons compounds, are discussed.
Abstract: Based on a series of ab initio studies we have pointed out the remarkable structural stability of nanotubular and quasiplanar boron clusters, and postulated the existence of novel layered, tubular, and quasicrystalline boron solids built from elemental subunits. The present study illustrates and predicts qualitative structural and electronic properties for various models of nanotubular and layered boron solids, and compares them to well-known tubular and layered forms of pure carbon and mixed boron compounds.
TL;DR: In this paper, a first-principles embedding theory that combines the salient features of density functional theory (DFT) and traditional quantum chemical methods is presented, which involves constructing a DFT-based embedding potential and then using it as a one-electron operator within a very accurate ab initio calculation.
Abstract: A first-principles embedding theory that combines the salient features of density functional theory (DFT) and traditional quantum chemical methods is presented. The method involves constructing a DFT-based embedding potential and then using it as a one-electron operator within a very accurate ab initio calculation. We demonstrate how DFT calculations can be systematically improved via this procedure. The scheme is tested using two closed shell systems, a toy model Li2Mg2, and the experimentally well characterized CO/Cu(111) system. Our results are in good agreement with near full configuration interaction calculations in the former case and experimental adsorbate binding energies in the latter. This method provides the means to systematically include electron correlation in a local region of a condensed phase.
TL;DR: In this article, the Tamm-Dancoff approximation to TDDFT was used to calculate the electronic excitation energies and oscillator strengths for a series of polycyclic aromatic hydrocarbon radical cations.
Abstract: Time-dependent density functional theory (TDDFT) and its modification, the Tamm–Dancoff approximation to TDDFT, are employed to calculate the electronic excitation energies and oscillator strengths for a series of polycyclic aromatic hydrocarbon radical cations. For the radical cations of naphthalene and anthracene, TDDFT using the Becke–Lee–Yang–Parr functional and the 6-31G** basis set provides the excitation energies that are roughly within 0.3 eV of the experimental data. The assignments of the electron transitions proposed by TDDFT accord with the previous assignments made by accurate ab initio calculations, except that TDDFT indicates the existence of a few additional transitions of π*←σ character among the several low-lying transitions. The calculated energies for these π*←σ transitions are found to be consistent with the onset of a σ electron ionization manifold in the photoelectron spectra. For the pyrene radical cation, TDDFT supports the previous assignments made by semiempirical calculations, whereas for the perylene radical cation, TDDFT suggests the energy ordering of the three lowest-lying excited states be changed from those of the semiempirical results.
TL;DR: A polarizable water pair potential was determined by fitting a potential form to microwave, terahertz, and mid-infrared (D2O)2 spectra through a rigorous calculation of the water dimer eigenstates and the calculated dimer structure and dipole moment are very close to those determined from microwave spectroscopy and high-level ab initio calculations.
Abstract: A polarizable water pair potential was determined by fitting a potential form to microwave, terahertz, and mid-infrared (D2O)2 spectra through a rigorous calculation of the water dimer eigenstates. It accurately reproduces most ground state vibration-rotation-tunneling spectra and yields excellent second viral coefficients. The calculated dimer structure and dipole moment are very close to those determined from microwave spectroscopy and high-level ab initio calculations. The dimer binding energy and acceptor switching and donor-acceptor interchange tunneling barriers are in excellent agreement with recent ab initio theory, as are cyclic water trimer and tetramer structures and binding energies.
TL;DR: In this article, the first time-dependent density functional theory (TDDFT) calculations on the spectra of molecules containing transition metals are reported, and three prototype systems are considered, of which the assignments are controversial:
Abstract: The first time-dependent density functional theory (TDDFT) calculations on the spectra of molecules containing transition metals are reported. Three prototype systems are considered, of which the assignments are controversial: MnO4-, Ni(CO)4, and Mn2(CO)10. The TDDFT results are shown to be comparable in accuracy to the most elaborate ab initio calculations and lead to new insights in the spectra of these molecules. In some cases, the presented TDDFT results differ substantially, in both the ordering and the values for the excitation energies, from the older DFT method for the calculation of excitation energies: the ΔSCF approach. For the Mn2(CO)10 molecule, the presented results are the highest-level theoretical results published so far. Over all, the results show that TDDFT can be a very useful tool in the calculation and interpretation of the spectra of transition metal compounds.
TL;DR: In this paper, the electronic ground state potential energy surface (PES) of hydrogen peroxide covering, in an almost global fashion, all six internal degrees of freedom by two different ab initio techniques.
Abstract: We report calculations of the electronic ground state potential energy surface (PES) of hydrogen peroxide covering, in an almost global fashion, all six internal degrees of freedom by two different ab initio techniques. Density functional theory (DFT) calculations using the Becke 3 parameter Lee-Yang-Parr (B3LYP) hybrid functional and multiconfigurational second order perturbation theory (CASPT2) calculations, both using large basis sets, are performed for a wide range of geometries (8145 DFT and 5310 CASPT2 single-point energies). We use a combined data set of mostly DFT with additional CASPT2 ab initio points and the complete CASPT2 surface to fit a total of four different 6D analytical representations. The resulting potentials contain 70-76 freely adjusted parameters and represent the ground state PES up to 40000 cm(-1) above the equilibrium energy with a standard deviation of 100-107 cm(-1) without any important artifacts. One of the model surfaces is further empirically refined to match the bond dissociation energy D-0 for HOOH --> 2OH. The potentials are designed for energy regions accessible by vibrational fundamental and overtone spectroscopy including the dissociation channel into hydroxyl radicals. Characteristic properties of the model surfaces are investigated by means of stationary point analyses, torsional barrier heights, harmonic frequencies, low-dimensional cuts and minimum energy paths for dissociation. Overall good agreement with high-level ab initio calculations, especially for the CASPT2 based potentials, is achieved. The drastic change in geometry at intermediate O-O distances, which reflects the transition from covalent to hydrogen bonding, is reproduced quantitatively. We calculate fully 6D anharmonic zero point energies and ground state torsional splittings with the diffusion quantum Monte Carlo method in perfect agreement, within statistical error bars, with experiment for the CASPT2 based potentials. Variational vibrational calculations in the (4+2)D adiabatic approximation yield energy levels and torsional splittings from the ground state up to predissociative states, satisfactorily reproducing the experimental transition wavenumbers. (C) 1999 American Institute of Physics. [S0021-9606(99)30205-1].
TL;DR: In this paper, the authors used time-dependent density functional theory (TDDFT) to analyze the electronic absorption spectrum of free base porphin (FBP) and showed that the Soret band is due to the two close-lying excitations 2'1B2u and 2'2B3u.
Abstract: Three high-level correlated ab initio studies have recently been performed on the electronic absorption spectrum of free base porphin (FBP), but significant differences between the various assignments of the low-lying bands remain. In view of the importance of FBP as the basic building block of the porphyrins, further reliable results are evidently required and are provided here, using time-dependent density functional theory (TDDFT). Our results strongly support the recent CASPT2 interpretation which is consistent with the traditional interpretation, stating that the intense B band (or Soret band) is due to the two close-lying excitations 2 1B2u and 2 1B3u. As in the CASPT2 paper, we attribute all low-lying bands to pairs of 1B2u–1B3u excitations. The interpretation of the combined B–N band system is discussed in some detail. The effects of basis set, geometry, and choice of exchange-correlation potential are considered as well.
TL;DR: In this article, a thermodynamic cycle was proposed to calculate absolute pKa values for a Bronsted acid in aqueous solution, where the solvent (water) was represented by a dielectric using the polarizable continuum model (PCM).
Abstract: A thermodynamical cycle is proposed to calculate absolute pKa values for a Bronsted acid in aqueous solution. The solvent (water) was represented by a dielectric using the polarizable continuum model (PCM), and the absolute pKa values of some aliphatic carboxylic acids were computed. The results indicate that the proposed methodology seems to be capable of predicting reasonably good absolute pKa values, although in some cases appreciable deviations are observed, which can be related to neglecting the molecular motion contributions (ΔGMm) to the solvation energy (ΔGsolv).