Extended Hartree Fock Approximate Correlation Energy Research Articles

Infrared radiative transition probabilities for the LiS(A2[formula omitted]-X2[formula omitted]) band system

In this work, the (A2Σ+-X2Π) band system of LiS is theoretically studied employing novel and accurate functional forms. For such, ab initio electronic energies are firstly calculated using an internally contracted multireference configuration interaction with Davidson modification (MRCI + Q) associated with aug-cc-pV(5 + d) Z basis set. These energies are subsequently fitted to the extended Hartree-Fock approximate correlation energy (EHFACE) model. Based on the obtained functions, molecular properties as spectroscopic parameters, rovibrational levels, and classical turning points were derived. For the first time, accurate Franck-Condon factors (FCF), r-centroids, and Einstein coefficients for spontaneous emission are reported. Calculated radiative lifetimes for the excited state A2Σ+ are of the order of milliseconds. The recoil temperature (Trecoil) is also calculated, indicating an ultracold temperature for this molecule. Besides, the results shown in this study can be considered as reliable guidelines for experimental measurements.

Potential energy curves, turning points, Franck–Condon factors and $r$-centroids for the astrophysically interesting $\mbox{S}_{2}$ molecule

We have theoretically studied the seven lowest triplet states of the disulfur species. For such investigations, analytical potential energy curves (APECs) for $X^{3}\varSigma _{g}^{-}$ ground and $A^{\prime\,3} \Delta _{u}$, $A^{3}\varSigma _{u}^{+}$, $B^{\prime\prime\,3}\varPi _{u}$, and $B^{3}\varSigma _{u}^{-}$ excited electronic states were constructed within the extended Hartree–Fock approximate correlation energy (EHFACE) model by Varandas. Once that these analytical representations are obtained, nuclear properties, such as vibrational energies, classical turning points, and spectroscopic constants were calculated. Particularly, comparisons between these vibrational levels with those obtained via the Rydberg–Klein–Rees (RKR) methodology as implemented in Le Roy’s RKR1 code are reported. The impact of tight $d$ augmented correlation consistent basis on the energies and frequencies is shown. We also re-examined the vibronic (vibration-electronic) transition parameters as Franck–Condon (FC) factors and $r$-centroids for the bands of the $B^{\prime\prime\,3}\varPi _{u} $–$X^{3}\varSigma _{g}^{-}$, $B^{3}\varSigma _{u}^{-} $–$X^{3}\varSigma _{g}^{-}$, $C^{3}\varSigma _{u}^{-} $–$X ^{3}\varSigma _{g}^{-}$, and $D^{3}\varPi _{u} - X^{3}\varSigma _{g}^{-}$ systems of the $\mbox{S}_{2}$ molecule. The vibrational levels and turning points of the two Rydberg states $C^{3}\varSigma _{u}^{-}$ and $D^{3}\varPi _{u}$ were computed only with the RKR method. Our results can be employed in rationalizations of astrochemical and astrophysical observations.

Accurate ab initio-based analytical potential energy function for S2 (ã1Δg) via extrapolation to the complete basis set limit**Project supported by the National Natural Science Foundation of China (Grant Nos. 11304185 and 11074151).

The potential energy curves (PECs) of the first electronic excited state of S2(ã1Δg) are calculated employing a multi-reference configuration interaction method with the Davidson correction in combination with a series of correlation-consistent basis sets from Dunning: aug-cc-pVXZ (X = T, Q, 5, 6). In order to obtain PECs with high accuracy, PECs calculated with aug-cc-pV(Q, 5)Z basis sets are extrapolated to the complete basis set limit. The resulting PECs are then fitted to the analytical potential energy function (APEF) using the extended Hartree-Fock approximate correlation energy method. By utilizing the fitted APEF, accurate and reliable spectroscopic parameters are obtained, which are consistent with both experimental and theoretical results. By solving the Schrödinger equation numerically with the APEFs obtained at the AV6Z and the extrapolated AV(Q, 5)Z level of theory, we calculate the complete set of vibrational levels, classical turning points, inertial rotation and centrifugal distortion constants.

Direct fit of extended Hartree–Fock approximate correlation energy model to spectroscopic data

Abstract By direct-fitting the parameters in the physically motivated extended Hartree–Fock approximate correlation energy model to spectroscopic data, potential curves have been obtained for a variety of diatomic systems that mimic the ro-vibrational data often within much less than 1 cm−1. The approach offers a simple, yet accurate, scheme for modeling the two-body terms that are the leading contributions in a cluster expansion of potential energy surfaces of larger dimensionality.

Potential Energy Curves for X1Σ+ and A1Π States of CO: The A1Π(v′=1–23)←X1Σ+(v″=0, 1) Transitions

We report new potential energy curves for the X1Σ+ and A1Π states of the CO molecule by employing the realistic extended Hartree–Fock approximate correlation energy model for diatomics with inclusion of the united-atom limit (EHFACE2U). Special care has been taken to describe the A1Π potential maximum, since this affects the vibrational wave functions of the highest vibrational levels. The corresponding vibronic spectroscopy and Franck–Condon factors (v″=0, 1) have also been calculated using accurate ab initio transition dipole moments. Comparison has been made with recent theoretical and experimental data.

Coupled ab initio potential energy surfaces for the two lowest 2A′ electronic states of the C2H molecule

Realistic two-valued potential energy surfaces for the reaction C(3P) + CH(X2Π) → C2 + H have been constructed from a set of high level ab initio data describing the first two 2A′ electronic states of the C2H system. These states have linear equilibrium configurations, known as the X 2Σ+ and A2Π states, and are coupled by a conical intersection. They lead to the formation of C2(X1Σ+ g) and C2(a3Πu) considering an adiabatic dissociation process. The ab initio calculations are of the multireference configuration interaction variety and were carried out using a polarized triple-zeta basis set. Using the ab initio adiabatic energies and the matrix elements of the dipole moment, a 2 × 2 diabatic representation of the electronic Hamiltonian was built. Each element of this Hamiltonian matrix was expressed within the double many-body expansion (DMBE) scheme which is based, in this case, on the extended Hartree-Fock approximate correlation energy model (EHFACE). The analytical adiabatic potential energy surfaces are then obtained as the eigenvalues of this matrix, and display correctly the Σ/Π conical intersection. Moreover, the non-adiabatic couplings given by our analytical model are compared with the ab initio ones, and good qualitative agreement is observed.

Analytical representations of high level ab initio potential energy curves of the C2 molecule

Realistic analytical representations of the twelve lowest singlet and triplet electronic adiabatic potential energy curves of C2 molecule are given in this article. The corresponding electronic states are correlated with C atoms both in their 3P state. A new set of high level MRCI calculations coupled with a double many-body expansion analytical fitting based on the extended Hartree–Fock approximate correlation energy model have been used in this work. Using RKR data available in the literature, comparison is made between our results and RKR turning points concerning the four lowest singlet states X1Σ+g, A1Πu, B1Δg and B′1Σ+g of C2. The agreement is very satisfying.

Global analytical representations of the three lowest potential energy surfaces of C2H, and rate constant calculations for the C(3P)+CH(2Π) reaction

Potential energy surfaces for the first three electronic states of the reaction C(3P)+CH(X 2Π)→C2+H have been constructed from a new set of high level ab initio data which are of the multireference configuration interaction variety and were carried out using a polarised triple-zeta basis set. These are the X 2Σ+ and the A 2Π states, and lead to the formation of C2(X 1Σg+) and C2(a 3Πu) considering an adiabatic dissociation process. Each adiabatic potential is expressed within the double many-body expansion (DMBE) scheme which is based, in this case, on the extended Hartree–Fock approximate correlation energy model (EHFACE). Moreover, a quasiclassical trajectory study of the title reaction has been performed for each of the three potential energy surfaces, yielding the corresponding rate constants.

Calculation of the asymptotic interaction and modelling of the potential energy curves of OH and OH+

The semiempirical extended Hartree—Fock approximate correlation energy model is used to represent the first ten electronic states of the hydroxyl radical, and the first two of the hydroxyl cation. Calculations of the asymptotic exchange energy, and of the leading C6 dispersion energy coefficient, are also reported. Using this asymptotic information, and existing spectroscopic RKR data or energies from ab initio electronic structure calculations to calibrate the model parameters, good quality potential energy curves are presented for all species considered.

Potential model for diatomic molecules including the united-atom limit and its use in a multiproperty fit for argon

The extended Hartree–Fock approximate correlation energy potential model recently reported for diatomics has been improved to account for the Coulombic behaviour at the collapsed diatomic limit for vanishingly small interatomic separations, R→ 0. Also discussed is how to impose the appropriate inverse exponential dependence on R into the extended Hartree–Fock energy contribution as R→∞. Test results are presented for the ground states of 13 chemically stable diatomics and the Ar2 van der Waals molecule. For Ar2, the potential-energy function is obtained from a multiproperty fit including spectroscopic, scattering, and second virial coefficient data. For the remaining diatomics, the calibration procedure uses only spectroscopic Rydberg–Klein–Rees (RKR) data. The new Ar2 potential is then used to calculate the thermophysical properties of gaseous argon over the temperature range 102⩽T/K⩽ 105. In all cases, the potential-energy function of the present work has been found to be very accurate.

Accurate diatomic curves for Ne2, Ar2, Kr2and Xe2form the extended Hartree–Fock approximate correlation energy model

The extended Hartree–Fock approximate correlation energy (EHFACE2) model described previously has been used to obtain accurate potential-energy curves for the rare-gas dimers Ne2, Ar2, Kr2 and Xe2. The potential-energy parameters have been determined from accurate ab initio SCF calculations, reliable estimates of dispersion energy coefficients and available experimental data on the second virial coefficients for the intert gases. For Kr2, the vibrational energy levels have also been used. The calculated second virial coefficients, transport properties and vibrational energy spacings are found to be in good agreement with experiment.