Core Correlation Effects Research Articles

Anchoring the potential energy surface of the diacetylene dimer

The diacetylene dimer, (H–C≡C–C≡C–H)2, serves as a useful prototype for delocalised π-type interactions. It mimics many of the important characteristics of aromatic π-stacking prototypes such as (C6H6)2 but is small enough that full geometry optimisations and vibrational frequency computations with analytic derivatives are feasible with sophisticated electronic structure techniques. Six stationary points on the potential energy surface (PES) of (H–C≡C–C≡C–H)2 have been identified and characterised with CCSD(T) computations. The relative energies of the stationary points on each PES have been anchored by combining explicitly correlated MP2-R12 computations with corrections for higher-order correlation effects as well as core correlation effects. These computations identify a Y-shaped global minimum with an electronic dissociation energy (D e ) of 1.84 kcal mol−1 at the all-electron CCSD(T) complete basis set limit. Higher-order correlation corrections are necessary to correctly identify the global minimum while core correlation effects have almost no influence on the relative energies of the stationary points.

A quantum-chemical study of the structure and conformational dynamics of the acrolein molecule in the ground electronic state

The geometric parameters (including vibrationally averaged parameters), energy differences (ΔE) between the s-cis and s-trans conformers, and barrier to internal rotation (Vt) were calculated for the acrolein molecule CH2=CHCHO by various quantum-chemical methods (MP2, DFT, CASSCF, QCISD, CCSD(T), and MR-AQCC). The MP2 and B3LYP methods were used to calculate internal rotation potential functions and vibrational frequencies; the calculations were performed in various anharmonic approximations. To refine the ΔE and Vt values, two-dimensional (using a basis set of atomic orbitals) VFPA extrapolation procedure was applied, which allowed the results to be estimated in the FCI/CBS approximation taking into account nonadiabaticity, core correlation effects, and changes in the difference between zero point energies.

The accuracy of rotational constants predicted by high-level quantum-chemical calculations. I. molecules containing first-row atoms

A statistical analysis of the accuracy of theoretically predicted rotational constants is presented based on the data for a total of 16 molecules and 97 isotopologues. Special focus is given on the treatment of electron correlation by using coupled-cluster methods up to quadruple excitations, core correlation, basis-set effects, zero-point vibrational corrections, and the electronic contribution to the rotational constants. The high accuracy achieved in the present investigation is demonstrated by the fact that at our best theoretical level, termed as CCSD(T)cc-pV infinity Z+Delta core+DeltaT+DeltaQ+DeltaB vib+DeltaB el, the mean absolute error is 0.04% and the standard deviation is 0.07% in comparison with the available experimental data. The importance of higher excitations, core correlation, and zero-point vibrational effects is emphasized, while the electronic contribution is found to be less important.

A comparative assessment of the perturbative and renormalized coupled cluster theories with a noniterative treatment of triple excitations for thermochemical kinetics, including a study of basis set and core correlation effects

The CCSD, CCSD(T), and CR-CC(2,3) coupled cluster methods, combined with five triple-zeta basis sets, namely, MG3S, aug-cc-pVTZ, aug-cc-pV(T+d)Z, aug-cc-pCVTZ, and aug-cc-pCV(T+d)Z, are tested against the DBH24 database of diverse reaction barrier heights. The calculations confirm that the inclusion of connected triple excitations is essential to achieving high accuracy for thermochemical kinetics. They show that various noniterative ways of incorporating connected triple excitations in coupled cluster theory, including the CCSD(T) approach, the full CR-CC(2,3) method, and approximate variants of CR-CC(2,3) similar to the triples corrections of the CCSD(2) approaches, are all about equally accurate for describing the effects of connected triply excited clusters in studies of activation barriers. The effect of freezing core electrons on the results of the CCSD, CCSD(T), and CR-CC(2,3) calculations for barrier heights is also examined. It is demonstrated that to include core correlation most reliably, a basis set including functions that correlate the core and that can treat core-valence correlation is required. On the other hand, the frozen-core approximation using valence-optimized basis sets that lead to relatively small computational costs of CCSD(T) and CR-CC(2,3) calculations can achieve almost as high accuracy as the analogous fully correlated calculations.

A new ab initio ground-state dipole moment surface for the water molecule

A valence-only (V) dipole moment surface (DMS) has been computed for water at the internally contracted multireference configuration interaction level using the extended atom-centered correlation-consistent Gaussian basis set aug-cc-pV6Z. Small corrections to these dipole values, resulting from core correlation (C) and relativistic (R) effects, have also been computed and added to the V surface. The resulting DMS surface is hence called CVR. Interestingly, the C and R corrections cancel out each other almost completely over the whole grid of points investigated. The ground-state CVR dipole of H(2) (16)O is 1.8676 D. This value compares well with the best ab initio one determined in this study, 1.8539+/-0.0013 D, which in turn agrees well with the measured ground-state dipole moment of water, 1.8546(6) D. Line intensities computed with the help of the CVR DMS shows that the present DMS is highly similar to though slightly more accurate than the best previous DMS of water determined by Schwenke and Partridge [J. Chem. Phys. 113, 16 (2000)]. The influence of the precision of the rovibrational wave functions computed using different potential energy surfaces (PESs) has been investigated and proved to be small, due mostly to the small discrepancies between the best ab initio and empirical PESs of water. Several different measures to test the DMS of water are advanced. The seemingly most sensitive measure is the comparison between the ab initio line intensities and those measured by ultralong pathlength methods which are sensitive to very weak transitions.

Electron affinity of the O2 molecule: CCSD(T) calculations using the optimized virtual orbitals space approach

AbstractThe electron affinity (EA) of the oxygen molecule is calculated by the CCSD(T) method using the optimized virtual orbitals space (OVOS) technique by which the dimension of the original space of virtual orbitals can be significantly reduced. Extended basis sets, up to the doubly augmented correlation consistent d‐aug‐cc‐pV6Z basis sets for O2 and O at their experimental geometries are used. We demonstrate that even when the space of virtual orbitals is reduced to 50% of the full space, the resulting EA of the O2 molecule is accurate to within 0.01 eV. At the same time, the computational effort is reduced by about an order of magnitude. With OVOS reduced to 60% of the full virtual space, results are almost accurate. Considering the complete basis set limit with so reduced OVOS, corrections for the core correlation, vibrational and relativistic effects, the electron affinity of O2 is 0.452 ± 0.01 eV. This value agrees very well with the full virtual orbital space, EA = 0.446 eV, and with the recent experimental value, EA = 0.448 ± 0.006 eV. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Assessment of a computational strategy approaching spectroscopic accuracy for structure, magnetic properties and vibrational frequencies of organic free radicals: the F2CN and F2BO case

The molecular structure, vibrational frequencies, and hyperfine couplings of F(2)CN and F(2)BO have been computed at the coupled cluster level in conjunction with hierarchical series of correlation consistent basis sets. For geometrical and hyperfine parameters, extrapolation to the complete basis set limit and inclusion of core correlation effects have been considered. While a remarkable agreement with most of vibrational frequencies supports the reliability of the computational approach followed, experimental geometries cannot be used for validation purposes due to their limited accuracy. In view of previous experience with related radicals, the computed hyperfine coupling constants as well as molecular structures are expected to be very reliable and accurate.

Ab Initio Study of Low-Lying Electronic States of SnCl2+

Complete active space self-consistent field (CASSCF), multireference configuration interaction (MRCI), and restricted-spin coupled-cluster singles-doubles with perturbative triples [RCCSD(T)] calculations have been carried out on low-lying doublet and quartet states of SnCl2+, employing basis sets of up to aug-cc-pV5Z quality. Effects of core correlation and off-diagonal spin-orbit interaction on computed vertical ionization energies were investigated. The best theoretical estimate of the adiabatic ionization energy (including zero-point vibrational energy correction) to the X2A1 state of SnCl2+ is 10.093+/-0.010 eV. The first photoelectron band of SnCl2 has also been simulated by employing RCCSD(T)/aug-cc-pV5Z potential energy functions and including Duschinsky rotation and anharmonicity.

Rigorous ab initio study of Allowed and Forbidden transition amplitudes of T12+

We report here 'allowed' and 'forbidden' electromagnetic transition amplitudes among various low lying states of doubly ionized Thallium (T12+) using relativistic coupled cluster method based on Dirac-Fock hybrid basis functions. These transitions are of importance in astrophysics and plasma research. The effect of core correlation, pair correlation and core polarization on these transitions are estimated.

Protonated carbonyl sulfide: Prospects for the spectroscopic observation of the elusive HSCO+ isomer

Spurred by the apparent conflict between ab initio predictions and infrared spectroscopic evidence regarding the relative stability of isomers of protonated carbonyl sulfide, key stationary points on the isomerization surface of HOCS(+) have been examined via systematic extrapolations of ab initio energies. Electron correlation has been accounted for using second-order Møller-Plesset perturbation theory and coupled cluster theory through triple excitations [CCSD, CCSD(T), and CCSDT] in conjunction with the correlation consistent hierarchy of basis sets, cc-pVXZ (X=D,T,Q,5,6). HSCO(+) is predicted to lie lower in energy than HOCS(+) by 4.86 kcal mol(-1), computed using the focal point extrapolation scheme of Allen and co-workers [J. Chem. Phys. 99, 4638 (1993)] with corrections for anharmonic zero-point vibrational energy, core correlation, non-Born-Oppenheimer, and scalar relativistic effects. A transition state has been located, constituting the barrier to isomerization of HSCO(+) to HOCS(+), lying 68.9 kcal mol(-1) higher in energy than HSCO(+). This is well above predicted exothermicity [DeltaH(r) (o)(0 K)=48.1 kcal mol(-1), cc-pVQZ CCSD(T)] for the reaction considered in the experiments (HSCO(+)+H(2)-->OCS+H(3) (+)). Though proton tunneling will lead to a lower effective barrier, this prediction is consistent with the lack of HSCO(+) in electrical discharges in H(2)OCS, since the relative populations of HOCS(+) and HSCO(+) will depend on the experimental details of the protonation route rather than the relative thermodynamic stability of the isomers. Anharmonic vibrational frequencies and vibrationally corrected rotational constants from cc-pVTZ CCSD(T) cubic and quartic force constants are provided, to aid in the spectroscopic observation of the energetically favorable but apparently elusive HSCO(+) isomer.

The small planarization barriers for the amino group in the nucleic acid bases

The amino group in the nucleic acid bases frequently interacts with other bases or with other molecular systems. Thus any nonplanarity of the amino group may affect the molecular recognition of nucleic acids. Ab initio Hartree-Fock (HF) and second-order Moller-Plesset perturbation (MP2) levels of theory have been used to obtain the equilibrium geometries of the C(l) and C(s) structures for five common nucleic acid bases. The energy barriers between the C(l) and C(s) structures have also been predicted. A series of correlation consistent basis sets up to cc-pCVQZ and aug-cc-pVQZ has been used to systematically study the dependence of the amino group nonplanarity. The equilibrium geometries of the nucleic acid bases with an amino group, including adenine, guanine, and cytosine, are examined carefully. At the MP2 level of theory, larger basis sets decrease the extent of nonplanarity of the amino group, but the decrease slows down when the QZ basis sets are used, demonstrating the intrinsic property of nonplanarity for guanine. For adenine and cytosine the situation is less clear; as the HF limit is approached, these two structures become planar. Addition of core correlation effects or diffuse functions further decreases the degree of nucleic acid base nonplanarity, in comparison to the original cc-pVXZ (X=D, T, and Q) basis sets. The aug-cc-pVXZ basis shows smaller degrees of nonplanarity than the cc-pCVXZ sets. The aug-cc-pVXZ basis is less size dependent than the cc-pVXZ and cc-pCVXZ sets in the prediction of the amino-group-related bond angles and dihedral angles and energy barriers for adenine, guanine, and cytosine. The cc-pCVQZ and aug-cc-pVQZ MP2 results may be regarded as benchmark predictions for the five common bases. The predicted classical barriers to planarization are 0.02 (adenine), 0.74 (guanine), and 0.03(cytosine) kcal mol(-1).

Accurate ab Initio Binding Energies of Alkaline Earth Metal Clusters

The effects of basis set superposition error (BSSE) and core-correlation on the electronic binding energies of alkaline earth metal clusters Y(n) (Y = Be, Mg, Ca; n = 2-4) at the Moller-Plesset second-order perturbation theory (MP2) and the single and double coupled cluster method with perturbative triples correction (CCSD(T)) levels are examined using the correlation consistent basis sets cc-pVXZ and cc-pCVXZ (X = D, T, Q, 5). It is found that, while BSSE has a negligible effect for valence-electron-only-correlated calculations for most basis sets, its magnitude becomes more pronounced for all-electron-correlated calculations, including core electrons. By utilizing the negligible effect of BSSE on the binding energies for valence-electron-only-correlated calculations, in combination with the negligible core-correlation effect at the CCSD(T) level, accurate binding energies of these clusters up to pentamers (octamers in the case of the Be clusters) are estimated via the basis set extrapolation of ab initio CCSD(T) correlation energies of the monomer and cluster with only the cc-pVDZ and cc-pVTZ sets, using the basis set and correlation-dependent extrapolation formula recently devised. A comparison between the CCSD(T) and density functional theory (DFT) binding energies is made to identify the most appropriate DFT method for the study of these clusters.

Model Identity SN2 Reactions CH3X + X- (X = F, Cl, CN, OH, SH, NH2, PH2): Marcus Theory Analyzed

The structures of seven gas phase identity S(N)2 reactions of the form CH(3)X + X(-) have been characterized with seven distinct theoretical methods: RHF, B3LYP, BLYP, BP86, MP2, CCSD, and CCSD(T), in conjunction with basis sets of double and triple zeta quality. Additionally, the energetics of said reactions have been definitively computed using focal point analyses utilizing extrapolation to the one-particle limit for the Hartree-Fock and MP2 energies using basis sets of up to aug-cc-pV5Z quality, inclusion of higher order correlation effects [CCSD and CCSD(T)] with basis sets of aug-cc-pVTZ quality, and additional auxiliary terms for core correlation and scalar relativistic effects. Final net activation barriers for the reactions are E(b)(F,F)= -0.8, E(b)(Cl,Cl)= 1.6, E(b)(CN,CN)= 28.7, E(b)(OH,OH)= 14.3, E(b)(SH,SH)= 13.8, E(b)(NH2,NH2)= 28.6, and E(b)(PH2,PH2)= 25.7 kcal mol(-1). General trends in the energetics, specifically the performance of the density functionals, and the component energies of the focal point analyses are discussed. The utility of classic Marcus theory as a technique for barrier predictions has been carefully analyzed. The standard Marcus theory results show disparities of up to 9 kcal mol(-1) with respect to explicitly computed results. However, when alternative approaches to Marcus theory, independent of the well-depths, are considered, excellent performance is achieved, with the largest deviations being under 3 kcal mol(-1).

Applications of Simple Estimation Scheme of Electron Correlation Energy to Strong Ionic Compounds

AbstractThe calculation results of electron correlation energies of KF and (KF)2 were reported. The transferability of lsK2, lsF2 and the inner core correlation effects of K and F in both K, K+, KF and F, F‐, KF systems were investigated respectively. The correlation energy contributions of K and F component to KF system were calculated. By applying the simple estimation scheme to the calculation of the correlation energy of the strong ionic compound KF and (KF) 2, it was shown that such a powerful scheme could not only reach the chemical accuracy but also need little computational work.

Fine‐Structure Excitation of C+and Si+by Atomic Hydrogen

We present calculations of cross sections for fine-structure excitation in collisions of carbon and silicon ions in the 2 P state with atomic hydrogen in the ground state. The results are based on accurate calculations of CH + and SiH + molecular potentials, including electronic core correlation and relativistic effects. We find that the energy dependence of the excitation cross sections is largely determined by shape resonances. Our work improves on the results of previous calculations with less accurate potentials. Analytical expressions for the cooling efficiency of C + ( 2 P1=2 )a nd Si + ( 2 P1=2) are given for the temperature interval 15‐2000 K. Subject headingg ISM: atoms — ISM: molecules — scattering

Anchoring the potential energy surface of the cyclic water trimer

Six cyclic stationary points on the water trimer potential energy surface have been fully optimized at the MP2 level with the aug-cc-pVQZ basis set. In agreement with previous work, harmonic vibrational frequencies indicate that two structures are minima, three are transition states connecting minima on the surface while the remaining stationary point is a higher-order saddle point. The 1- and n-particle limits of the electronic energies of each of these six structures were estimated by systematically varying both the basis sets and theoretical methods. The former limit was approached with the cc-pVXZ and aug-cc-pVXZ families of basis sets (X=2-7) while MP2, CCSD(T), and BD(TQ) calculations helped examine the latter. Core correlation effects have also been assessed at the MP2 level with the cc-pCVXZ series of basis sets (X=2-5). These data have been combined to provide highly accurate relative energies and dissociation energies for these stationary points.

The CIPT2 method: Coupling of multi-reference configuration interaction and multi-reference perturbation theory. Application to the chromium dimer

The potential energy function of is computed using the MRCI+Q (multi-reference configuration interaction with Davidson correction) and CASPT2 (complete active space second-order multi-reference perturbation theory) methods and very large basis sets. In addition, a new hybrid method denoted CIPT2, in which excitations solely from the active space are treated by MRCI and the remaining ones by perturbation theory, is proposed and applied. These methods are used to analyse valence and core correlation effects on the potential energy function. MRCI calculations which fully include correlation of the 3p, 3d, and 4s electrons yield excellent agreement with experimental values for the equilibrium distance Re and the harmonic vibrational constant ω e . Correlation of the (3p) orbitals is found to be very important. At the MRCI+Q level this reduces Re by 0.08 Å and increases ω e by about 200 cm−1. These effects are not correctly reproduced in CASPT2 and CIPT2. It is shown that in CASPT2 the valence correlation effects are significantly overestimated and the distance dependence of the core correlation is incorrect. The basis set limit of the dissociation energies for MRCI+Q and CASPT 2 is estimated to be 1.3 eV and 1.9 eV, respectively. The remaining error (≈ 0.2 − 0.3 eV) of the MRCI+Q dissociation energy is attributed to unlinked cluster and higher excitation effects, which are only approximately accounted for by the Davidson correction.

Spin-orbit ab initio study of alkyl halide dissociation via electronic curve crossing.

An ab initio study of the role of electronic curve crossing in the photodissociation dynamics of the alkyl halides is presented. Recent experimental studies show that curve crossing plays a deterministic role in deciding the channel of dissociation. Coupled repulsive potential energy curves of the low-lying n-sigma(*) states are studied including spin-orbit and relativistic effects. Basis set including effect of core correlation is used. Ab initio vertical excitation spectra of CH(3)I and CF(3)I are in agreement with the experimental observation. The curve crossing region is around 2.371 A for CH(3)I and CF(3)I. The potential curves of the repulsive excited states have larger slope for CF(3)I, suggesting a higher velocity and decreased intersystem crossing probability on fluorination. We also report the potential curves and the region of curve crossing for CH(3)Br and CH(3)Cl.

A theoretical study of diazirine (H2CN2), diazirinyl radical (HCN2) and their related cations (H2CN2+,HCN2+): molecular structure, energetics and ionization potential

The molecular structures and energetics of diazirine (H2CN2), diazirinyl radical (HCN2) and their related cations (H2CN2+,HCN2+) have been investigated by means of the coupled-cluster method in conjunction with different basis sets. Core correlation effects and basis set incompleteness have been taken into account in order to obtain best estimates of their equilibrium geometries, C–H bond dissociation energies of diazirine and diazirine cation (H2CN2+), and ionization potentials of diazirine and diazirinyl radical. Adiabatic as well as vertical ionization potentials have been considered. For all the species studied, harmonic vibrational frequencies have also been evaluated in order to obtain zero-point corrections to both ionization potentials and C–H dissociation energies.

An ab initio study of diazirine: equilibrium structure and molecular properties

The equilibrium structure and two important molecular properties, the dipole moment and the nuclear quadrupole coupling constants, of diazirine have been investigated theoretically at high level of theory. Very accurate results, which can be favourably compared to the experiment, are presented. As far as the equilibrium structure is concerned, coupled-cluster approach with perturbative inclusion of triples and very large basis sets have been employed. Core correlation effects and basis set incompleteness have been taken into account in order to obtain best estimates of equilibrium geometry. The molecular dipole moment has been calculated at coupled-cluster level using bases of different quality including both diffuse and tight functions. Extrapolation to the infinite basis set limit has also been performed. In addition, the complete inertial nuclear quadrupole tensors, evaluated from the electric field gradients at nitrogen and hydrogen nuclei, have been computed at different level of theory: the multi-configuration self-consistent field, the Møller–Plesset many-body perturbation to second order and the coupled-cluster methods have been employed in conjunction with core-valence basis sets.