Showing papers on "Ab initio quantum chemistry methods published in 1989"

A fast intrinsic localization procedure applicable for ab initio and semiempirical linear combination of atomic orbital wave functions

Abstract: A new intrinsic localization algorithm is suggested based on a recently developed mathematical measure of localization. No external criteria are used to define a priori bonds, lone pairs, and core orbitals. It is shown that the method similarly to Edmiston–Ruedenberg’s localization prefers the well established chemical concept of σ–π separation, while on the other hand, works as economically as Boys’ procedure. For the application of the new localization algorithm, no additional quantities are to be calculated, the knowledge of atomic overlap intergrals is sufficient. This feature allows a unique formulation of the theory, adaptable for both ab initio and semiempirical methods, even in those cases where the exact form of the atomic basis functions is not defined (like in the EHT and PPP calculations). The implementation of the procedure in already existing program systems is particularly easy. For illustrative examples, we compare the Edmiston–Ruedenberg and Boys localized orbitals with those calculated b...

Is coupled cluster singles and doubles (CCSD) more computationally intensive than quadratic configuration interaction (QCISD)

Abstract: It is shown that the recently proposed QCI method including all single and double substitutions has essentially the same computational requirements as the more complete CCSD approach. If properly formulated, the CCSD equations contain at most quadratic terms in the excitation amplitudes.

Density-functional calculation of the parameters in the Anderson model: Application to Mn in CdTe

Abstract: We discuss methods for ab initio calculations of the parameters in the Anderson model. First, we present a very simple method for calculating the appropriate combination of hopping matrix elements needed in the impurity Anderson model. For a substitutional impurity, we show that to a good approximation it is sufficient to know the potential of the impurity atom and the local density of states of the unperturbed host. Calculations are performed for Mn substituting Cd in CdTe. As expected, the Mn 3d orbitals have a strong coupling to the Te 5p--derived valence band, but there is also a strong coupling to the conduction band. The dependence of the hopping matrix elements on the Mn configuration is studied. While there is a strong dependence on the Mn net charge, we find that the creation of, e.g., a core hole has a fairly small effect on the matrix elements, provided that the 3d occupancy is allowed to relax. Second, the Coulomb integrals between two Mn 3d orbitals and between a 3d orbital and a core orbital are calculated. The renormalization of these quantities due to the radial relaxation of the Mn 3d, 4s, and 4p orbitals, and due to charge-transfer effects, are analyzed in detail. Because of the nonmetallic character of CdTe, a change in the number of Mn 3d electrons is only partly screened by a charge transfer to the Mn 4s and 4p orbitals. Because of the moderate size of the band gap, this screening is, nevertheless, important. The radial relaxation of the Mn 3d, 4s, and 4p wave functions is also important. The relaxation of the neighboring atoms plays a rather small role. Results for the photoemission spectra are calculated including multiplet effects. The results are found to be in rather good agreement with experiment.

Ion and Cluster Ion Spectroscopy and Structure

Abstract: Structures of Molecules and Clusters as Determined by Coulomb Explosions (Z. Vager et al.). Microwave Spectroscopy of Molecular Ions (R.C. Woods). Spectroscopic Properties of Polyatomic Cations and Anions for Ab Initio Calculations (P. Botschwina). The Infrared Spectra of Polyatomic Molecular Ions: A Profitable Alliance between Theory and Experiment (H.F. Schaeffer III). Infrared Laser Spectroscopy of Molecular Ions (J.V. Coe, R.J. Saykally). High Resolution Photodetachment Studies of Molecular Negative Ions (D.M. Neumark). Electronic Spectroscopy of Open-Shell Cations (J.P. Maier). Laser Induced Fluorescence and Mass Spectrometric Detection and Characterization of Ionic Clusters (R.A. Kennedy et al.). Photodissociation Dynamics of Small Cluster Ions (M.T. Bowers). Structure of Solvated Cluster Ions (R.G. Keese, A.W. Castleman Jr.). Ion/Molecule Complexes as Central Intermediates in Unimolecular Decompositions of Metastable Radical Cations of Some Keto/Enol Tautomers: Theory and Experiment in Concert (N. Heinrich, H. Schwarz). UPS of Metal and Semiconductor Clusters (O. Cheshnovsky et al.). Continuous Beam Photoelectron Spectroscopy of Cluster Anions (S.T. Arnold et al.). Subject Index.

"Absolute" deformation potentials: Formulation and ab initio calculations for semiconductors.

Abstract: The subjects of this paper are the proper inclusion of long-range electrostatic terms in the theory of electronic deformation potentials, the way to include these terms by using supercells in ab initio density-functional methods, and calculations for selected semiconductors. We describe the connection with the heterojunction problem, and compare our values with previous model theories and with experiment.

Quantum scattering studies of electronically inelastic collisions of CN (X 2Σ+, A 2Π) with He

Abstract: Using recent ab initio interaction potential energy surfaces for the CN (X 2Σ+, A 2Π)+He system [H.‐J. Werner, B. Follmeg, and M. H. Alexander, J. Chem. Phys. 89, 3139 (1988)], we have calculated fully quantum cross sections for inelastic transitions between individual rovibrational levels of the A 2Π and the X 2Σ+ states of CN. We have concentrated on the transitions studied experimentally by Dagdigian and co‐workers for CN+Ar, namely transitions between the rotational levels of the A, v=8 and X, v’=12, the A, v=7 and X, v’=11, and the A, v=3 and X, v’=7 vibrational manifolds. In the case of the 8→12 and 7→11 transitions the cross sections are large (0.1–1 A2), and the dependence on initial Λ doublet level and on final rotational quantum number displays the same subtle alternations as seen experimentally. In the case of the 3→7 transitions, for which the vibrational levels are energetically much more separated, the calculated cross sections for CN+He are extremely small (10−5 A2), far smaller than observ...

Theoretical study of the structures and electron affinities of the dimers and trimers of the group IB metals (Cu, Ag, and Au)

Abstract: The molecular structure of both the neutral and negatively charged diatomic and triatomic systems containing the Cu, Ag, and Au metals are determined from ab initio calculations. For the neutral triatomic systems, the lowest energy structure is found to be triangular. The relative stability of the 2A1 and 2B2 structures can be predicted simply by knowing the constituent diatomic bond distances and atomic electron affinities (EAs). The lowest energy structure is linear for all of the negative ions. For anionic clusters containing Au, the Au atom(s) preferentially occupy the terminal position(s). The EAs of the heteronuclear systems can be predicted relatively accurately from a weighted average of the corresponding homonuclear systems. Although the theoretical EAs are systematically too small, accurate predictions for the EAs of the triatomics are obtained by uniformly scaling the ab initio results using the accurate experimental EA values available for the atoms and homonuclear diatomics.

Bond ionicity of the halogen−silver interaction

Abstract: The nature of the bonding between halogen atoms (F, Cl, and Br) and the Ag (111) surface has been investigated by analyzing ab initio Hartree–Fock wave functions for cluster models of the Ag surface and a halogen atom. Using a variety of criteria, we conclude that the bonding is ionic and that the halogen ionicity is essentially −1. The measures of ionicity reported are (a) the expectation value of a projection operator which provides an indication of the total charge associated with the halogen atom, (b) the analysis of the dipole moment curve as function of distance, (c) the effect on the equilibrium bond distances of a uniform external electric field, and (d) the decomposition of the interaction energy into the sum of different contributions. This latter analysis shows that the bonding arises, almost entirely, from two effects: (1) the Coulomb attraction between the charged halogen and the metal and (2) the intraunit polarization of the metal and halogen subunits.

An ab initio determination of the potential‐energy surfaces and rotation–vibration energy levels of methylene in the lowest triplet and singlet states and the singlet–triplet splitting

Abstract: The potential‐energy surfaces and rotation–vibration energy levels of the ground (X 3B1) and first excited (a 1A1) electronic states of the methylene radical have been determined by purely ab initio means. The potential‐energy surfaces were determined by multireference configuration interaction calculations, using a full‐valence complete‐active‐space reference space, with an atomic‐natural‐orbital basis set of size [5s4p3d2f1g/3s2p1d]. The configuration interaction (CI) calculations were carried out at 45 points on the triplet surface and 24 points on the singlet surface. The Morse oscillator rigid bender internal dynamics (MORBID) procedure was used to calculate vibrational and rotational energy levels for 12CH2, 12CD2, 13CH2, and 12CHD. Also calculated were the zero‐point vibrational energies, the singlet–triplet splitting, and the dissociation energy. The zero‐point energy of 12CH2 is found to be 127 cm−1 (0.363 kcal/mol) greater in the triplet state than in the singlet. The singlet–triplet splitting in 12CH2 is computed as T0=3116 cm−1 (8.909 kcal/mol), compared with the experimentally derived value of 3156±5 cm−1 (9.024±0.014 kcal/mol). The dissociation energy of the ground state is obtained as D0=179.06 kcal/mol, compared to an experimental value of 179.2±0.8 kcal/mol. The fundamental frequencies for the triplet state are obtained as ν1=3015, ν2=974, and ν3=3236 cm−1 (the experimental value of ν2 is 963.10 cm−1). The corresponding values for the singlet (experimental values in parentheses) are ν1=2787 (2806), ν2=1351 (1353), and ν3=2839 (2865) cm−1.

Theoretical study of low‐lying 1Σ+ and 1Π states of CO. II. Transition dipole moments, oscillator strengths, and radiative lifetimes

Abstract: Transition dipole moments between five low‐lying adiabatic states of CO, X 1Σ+, A 1Π, 2 1Σ+ (B 1Σ+ and D’ 1Σ+), 3 1Σ+ (C 1Σ+ and C’ 1Σ+) and 2 1Π (E 1Π), have been obtained. Ab initio calculations of the electronic energies and wave functions for these states have been reported by us previously. The calculated oscillator strengths and radiative lifetimes are compared with various measurements of these quantities. These comparisons are complicated by the large discrepancies among the experimental measurements, apparent even in the most recent studies.

Photodissociation kinetics of aluminum cluster ions: Determination of cluster dissociation energies

Abstract: The photodissociation of aluminum clusters, Al+n (n=7–17), has been studied over a broad energy range (1.88–6.99 eV). Measurements of the lifetimes of the photoexcited clusters are described. Dissociation energies have been determined by comparing the measured lifetimes with the predictions of a simple RRKM model. The dissociation energies show an overall increase with cluster size, but there are substantial oscillations around n=7–8 and n=13–15. Cluster cohesive energies are derived from these results and from previous measurements of the dissociation energies of the smaller clusters. The cohesive energies of the larger clusters (n>6) are in good agreement with the predictions of a simple model based on the bulk cohesive energy and the cluster surface energy. However, the cohesive energies are substantially larger than the results of recent ab initio calculations. The photodissociation spectrum of Al+8 has been measured and shows a broad absorption feature with a maximum ∼470 nm.

Transition state structure, barrier height, and vibrational frequencies for the reaction Cl+CH4→CH3+HCl

Abstract: We have carried out ab initio calculations using second‐ and fourth‐order Mo/ller–Plesset perturbation theory, scaled electron correlation, and several basis sets for the reaction Cl+CH4→CH3+HCl. We found that including electron correlation is essential for obtaining accurate barrier heights and vibrational frequencies. Furthermore, scaling the correlation energy further improves the barrier height predictions provided that the basis set being used is correlation balanced for both bonds involved in the reaction. Geometries and transition state frequencies calculated at the MP2 and MP‐SAC2 levels with the most extensive and best balanced basis set are in good agreement with one another for all bound modes, but the unbound‐mode frequency changes by 214i cm−1.

Ab initio calculations of polarizability and second hyperpolarizability in benzene including electron correlation treated by Møller-Plesset theory

Abstract: We present an ab initio calculation of polarizability and second hyperpolarizability for the benzene molecule including electron–electron correlation. The finite field method is used. For each selected strength of the applied electric field the energy of the benzene molecule is calculated using the self‐consistent field method (SCF) as well as with its Mo/ller–Plesset correction in the second order (MP‐2). Then the microscopic optical nonlinear responses are calculated by fitting both the SCF energy and the MP‐2 energy to a polynomial in the field strength. We find that electron correlation significantly enhances the second hyperpolarizability. For the polarizability, our computed value shows an excellent agreement with the experimentally measured value. For the second hyperpolarizability, the computed value using MP‐2 energy shows a reasonable agreement with that reported by the electrical field induced second harmonic (EFISH) generation but a poor agreement with the result of degenerate four wave mixing...

Vibrational spectrum, dipole moment function, and potential energy surface of the CH chromophore in CHX3 molecules

Abstract: The Fermi‐resonance overtone spectra of the CH chromophore up to about 18 000 cm−1 are evaluated by variational vibrational calculations for the CHX3 molecules trideuteromethane (CHD3), trifluoromethane (CHF3), chloroform (CHCl3) and 1,1,1,3,3,3‐hexafluoro‐2‐trifluoromethylpropane [(CF3)3CH]. Using appropriate model potential functions in a normal coordinate subspace, one can derive parameters for the CH chromophore potential and empirical dipole moment functions. For CHD3 and CHF3 ab initio (SCF‐CI and vibrational variational) calculations are presented, the results of which compare well with the experiments and for CHD3 also with previous (MRD‐CI) ab initio results. For all cases an accurate similarity transformation to the equivalent tridiagonal form of the effective hamiltonian can be made and the corresponding spectroscopic parameters agree with previous results. Comparison is also made with results from an internal coordinate model Hamiltonian.

Global potential-energy surfaces for H2Cl

Abstract: We present two new analytic potential‐energy surfaces suitable for studying the competition between the abstraction reaction H+DCl→HD+Cl and the exchange reaction H+DCl→HCl+D. In the abstraction channel the surfaces are only slightly different from the Stern–Persky–Klein GSW surface, but the exchange barrier on both surfaces is raised by inclusion of a three‐center term fitted to ab initio extended‐basis‐set multireference configuration interaction calculations with scaled external correlation. The two surfaces differ significantly only for the steepness of H–Cl–H bend potential. The exchange and abstraction saddle points are characterized by harmonic analysis for H2Cl, HDCl, and D2Cl, and we also compute vibrationally adiabatic barrier heights including anharmonicity. We also report thermal rate constants and activation energies for both reactions mentioned above.

s-cis and s-trans Conformers of formic, thioformic and dithioformic acids. An ab initio study

Abstract: Ab initio SCF-MO calculations have been carried out for formic, thioformic and dithioformic acids using the 6–31G* basis set. Fully optimized geometries, atomic charges, relative stabilities and harmonic force fields for s-cis and s-trans conformers of these molecules have been determined and the effects of oxygen-by-sulphur substitution analysed. A realistic description of the molecular charge distribution can be reached by introducing a quantum-mechanical correction to the Mulliken atomic charges, derived from the ‘charge’-‘charge flux’-‘overlap’(CCFO) model. Unlike reported theoretical results, the present ab initio calculations yield relative stabilities of the thioformic acid conformers in agreement with experiment [s-cis(thiol) > s-trans(thiol) > s-cis(thione) > s-trans(thione)]. The success of these ab initio calculations should be partially ascribed to the inclusion of polarization functions on all non-hydrogen atoms.

The electronic and molecular structure of C6: Complete active space self‐consistent‐field and multireference configuration interaction

Abstract: Energies and equilibrium geometries have been calculated for different structures of the C6 molecule, using multiconfiguration self‐consistent‐field (MCSCF) and multireference configuration interaction (MRCI) methods with large basis sets of ANO type. The ground state is linear with a cumulene‐like electronic structure of 3Σ−g symmetry. This result is consistent with recent experimental findings, but disagrees with other recent ab initio calculations using perturbation methods.

Theoretical studies of the complexes of benzene and pyrene with water and of benzene with formic acid, ammonia, and methane

Abstract: We present ab initio quantum‐chemical calculations including correlation effects via second‐order Mo/ller–Plesset perturbation theory, of the structures of the complexes that benzene forms with water, ammonia, methane, and formic acid, and of the pyrene–water complex. We describe our results for a number of complex structures, including symmetric and nonsymmetric configurations, in which the aromatic molecule acts as proton acceptor. In some instances, configurations where benzene is the proton donor are also investigated. Series of calculations are performed in order to assess the energy required for the Lewis acid molecule to move on top of benzene or pyrene. Our results are compared to available experimental data on the geometries and relative binding energies of these complexes.

Propargylene: A C3H2 isomer with unusual bonding

Abstract: Propargylene was identified in a matrix as a product of photolysis of cyclopropenylidene and diazopropyne. The molecule is a triplet. The optimum geometry predicted by ab initio calculations corresponds to a structure HC≡C–CH. The transition structure in the interconversion HC≡C–CH⇄HĊ=C=ĊH⇄HC–C≡CH is very low in energy and close to the energy of the vibrational ground state. Owing to this nonrigidity, computed infrared (IR) frequencies based on a harmonic treatment do not match the experimental spectrum. When this nonrigidity is taken into account by using a nonharmonic approximation calculated UMP2/6‐31G** IR spectra are in good agreement with the observed spectra of HCCCH and DCCCD.

First-principles calculation of impurity-solution energies in Cu and Ni

Abstract: We present ab initio calculations for the solution energies of 3d impurities in Cu and Ni hosts. The calculations are based on density-functional theory and the KKR Green's-function method. We apply a grand-canonical energy functional which is extremal against non-particle-conserving charge variations and give an extension of Lloyd's formula for complex energies. The full nonsphericity of the charge density is used in the double counting terms. Test calculations show that it is sufficient to take the perturbation of one shell of host atoms around the impurity into account. The calculated solution energies of 3d impurities in Cu and Ni are in good agreement with the experimental data and with the values predicted by Miedema's model.

The electronic spectrum of NOCl: Photofragment spectroscopy, vector correlations, and ab initio calculations

Abstract: The electronic absorption spectrum of NOCl in the region 620–180 nm is assigned by using vector properties of the NO photofragment and the results of ab initio calculations at the CI level. In assigning the electronic spectrum, we take into account the recoil anisotropy, rotational alignment, and Λ‐doublet populations of NO, as well as the calculated vertical excitation energies, oscillator strengths, and the nature of the orbitals involved in the transitions. In the experiments, we use expansion‐cooled samples and measure the recoil anisotropy parameters from the Doppler profiles of selected NO A 2Σ+←X 2Π rotational lines. The alignment parameters and Λ‐doublet populations are derived from the rotational spectra using different laser polarizations and excitation–detection geometries. The theoretical calculations treat all low‐lying singlet and triplet states. The calculations yield least energy paths for the excited states, with optimized rNO and ClNO angle as a function of rClN, as well as the angular dependences of the potentials and oscillator strengths of the singlet–singlet transitions. The following assignments are proposed for the main absorption bands: (1) E band—T1(1 3A‘)←S0(1 1A’); the transition borrows intensity by mixing with remote singlet states, predominantly the 4 1A’ state; (2) D and C bands—S1(1 1A‘)←S0(1 1A’); the C band corresponds to excitation of ν’1 in S1; (3) B band—S3(2 1A’)←S0(1 1A’); (4) A band—S5(4 1A’)→S0(1 1A’). The assignments proposed here are in full agreement with all the experimental observations and the results of the calculations. Despite the shallow minima calculated for the T1 and S1 surfaces, dissociation on all the surfaces is fast, and the implications of the results to the dissociation dynamics are discussed.

Quadratic configuration interaction: Reply to comment by Paldus, Cizek, and Jeziorski

Abstract: Replique a des critiques de Paldus, Cizek et Jeziorski concernant la terminologie et la presentation de la theorie de l'interaction de configurations quadratiques

Ab initio heats of formation of medium-sized hydrocarbons. 11. The benzenoid aromatics

Abstract: Ab initio calculations have been performed at the SCF level in the STO-3G, 3-21G, and 6-31G* basis sets on a variety of benzenoid and nonbenzenoid aromatic hydrocarbons, ranging from benzene to coronene. It is shown that (1) the enthalpy changes for homodesmic reactions involving only benzenoid aromatics can be computed accurately in all three bases and (2) there exist group equivalents for each basis set that enable conversion of ab initio total SCF energies to accurate heats of formation. Several examples demonstrate the extension of the method to substituted benzenes. The implications of these results concerning the correlation energies of aromatic hydrocarbons and the relationship of finite planar aromatics to the infinite graphitic sheet are discussed.

Photodissociation dynamics of water in the second absorption band. II. Ab initio calculation of the absorption spectra for H2O and D2O and dynamical interpretation of ‘‘diffuse vibrational’’ structures

Abstract: We calculated the absorption spectra of H2O and D2O in the second absorption band around 128 nm using a two‐dimensional ab initio potential energy surface for the B(1A1) electronic state. Nonadiabatic coupling to the lower states A and X and the vibrational degree of freedom of the OH fragment are completely neglected. Despite these limitations the agreement with the measured spectra is very satisfactory. The overall shape, the width, and the energetical position of the maximum are well described. Most important, however, is the reproduction of the diffuse vibrational structures superimposed on the broad background. It is demonstrated that this structure is not caused by pure bending‐excitation in the B state with associated bending quantum numbers ν’2=1,2,3,... as originally assumed. Because the equilibrium HOH bending angle and the equilibrium H–OH distance are very different in the ground and in the excited state, the main part of the spectrum and especially the diffuse structures occur at high ene...

Low temperature second virial coefficients of para-H2 gas obtained from quantum mechanical pair correlation functions

Abstract: The successfully tested interaction potential of H2-H2 obtained from quantum chemical calculations (M 80) has been used again as input in the attempt of determining a fit interaction potential of the H2-H2 system. It does not only give quantitative agreement with measurements as obtained previously, but also reproduces the second virial coefficients at low temperatures, calculated via quantum mechanical pair correlation functions, as measured in several independent experiments. Since the unique determination of this fit is not possible in principle, we have chosen a final version by means of plausible estimates of the limitations of previous ab initio calculations. The new potential is included in this paper in a table. Our isotropic rigid rotor potential term will be discussed in comparisons with experimentally determined potential fits. More tests of our fit potential have already been completed and can be used for a final judgement.

An ab initio semirigid bender calculation of the rotation and trans-tunneling spectra of (HF) 2 and (DF) 2

Abstract: Using a purely ab initio minimum energy path for the trans‐tunneling motion in the HF dimer, the energy levels for the K‐type rotation and trans‐tunneling motion for (HF)2 and (DF)2 are calculated with a one‐dimensional semirigid bender Hamiltonian and no adjustable parameters. The transition moments for rotation‐tunneling transitions are calculated, using our ab initio value for the dipole moment of an isolated HF molecule, and we also calculate B values. The energy levels we obtain are in close agreement with experiment; for example, the K=0 tunneling splitting in (HF)2 is calculated as 0.65 cm−1 compared to the experimental value of 0.658 69 cm−1. As well as showing that our ab initio minimum energy path is very good, the calculation demonstrates that the semirigid bender formalism is able to account quantitatively for the unusual Kdependence of the rotational energies resulting from the quasilinear behavior, and that the trans‐tunneling motion is separable from the other degrees of freedom. We use th...