Core Electron Correlation Research Articles

Ground and Excited Electronic Structure Analysis of FeH with Correlated Wave Function Theory and Density Functional Approximations.

FeH is one of the most challenging diatomic molecules to study under electronic structure theory. Here, we have successfully studied 22 electronic states of FeH using ab initio multireference configuration interaction (MRCI), Davidson-corrected MRCI (MRCI+Q), and coupled cluster singles, doubles, and perturbative triples [CCSD(T)] levels of theory. We report their potential energy curves (PECs), excitation energies, dissociation energies, equilibrium electronic configurations, and a series of spectroscopic constants with the use of augmented triple-ζ, quadruple-ζ, and quintuple-ζ quality correlation consistent basis sets. The scalar relativistic effects and active space and core electron correlation contribution on the properties of FeH are also explored. The use of a large CASSCF active space that includes 4s, 4p, 3d, and 4d orbitals of Fe and the 1s of H is critical for producing accurate full PECs with proper dissociations and predicting the exact order of the electronic states. Our findings are in harmony with the experimental results available in the literature and will serve as reference values for future studies of FeH. Furthermore, with the use of PECs, the total internal partition function sum (TIPS) of FeH was calculated across a range of temperatures. Finally, we exploited the single-reference nature of the a6Δ of FeH and its ionized product FeH+ (X5Δ) to evaluate the associated density functional theory (DFT) errors on their dissociation energies and spectroscopic parameters.

Second order Rayleigh–Schrödinger perturbation theory for the Grasp2018 package: Core–core correlations

GRASP package is based on the relativistic configuration interaction in which accurate calculations, accounting for valence, valence–valence, core–valence, core and core–core electron correlations, often rely on massive CSF expansions. This paper presents a further development of the method based on the second-order perturbation theory for finding the most important CSFs that have the greatest influence on the core–valence, core and core–core correlations. This method is based on a combination of the relativistic configuration interaction method and the stationary second-order Rayleigh–Schrödinger many-body perturbation theory in an irreducible tensorial form [G. Gaigalas, P. Rynkun, and L. Kitovienė, Second-order Rayleigh–Schrödinger perturbation theory for the GRASP2018 package: Core–valence correlations, Lith. J. Phys. 64(1), 20–39 (2024), https://doi.org/10.3952/physics.2024.64.1.3, and G. Gaigalas, P. Rynkun, and L. Kitovienė, Second-order Rayleigh–Schrödinger perturbation theory for the GRASP2018 package: Core correlations, Lith. J. Phys. 64(2), 73–81 (2024), https://doi.org/10.3952/physics.2024.64.2.1]. In this extension, the perturbation theory takes into account electron core–valence, core and core–core correlations, where an atom or ion has any number of valence electrons, for calculation of energy spectra and other properties. Meanwhile, the rest of the correlations are taken into account in a traditional way. This allows a significant reduction of the space of the configuration state function for complex atoms and ions. We also demonstrate how this method works for calculations of the energy structure and E1 transition properties of Fe XV ion.

Probing Antioxidant-Related Properties for Phenolic Compounds.

In this work, properties related to antioxidant-potential mechanisms (such as the bond dissociation enthalpy, BDE, for the homolytic cleavage of the O-H bond and ionization energies, IEs) were determined for phenol, pyrocatechol, and gallic acid (GA). Both the protonated and deprotonated forms of GA were investigated. The Feller-Peterson-Dixon (FPD) composite method was employed with a variety of computational approaches, i.e., density functional theory, Möller-Plesset perturbation theory, and coupled-cluster-based methods, in combination with large correlation consistent basis sets with extrapolation to the complete basis set limit and consideration of core electron correlation effects. FPD results were compared to experimental and computational data available in the literature, presenting good agreement. For example, the FPD BDE (298 K) obtained for phenol, which was based on valence-correlated MP2/CBS calculations with contributions from correlating all electrons, was determined to be 87.56 kcal/mol, a value that is 0.42 kcal/mol lower than the result obtained in the most recent experiments, 87.98 ± 0.62. Calibration against coupled-cluster calculations was also carried out for phenol. We expect that the outcomes gathered here may help in establishing a general protocol for computational chemists that are interested in determining antioxidant-related properties for phenolic compounds with considerable accuracy as well as to motivate future IE measurements (particularly for GA) to be accomplished in the near future.

Accurate ab initio spectroscopic studies of promising interstellar ethanolamine iminic precursors

Context. The detection of NH2CH2CH2OH (ethanolamine) in molecular cloud G+0.693-0.027 adds an additional player to the pre-biotic molecules discovered so far in the interstellar medium (ISM). As this molecule might be formed through condensed-phase hydrogenation steps, detecting one or more of the molecules involved might help to elucidate the chemical pathway leading to its production. Aims. The chemical path involves the formation of four chemical species. In this work, we study the energies of the isomers involved, indicate the best candidates for detection purposes, and provide the distortion constants of the most energetically favoured isomers undetected so far. Methods. We used highly accurate CCSD(T)-F12/cc-pCVTZ-F12 computations to predict the lowest energy isomers as well as their spectroscopic constants, taking corrections for core electron correlation and scalar relativity into account. Results. We studied 14 isomers. We find that the lowest energy isomer proposed in previous studies is not the actual minimum. We provide a set of rotational and distortion constants of the two new most stable isomers together with their fundamental vibrational frequencies in order to guide the search for these important astrochemical precursors of prebiotic molecules in the ISM.

Fulminic acid: a quasibent spectacle.

Fulminic acid (HCNO) played a critical role in the early development of organic chemistry, and chemists have sought to discern the structure and characteristics of this molecule and its isomers for over 200 years. The mercurial nature of the extremely flat H-C-N bending potential of fulminic acid, with a nearly vanishing harmonic vibrational frequency at linearity, remains enigmatic and refractory to electronic structure theory, as dramatic variation with both orbital basis set and electron correlation method is witnessed. To solve this problem using rigorous electronic wavefunction methods, we have employed focal point analyses (FPA) to ascertain the ab initio limit of optimized linear and bent geometries, corresponding vibrational frequencies, and the HCN + O(3P) → HCNO reaction energy. Electron correlation treatments as extensive as CCSDT(Q), CCSDTQ(P), and even CCSDTQP(H) were employed, and complete basis set (CBS) extrapolations were performed using the cc-pCVXZ (X = 4-6) basis sets. Core electron correlation, scalar relativistic effects (MVD1), and diagonal Born-Oppenheimer corrections (DBOC) were all included and found to contribute significantly in determining whether vibrationless HCNO is linear or bent. At the all-electron (AE) CCSD(T)/CBS level, HCNO is a linear molecule with ω5(H-C-N bend) = 120 cm-1. However, composite AE-CCSDT(Q)/CBS computations give an imaginary frequency (51i cm-1) at the linear optimized geometry, leading to an equilibrium structure with an H-C-N angle of 173.9°. Finally, at the AE-CCSDTQ(P)/CBS level, HCNO is once again linear with ω5 = 45 cm-1, and inclusion of both MVD1 and DBOC effects yields ω5 = 32 cm-1. A host of other topics has also been investigated for fulminic acid, including the dependence of re and ωi predictions on a variety of CBS extrapolation formulas, the question of multireference character, the N-O bond energy and enthalpy of formation, and issues that give rise to the quasibent appellation.

Ab initio electronic structure analysis of ground and excited states of HfN0,.

High-level ab initio electronic structure analysis of third-row transition metal (TM)-based diatomic species is challenging and has been perpetually lagging. In this work, fourteen and eighteen electronic states of HfN and HfN+ respectively are studied, employing multireference configuration interaction (MRCI) and coupled cluster singles doubles and perturbative triples [CCSD(T)] theories under larger correlation-consistent basis sets. Their potential energy curves (PECs), energetics, and spectroscopic parameters are reported. Core electron correlation effects on their properties are also investigated. Chemical bonding patterns of several low-lying electronic states are introduced based on the equilibrium electron configurations. The ground state of HfN (X2Σ+) has the 1σ22σ23σ11π4 electronic configuration, and the ionization of the 3σ1 electron produces the ground state of HfN+ (X1Σ+). Ground states of both HfN and HfN+ are triple bonded in nature and bear 124.86 and 109.10 kcal mol-1 binding energies with respect to their ground state fragments. The findings of this work agree well with the limited experimental literature available and provide useful reference values for future experimental analysis of HfN and HfN+.

The fundamental vibrational frequencies and spectroscopic constants of the C2O2H2 isomers: molecules known in simulated interstellar ice analogues.

While trans-glyoxal may not be easily observable in astronomical sources through either IR or radioastronomy due to its C2h symmetry, its cis conformer along with the cyc-H2COCO epoxide isomer should be ready targets for astrochemical detection. The present quantum chemical study shows that not only are both molecular isomers strongly polar, they also have notable IR features and low isomerisation energies of 4.1 kcal mol-1 and 10.7 kcal mol-1, respectively. These three isomers along with two other C2O2H2 isomers have had their full set of fundamental vibrational frequencies and spectroscopic constants characterised herein. These isomers have previously been shown to occur in simulated astrophysical ices making them worthy targets of astronomical search. Furthermore, the hybrid quartic force field (QFF) approach utilized herein to produce the needed spectral data has a mean absolute percent error compared to the experimentally-available, gas phase fundamental vibrational frequencies of 0.6% and rotational constants to better than 0.1%. The hybrid QFF is defined from explicitly correlated coupled cluster theory at the singles, doubles, and perturbative triples level [CCSD(T)-F12b] including core electron correlation and a canonical CCSD(T) relativity correction for the harmonic (quadratic) terms in the QFF and simple CCSD(T)-F12b/cc-pVDZ energies for the cubic and quartic terms, the so-called "F12-TcCR+DZ QFF." This method is producing spectroscopically-accurate predictions for both fundamental vibrational frequencies and principal spectroscopic constants. Hence, the values computed in this work should be notably accurate and, hence, exceptionally useful to the spectroscopy and astrochemistry communities.

The performance of hybrid and F12∗/F12c explicitly correlated coupled cluster methods for use in anharmonic vibrational frequency computations

AbstractA hybrid quartic force field approach produces the same accuracies as non‐hybrid methods but for less than one quarter of the computational time. This method utilizes explicitly correlated coupled cluster theory at the singles and doubles level inclusive of perturbative triples (CCSD(T)‐F12b) in conjunction with a triple‐ basis set, core electron correlation, and scalar relativity for the harmonic terms and CCSD(T)‐F12b with a valence double‐ basis set for the cubic and quartic terms. There is no sacrifice in the prediction of fundamental anharmonic vibrational frequencies or vibrationally‐averaged rotational constants as compared to experiment, but the time saved is notable. Other hybrid methods are examined involving different sizes of basis sets and composite terms included or excluded. Not one is more accurate; only one is faster. F12 (also called F12c) is tested as well, but it has an increase in computational time for no increase in accuracy. As such, this work reports a hybrid and composite approach (F12‐TcCR+DZ) in the computation of rovibrational spectral data which can be applied to the observation of novel molecules in the gas phase in the laboratory and potentially even in astrophysical environments.

Ab initio potential energy surface and vibration-rotation energy levels of magnesium monohydroxide revisited

The accurate potential energy surface of magnesium monohydroxide, MgOH, in its ground electronic state X˜2Σ+ has been determined from ab initio calculations using the coupled-cluster approach in conjunction with the correlation-consistent basis sets up to septuple-zeta quality. The core-electron correlation, higher-order electron correlation, scalar relativistic, and adiabatic effects were taken into account. The equilibrium configuration of the MgOH molecule was confirmed to be linear, although with the bending potential energy function being very flat near the minimum. The vibration–rotation-spin energy levels of the MgOH, MgOD, 25MgOH, and 26MgOH isotopologues were predicted using a variational approach. The spectroscopic constants of these isotopologues were determined to high accuracy.

Single-photon ionization of SiC in the gas phase: experimental and ab initio characterization of SiC.

We report the first experimental observation of single-photon ionization transitions of the SiC radical between 8.0 and 11.0 eV performed on the DESIRS beamline at the SOLEIL synchrotron facility. The SiC radical, very difficult to synthesize in the gas phase, was produced through chemical reactions between CHx (x = 0-3) and SiHy (y = 0-3) in a continuous microwave discharge flow tube, the CHx and SiHy species being formed by successive hydrogen-atom abstractions induced by fluorine atoms on methane and silane, respectively. Mass-selected ion yield and photoelectron spectra were recorded as a function of photon energy using a double imaging photoelectron/photoion coincidence spectrometer. The photoelectron spectrum enables the first direct experimental determinations of the X+ 4Σ- ← X 3Π and 1+ 2Π ← X 3Π adiabatic ionization energies of SiC (8.978(10) eV and 10.216(24) eV, respectively). Calculated spectra based on Franck-Condon factors are compared with the experimental spectra. These spectra were obtained by solving the rovibrational Hamiltonian, using the potential energy curves calculated at the multireference single and double configuration interaction level with Davidson correction (MRCI + Q) and the aug-cc-pV5Z basis set. MRCI + Q calculations including the core and core-valence electron correlation were performed using the aug-cc-pCV6Z basis set to predict the spectroscopic properties of the six lowest electronic states of SiC+. Complete basis set extrapolations and relativistic energy corrections were also included in the determination of the energy differences characterizing the photoionization process. Using our experimental and theoretical results, we derived semi-experimental values for the five lowest ionization energies of SiC.

C-AlO[formula omitted], c-HAlO[formula omitted], and c-(HN)OAlH spectroscopic constants and anharmonic frequencies

Aluminum forms strong bonds to oxygen, nitrogen, and hydrogen, but these motifs are relatively rare in the interstellar medium (ISM). However, no chemical rationale accounts for this observation. The cyclic molecules AlO2, HAlO2, and (HN)OAlH are quintessential examples of the aforementioned phenomenon, and the latter two are isoelectronic with each other. Matrix-isolation spectroscopic data and prior theoretical computations exist for cyclic AlO2, although it has yet to be observed astronomically. However, high-level theoretical data provided herein generate useful predictions for and insights into spectral data for these three molecules. Anharmonic vibrational frequencies and rotational constants are determined in this work for these molecules via quartic force fields at the CCSD(T)-F12b/cc-pVTZ-F12 level of theory and with canonical CCSD(T) for consideration of basis set convergence, core electron correlation, and relativity (CcCR). The present work finds that these aluminum oxides are weak-to-moderate infrared emitters but are much stronger microwave emitters with dipole moments of 4.95 D, 4.55 D, and 3.76 D respective of cyclic AlO2 (c-AlO2), cyclic HAlO2 (c-HAlO2), and cyclic (HN)OAlH (c-(HN)OAlH). Correlation to argon-matrix experiments for c-AlO2 is within the expected matrix shift for the ν3 bend computed here to be 527.8 cm−1. Astrophysical detection of these molecules could imply their role in the creation or degradation of aluminum-containing nanocrystals and interstellar dust of importance for the formation of rocky bodies, and the spectral data computed in this work should be able to assist in such classification.

Ab initio characterization of the aluminum dimer in its X and A electronic states.

The potential energy functions of the aluminum dimer, Al2 , in its lowest-energy electronic states, X and A have been determined from ab initio calculations using the multi-reference averaged coupled-pair functional method in conjunction with the correlation-consistent basis sets up to septuple-zeta quality. The core-electron correlation, scalar relativistic, and spin-orbit effects were taken into account. The vibration-rotation energy levels for both the states of Al2 were calculated to near the "spectroscopic" accuracy. The state was unequivocally confirmed to be the electronic ground state of Al2 , and the electronic term value of the state was predicted to be 247 cm-1 . The energies and intensities of direct electronic-vibration transitions X ↔A were predicted.

Ab initio investigation of the ground and excited states of TcO+ and RhO+

High-level quantum calculations are utilized to analyze nineteen and thirteen lowest energy electronic states of TcO+ and RhO+. Their potential energy curves, equilibrium bond lengths, electronic excitation energies, harmonic vibrational frequencies, and anharmonicities are reported using quadruple-ζ and quintuple-ζ quality correlation consistent basis sets. The core electron correlation effects and equilibrium chemical bonding patterns of several low-energy electronic states are also considered. The ground state of TcO+ (11Ʃ+) (1σ21π41δ4) is triple bonded in nature, results adiabatically from the high energy Tc+(3P) + O(3P) fragments, and relates to the first excited state of TcO (2Σ+) (1σ22σ11π41δ4). The chemical bonding of TcO+(11Ʃ+) is identical to its isoelectronic RuO2+ (X1Ʃ+) ground state. The ground state of RhO+ (3Σ−) features a 1σ21π41δ42π2 electronic configuration. Moving from TcO+ to RhO+ the binding energy decreases by ∼15−20 kcal/mol, the metal-oxygen bond shortens by 0.07 Å, and the harmonic stretching frequency decreases by ∼170 cm−1. Bonding patterns and spectroscopic constants are compared for the neighboring species of RhO+ and TcO+.

Ionization potentials for the H2CO trimer.

In this work, a computational study on the ionization potentials (IPs) of the formaldehyde trimer, (H2CO)3, is presented. Twelve lowest-lying vertical IPs were determined through the use of the coupled-cluster level of theory using correlation consistent basis sets with extrapolation to the complete basis set limit and consideration of core electron correlation effects. Specifically, the equation-of-motion ionization potential coupled-cluster with single and double excitations method with the aug-cc-pVnZ and aug-cc-pCVnZ (n = D and T) basis sets was used. The Feller-Peterson-Dixon (FPD) composite approach was employed to provide accurate IPs, and eight conformations of (H2CO)3 were considered. The FPD IPs determined for (H2CO)3 were found to be systematically lower than those computed for the dimer and monomer of H2CO in the pattern IP(monomer) > IP(dimer) > IP(trimer) for a given IP. In addition, the IPs calculated when considering only the more stable conformation (C0) are in good agreement with those obtained using the eight conformations of the H2CO trimer, and thus, the actual conformation played only a minor role in determining such properties in the present case. By providing first accurate IP results for the H2CO trimer, we hope to motivate future experimental and computational investigations (e.g., studies involving photoionization) that rely on such quantities.

Energy and transition data computations for P-like ions: As, Kr, Sr, Zr, Mo, and W

The multiconfiguration Dirac–Hartree–Fock (MCDHF) and relativistic configuration interaction (RCI) methods were used to compute excitation energies and transition data for the 147 lowest states of the even 3s3p4, 3s23p23d, 3p43d, 3s3p23d2 configurations and for the 124 lowest states of the odd 3s23p3, 3p5, 3s3p33d, 3s23p3d2, 3p33d2 configurations for the P-like ions: As XIX, Kr XXII, Sr XXIV, Zr XXVI, Mo XXVIII, and W LX. E1 transition rates and weighted oscillator strengths among these states are given. Valence–valence, core–valence and core–core electron correlation effects are included. Computed excitation energies and transition data are compared with the NIST recommended values and experimental or theoretical results of other authors. All calculations were performed using the general relativistic atomic structure package GRASP2018.

Molecular frame dipole moment of diatomic molecules within relativistic coupled‐cluster framework: A comparative study of expectation value versus energy derivative approach

AbstractThe ground state molecular frame dipole moments of alkaline earth metal monofluorides and Group ІІB‐monohydrides have been computed using two analytic methods, namely the Z‐vector technique and the linear expectation value method within the four‐component relativistic coupled cluster singles and doubles framework. We have compared our results with the experimentally measured permanent dipole moments wherever available. It is found that the Z‐vector method, which is an energy‐derivative approach predicts the molecular permanent dipole moment more accurately than the linear expectation value approach in DZ and TZ quality basis set. Further, our study shows that the high‐energy virtual spinors seem to have almost no influence on the permanent dipole moment whereas the core electron correlation slightly affects this molecular property.

Theoretical Investigation of a Vital Step in the Gas-Phase Formation of Interstellar Ammonia NH2+ + H2 → NH3+ + H.

A crucial step in the gas-phase formation of ammonia in the interstellar medium (ISM) is the reaction of NH2+ with molecular hydrogen. Understanding the electronic structure of the participating species in this reaction and the evaluation of the rate coefficients at interstellar temperatures are, therefore, critical to gain new insights into the mechanisms of formation of interstellar ammonia. We present here the first theoretical results of the rate coefficients of this reaction as a function of temperatures relevant to the ISM, computed using transition-state theory. The results are in reasonable agreement with recent experimental data. This exothermic reaction features a tiny barrier which is primarily a consequence of zero-point energy corrections. The results demonstrate that quantum mechanical tunneling and core-electron correlations play significant roles in determining the rate of the reaction. The noteworthy failure of popular density functionals to describe this reaction is also highlighted.

Ab initioproperties of the NaLi molecule in thea3Σ+electronic state

Ultracold polar and magnetic ${}^{23}$Na${}^6$Li molecules in the rovibrational ground state of the lowest triplet $a^3\Sigma^+$ electronic state have been recently produced. Here, we calculate the electronic and rovibrational structure of these 14-electron molecules with spectroscopic accuracy ($<0.5\,$cm$^{-1}$) using state-of-the-art ab initio methods of quantum chemistry. We employ the hierarchy of the coupled-cluster wave functions and Gaussian basis sets extrapolated to the complete basis set limit. We show that the inclusion of higher-level excitations, core-electron correlation, relativistic, QED, and adiabatic corrections is necessary to reproduce accurately scattering and spectroscopic properties of alkali-metal systems. We obtain the well depth, $D_e=229.9(5)\,$cm$^{-1}$, the dissociation energy, $D_0=208.2(5)\,$cm$^{-1}$, and the scattering length, $a_s=-84^{+25}_{-41}\,$bohr, in good agreement with recent experimental measurements. We predict the permanent electric dipole moment in the rovibrational ground state, $d_0=$0.167(1)$\,$debye. These values are obtained without any adjustment to experimental data, showing that quantum chemistry methods are capable of predicting scattering properties of many-electron systems, provided relatively weak interaction and small reduced mass of the system.

The structure of ScC2 (X̃2A1): A combined Fourier transform microwave/millimeter-wave spectroscopic and computational study.

Pure rotational spectra of Sc13C2 (X̃2A1) and Sc12C13C (X̃2A') have been measured using Fourier transform microwave/millimeter-wave methods. These molecules were synthesized in a DC discharge from the reaction of scandium vapor, produced via laser ablation, with 13CH4 or 13CH4/12CH4, diluted in argon. The NKa,Kc = 10,1 → 00,0, 20,2 → 10,1, 30,3 → 20,2, and 40,4 → 30,3 transitions in the frequency range of 14 GHz-61 GHz were observed for both species, each exhibiting hyperfine splittings due to the nuclear spins of 13C (I = 1/2) and/or Sc (I = 7/2). These data have been analyzed with an asymmetric top Hamiltonian, and rotational, spin-rotation, and hyperfine parameters have been determined for Sc13C2 and Sc12C13C. In addition, a quartic force field was calculated for ScC2 and its isotopologues using a highly accurate coupled cluster-based composite method, incorporating complete basis set extrapolation, scalar relativistic corrections, outer core and inner core electron correlation, and higher-order valence correlation effects. The agreement between experimental and computed rotational constants, including the effective constant (B + C), is ∼0.5% for all three isotopologues. This remarkable agreement suggests promise in predicting rotational spectra of new transition metal-carbon bearing molecules. In combination with previous work on Sc12C2, an accurate structure for ScC2 has been established using combined experimental (B, C) and theoretical (A) rotational constants. The radical is cyclic (or T-shaped) with r(Sc-C) = 2.048(2) Å, r(C-C) = 1.272(2) Å, and ∠(C-Sc-C) = 36.2(1)°. The experimental and theoretical results also suggest that ScC2 contains a C2 - moiety and is largely ionic.

Probing the ionization potentials of the formaldehyde dimer.

In this work, we present a computational investigation on the ionization potentials (IPs) of the formaldehyde dimer, (H2CO)2. Twelve lowest lying IPs (corresponding to the entire valence orbitals) for both C2h and Cs symmetry conformers have been computed at the coupled cluster level of theory using large correlation consistent basis sets with extrapolation to the complete basis set limit and consideration of core electron correlation effects. Specifically, the equation-of-motion ionization potential coupled-cluster with single and double (EOMIP-CCSD) excitations method with the aug-cc-pVXZ and aug-cc-pCVXZ (X = T, Q, and 5) basis sets combined with the Feller-Peterson-Dixon approach was employed, as well as CCSD with perturbative triples [CCSD(T)] with the aug-cc-pVTZ basis sets. In general, excellent agreement was observed from the comparison between the results obtained through the use of these approaches. In addition, the IPs for the formaldehyde monomer were also obtained using such methodologies and the results compared with existing experimental data; excellent agreement was also observed in this case. To the best of our knowledge, this work represents the first of its kind to determine the IPs for all these systems using a high level theory approach and is presented to motivate experimental investigations, e.g., studies involving photoionization, particularly for the formaldehyde dimer. The equilibrium binding energy of the C2h dimer is calculated in this work at the CCSD(T)/aug-cc-pVTZ level of theory to be -4.71 kcal/mol. At this same level of theory, the equilibrium isomerization energy between C2h and Cs conformers is 0.76 kcal/mol (Cs conformer being more stable).