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Zhi He  One of the best experts on this subject based on the ideXlab platform.

Sizeextensive quadratic CI methods including quadruple excitations: QCISDTQ and QCISDTQ(6) – On the importance of fourelectron correlation effects
Chemical Physics Letters, 2000CoAuthors: Yuan He, Zhi He, Dieter CremerAbstract:Abstract Sizeextensive Quadratic CI with single (S), double (D), triple (T), and quadruple (Q) excitations (QCISDTQ) and its noniterative extension to a method correct at sixthorder perturbation theory, QCISDTQ(6), are developed and applied to electronic systems, for which full CI (FCI) results are available. It is shown that QCISDTQ results are more accurate than either QCISDT or CCSDT. In particular, QCISDTQ(6) correlation energies differ from FCI correlation energies on the average by just 0.04 mhartree. The improvement is caused preferentially by the balanced addition of connected and disconnected fourelectron correlation effects, which confirms observations that fourelectron correlation effects are important for a balanced description of electron correlation.

application of quadratic ci with singles doubles and triples QCISDT an attractive alternative to ccsdt
International Journal of Quantum Chemistry, 1996CoAuthors: Zhi He, Elfi Kraka, Dieter CremerAbstract:The sizeextensive quadratic CI method with single(s), double(D), and triple (T) excitations, QCISDT, is compared with QCISD, QCISD(T), CCSDTn, and CCSDT. It is shown that QCISDT results are more accurate than are either QCISD or QCISD(T) results. In particular, QCISDT turns out to be more stable than are QCISD and QCISD(T) in cases with considerable multireference character. QCISDT and CCSDT results are of similar accuracy with slight advantages for the former method. Since QCISDT is much easier to implement on a computer than is CCSDT, it is an attractive alternative to CCSDT. © 1996 John Wiley & Sons, Inc.

size extensive QCISDT implementation and application
Chemical Physics Letters, 1994CoAuthors: Dieter Cremer, Zhi HeAbstract:Abstract A sizeextensive quadratic CI method with single (S), double (D), and triple (T) excitations, QCISDT, has been derived by appropriate cancellation of disconnected terms in the CISDT projection equations. Matrix elements of the new QCI method have been evaluated in terms of twoelectron integrals and applied to a number of atoms and small molecules. While QCISDT results are of similar accuracy to CCSDT results, the new method is easier to implement, converges in many cases faster and, thereby, leads to advantages compared to CCSDT.

Sizeextensive QCISDT — implementation and application
Chemical Physics Letters, 1994CoAuthors: Dieter Cremer, Zhi HeAbstract:Abstract A sizeextensive quadratic CI method with single (S), double (D), and triple (T) excitations, QCISDT, has been derived by appropriate cancellation of disconnected terms in the CISDT projection equations. Matrix elements of the new QCI method have been evaluated in terms of twoelectron integrals and applied to a number of atoms and small molecules. While QCISDT results are of similar accuracy to CCSDT results, the new method is easier to implement, converges in many cases faster and, thereby, leads to advantages compared to CCSDT.

Analysis of coupled cluster methods
Theoretica chimica acta, 1994CoAuthors: Dieter Cremer, Zhi HeAbstract:A procedure is developed that leads from CI to sizeextensive CI (ECI) by stepwise cancellation of disconnected terms in the CI equations. The ECI methods thus obtained are identical with the corresponding coupled cluster (CC) methods with the exception of CISD and CISDT, which convert to sizeextensive quadratic CI (QCISD) and ECISDT. The latter method has similar properties as CCSDT, but does not offer any significant timesavings as compared to CCSDT. Therefore, the idea of extending CI methods to sizeextensive CI methods does not lead to a hierarchy of independent CC methods. However, the procedure of obtaining ECI methods lays the basis for deriving QCI methods that are truly sizeextensive. This is accomplished by (a) deleting the first linear term of the p fold CI excitation equations ( p ≥ 3) since this term always represents a disconnected term and (b) including just the connected part of appropriate quadratic correction terms in all but the energy equation. In this way, sizeextensive QCISDT and QCISDTQ are obtained and their properties are discussed in comparison with QCISD(T) and CCSDT.
Dieter Cremer  One of the best experts on this subject based on the ideXlab platform.

Sizeextensive quadratic CI methods including quadruple excitations: QCISDTQ and QCISDTQ(6) – On the importance of fourelectron correlation effects
Chemical Physics Letters, 2000CoAuthors: Yuan He, Zhi He, Dieter CremerAbstract:Abstract Sizeextensive Quadratic CI with single (S), double (D), triple (T), and quadruple (Q) excitations (QCISDTQ) and its noniterative extension to a method correct at sixthorder perturbation theory, QCISDTQ(6), are developed and applied to electronic systems, for which full CI (FCI) results are available. It is shown that QCISDTQ results are more accurate than either QCISDT or CCSDT. In particular, QCISDTQ(6) correlation energies differ from FCI correlation energies on the average by just 0.04 mhartree. The improvement is caused preferentially by the balanced addition of connected and disconnected fourelectron correlation effects, which confirms observations that fourelectron correlation effects are important for a balanced description of electron correlation.

application of quadratic ci with singles doubles and triples QCISDT an attractive alternative to ccsdt
International Journal of Quantum Chemistry, 1996CoAuthors: Zhi He, Elfi Kraka, Dieter CremerAbstract:The sizeextensive quadratic CI method with single(s), double(D), and triple (T) excitations, QCISDT, is compared with QCISD, QCISD(T), CCSDTn, and CCSDT. It is shown that QCISDT results are more accurate than are either QCISD or QCISD(T) results. In particular, QCISDT turns out to be more stable than are QCISD and QCISD(T) in cases with considerable multireference character. QCISDT and CCSDT results are of similar accuracy with slight advantages for the former method. Since QCISDT is much easier to implement on a computer than is CCSDT, it is an attractive alternative to CCSDT. © 1996 John Wiley & Sons, Inc.

size extensive QCISDT implementation and application
Chemical Physics Letters, 1994CoAuthors: Dieter Cremer, Zhi HeAbstract:Abstract A sizeextensive quadratic CI method with single (S), double (D), and triple (T) excitations, QCISDT, has been derived by appropriate cancellation of disconnected terms in the CISDT projection equations. Matrix elements of the new QCI method have been evaluated in terms of twoelectron integrals and applied to a number of atoms and small molecules. While QCISDT results are of similar accuracy to CCSDT results, the new method is easier to implement, converges in many cases faster and, thereby, leads to advantages compared to CCSDT.

Sizeextensive QCISDT — implementation and application
Chemical Physics Letters, 1994CoAuthors: Dieter Cremer, Zhi HeAbstract:Abstract A sizeextensive quadratic CI method with single (S), double (D), and triple (T) excitations, QCISDT, has been derived by appropriate cancellation of disconnected terms in the CISDT projection equations. Matrix elements of the new QCI method have been evaluated in terms of twoelectron integrals and applied to a number of atoms and small molecules. While QCISDT results are of similar accuracy to CCSDT results, the new method is easier to implement, converges in many cases faster and, thereby, leads to advantages compared to CCSDT.

Analysis of coupled cluster methods
Theoretica chimica acta, 1994CoAuthors: Dieter Cremer, Zhi HeAbstract:A procedure is developed that leads from CI to sizeextensive CI (ECI) by stepwise cancellation of disconnected terms in the CI equations. The ECI methods thus obtained are identical with the corresponding coupled cluster (CC) methods with the exception of CISD and CISDT, which convert to sizeextensive quadratic CI (QCISD) and ECISDT. The latter method has similar properties as CCSDT, but does not offer any significant timesavings as compared to CCSDT. Therefore, the idea of extending CI methods to sizeextensive CI methods does not lead to a hierarchy of independent CC methods. However, the procedure of obtaining ECI methods lays the basis for deriving QCI methods that are truly sizeextensive. This is accomplished by (a) deleting the first linear term of the p fold CI excitation equations ( p ≥ 3) since this term always represents a disconnected term and (b) including just the connected part of appropriate quadratic correction terms in all but the energy equation. In this way, sizeextensive QCISDT and QCISDTQ are obtained and their properties are discussed in comparison with QCISD(T) and CCSDT.
Joseph S. Francisco  One of the best experts on this subject based on the ideXlab platform.

role of oh stretch torsion coupling and quantum yield effects in the first oh overtone spectrum of cis cis hoono
Journal of Chemical Physics, 2005CoAuthors: Anne B Mccoy, Joseph S. Francisco, Andrew K Mollner, Mitchio OkumuraAbstract:A joint theoretical and experimental investigation is undertaken to study the effects of OHstretch/HOON torsion coupling and of quantum yield on the previously reported first overtone action spectrum of ciscis HOONO (peroxynitrous acid). The minimum energy path along the HOON dihedral angle is computed at the coupled cluster singles and doubles with perturbative triples level with correlation consistent polarized quadruple zeta basis set, at the structure optimized using the triple zeta basis set (CCSD(T)/ccpVQZ//CCSD(T)/ccpVTZ). The twodimensional ab initio potential energy and dipole moment surfaces for ciscis HOONO are calculated as functions of the HOON torsion and OH bond length about the minimum energy path at the CCSD(T)/ccpVTZ and QCISD/AUGccpVTZ (QCISD—quadratic configuration interaction with single and double excitation and AUGaugmented with diffuse functions) level of theory/basis, respectively. The OHstretch vibration depends strongly on the torsional angle, and the torsional potential possesses a broad shelf at ~90°, the cisperp conformation. The calculated electronic energies and dipoles are fit to simple functional forms and absorption spectra in the region of the OH fundamental and first overtone are calculated from these surfaces. While the experimental and calculated spectra of the OH fundamental band are in good agreement, significant differences in the intensity patterns are observed between the calculated absorption spectrum and the measured action spectrum in the 2nuOH region. These differences are attributed to the fact that several of the experimentally accessible states do not have sufficient energy to dissociate to OH+NO2 and therefore are not detectable in an action spectrum. Scaling of the intensities of transitions to these states, assuming D0=82.0 kJ/mol, is shown to produce a spectrum that is in good agreement with the measured action spectrum. Based on this agreement, we assign two of the features in the spectrum to Delta n=0 transitions (where n is the HOON torsion quantum number) that are blue shifted relative to the origin band, while the large peak near 7000 cm^–1 is assigned to a series of Delta n=+1 transitions, with predominant contributions from torsionally excited states with substantial cisperp character. The direct absorption spectrum of ciscis HOONO (6300–6850 cm^–1) is recorded by cavity ringdown spectroscopy in a discharge flow cell. A single band of HOONO is observed at 6370 cm^–1 and is assigned as the origin of the first OH overtone of ciscis HOONO. These results imply that the origin band is suppressed by over an order of magnitude in the action spectrum, due to a reduced quantum yield. The striking differences between absorption and action spectra are correctly predicted by the calculations.

cis cis and trans perp hoono action spectroscopy and isomerization kinetics
Journal of Chemical Physics, 2004CoAuthors: Sergey A Nizkorodov, Joseph S. Francisco, Mitchio Okumura, C M Roehl, Paul O. WennbergAbstract:The weakly bound HOONO product of the OH + NO_2 + M reaction is studied using the vibrational predissociation that follows excitation of the first OH overtone (2nu1). We observe formation of both ciscis and transperp conformers of HOONO. The transperp HOONO 2nu1 band is observed under thermal (223–238 K) conditions at 6971 cm^(–1). We assign the previously published (warmer temperature) HOONO spectrum to the 2nu1 band at 6365 cm^(–1) and 2nu1containing combination bands of the ciscis conformer of HOONO. The band shape of the transperp HOONO spectrum is in excellent agreement with the predicted rotational contour based on previous experimental and theoretical results, but the apparent origin of the ciscis HOONO spectrum at 6365 cm^(–1) is featureless and significantly broader, suggesting more rapid intramolecular vibrational redistribution or predissociation in the latter isomer. The thermally less stable transperp HOONO isomerizes rapidly to ciscis HOONO with an experimentally determined lifetime of 39 ms at 233 K at 13 hPa (in a buffer gas of predominantly Ar). The temperature dependence of the transperp HOONO lifetime in the range 223–238 K yields an isomerization barrier of 33±12 kJ/mol. New ab initio calculations of the structure and vibrational mode frequencies of the transition state perpperp HOONO are performed using the coupled cluster singles and doubles with perturbative triples [CCSD(T)] model, using a correlation consistent polarized triple zeta basis set (ccpVTZ). The energetics of ciscis, transperp, and perpperp HOONO are also calculated at this level [CCSD(T)/ccpVTZ] and with a quadruple zeta basis set using the structure determined at the triple zeta basis set [CCSD(T)/ccpVQZ//CCSD(T)/ccpVTZ]. These calculations predict that the anti form of perpperp HOONO has an energy of DeltaE0 = 42.4 kJ/mol above transperp HOONO, corresponding to an activation enthalpy of DeltaH 298 [doubledagger]0 = 41.1 kJ/mol. These results are in good agreement with statistical simulations based on a model developed by Golden, Barker, and Lohr. The simulated isomerization rates match the observed decay rates when modeled with a transperp to ciscis HOONO isomerization barrier of 40.8 kJ/mol and a strong collision model. The quantum yield of ciscis HOONO dissociation to OH and NO2 is also calculated as a function of photon excitation energy in the range 3500–7500 cm^(–1), assuming D0 = 83 kJ/mol. The quantum yield is predicted to vary from 0.15 to 1 over the observed spectrum at 298 K, leading to band intensities in the action spectrum that are highly temperature dependent; however, the observed relative band strengths in the ciscis HOONO spectrum do not change substantially with temperature over the range 193–273 K. Semiempirical calculations of the oscillator strengths for 2nu1(ciscis HOONO) and 2nu1(transperp HOONO) are performed using (1) a onedimensional anharmonic model and (2) a Morse oscillator model for the OH stretch, and ab initio dipole moment functions calculated using Becke, Lee, Yang, and Parr density functional theory (B3LYP), MollerPlesset pertubation theory truncated at the second and third order (MP2 and MP3), and quadratic configuration interaction theory using single and double excitations (QCISD). The QCISD level calculated ratio of 2nu1 oscillator strengths of transperp to ciscis HOONO is 3.7:1. The observed intensities indicate that the concentration of transperp HOONO early in the OH + NO2 reaction is significantly greater than predicted by a Boltzmann distribution, consistent with statistical predictions of high initial yields of transperp HOONO from the OH + NO_2 + M reaction. In the atmosphere, transperp HOONO will isomerize nearly instantaneously to ciscis HOONO. Loss of HOONO via photodissociation in the nearIR limits the lifetime of ciscis HOONO during daylight to less than 45 h, other loss mechanisms will reduce the lifetime further.

relative vibrational overtone intensity of cis cis and trans perp peroxynitrous acid
Journal of Chemical Physics, 2004CoAuthors: Jamie Matthews, Amitabha Sinha, Joseph S. FranciscoAbstract:The vibrational overtone spectrum of HOONO is examined in the region of the 2νOH and 3νOH bands using action spectroscopy in conjunction with ab initio intensity calculations. The present measurements indicate that the oscillator strength associated with the higher energy trans–perp conformer of HOONO is stronger relative to the lower energy cis–cis conformer for both these vibrational overtone levels. Ab initio intensity calculations carried out at the QCISD level of theory suggest that this disparity in oscillator strength apparently arises from differences in the second derivative of the transition dipole moment function of the two isomers. The calculations indicate that the oscillator strength for the trans–perp isomer is ∼5.4 times larger than that of the cis–cis isomer for the 2νOH band and ∼2 times larger for 3νOH band. The band positions and intensities predicted by the calculations are used to aid in the assignment of features in the experimental action spectra associated with the OH stretching o...

Relative vibrational overtone intensity of cis–cis and trans–perp peroxynitrous acid
Journal of Chemical Physics, 2004CoAuthors: Jamie Matthews, Amitabha Sinha, Joseph S. FranciscoAbstract:The vibrational overtone spectrum of HOONO is examined in the region of the 2νOH and 3νOH bands using action spectroscopy in conjunction with ab initio intensity calculations. The present measurements indicate that the oscillator strength associated with the higher energy trans–perp conformer of HOONO is stronger relative to the lower energy cis–cis conformer for both these vibrational overtone levels. Ab initio intensity calculations carried out at the QCISD level of theory suggest that this disparity in oscillator strength apparently arises from differences in the second derivative of the transition dipole moment function of the two isomers. The calculations indicate that the oscillator strength for the trans–perp isomer is ∼5.4 times larger than that of the cis–cis isomer for the 2νOH band and ∼2 times larger for 3νOH band. The band positions and intensities predicted by the calculations are used to aid in the assignment of features in the experimental action spectra associated with the OH stretching o...

bimolecular reaction of molecular oxygen with overtone excited hooh implications for recycling ho2 in the atmosphere
Physical Chemistry Chemical Physics, 2003CoAuthors: D J Donaldson, Joseph S. FranciscoAbstract:We report ab initio calculations of important regions of the potential energy surface for the reaction: O2(3Σ−g) + HOOH → 2 HO2. The reaction is endothermic by 37.5 ± 3 kcal mol−1, as calculated at the QCISD(T)/6311++G(3df,3pd)//QCISD/6311G(2df,2p) level of theory. The predicted mechanism is via a simple hydrogen abstraction, with a barrier of 35.3 kcal mol−1, followed by formation of a sixmembered complex, which breaks up into two HO2 radicals. If an OH stretching type local vibrational mode of the hydrogen peroxide molecule is excited to the overtone level vOH = 4, the reaction becomes energetically feasible. This overtonedriven reaction will occur in the atmosphere; we estimate the amount of “extra” HOx production by this mechanism to be about 20–30% of the production by H2O2 ultraviolet absorption when the sun is low in the sky, and about 1% when the sun is overhead.
Kaito Takahashi  One of the best experts on this subject based on the ideXlab platform.

temperature dependent rate coefficient for the reaction of ch3sh with the simplest criegee intermediate
Journal of Physical Chemistry A, 2019CoAuthors: Kaito Takahashi, Yulin Li, Cheyu Chiang, Yuanpin ChangAbstract:The kinetics of the reaction of the simplest Criegee intermediate CH2OO with CH3SH was measured with transient IR absorption spectroscopy in a temperaturecontrolled flow reaction cell, and the bimolecular rate coefficients were measured from 278 to 349 K and at total pressure from 10 to 300 Torr. The measured bimolecular rate coefficient at 298 K and 300 Torr is (1.01 ± 0.17) × 10–12 cm3 s–1. The results exhibit a weak negative temperature dependence: the activation energy Ea (k = Ae–Ea/RT) is −1.83 ± 0.05 kcal mol–1, measured at 30 and 100 Torr. Quantum chemistry calculations of the reaction rate coefficient at the QCISD(T)/CBS//B3LYP/6311+G(2d,2p) level (1.6 × 10–12 cm3 s–1 at 298 K; Ea = – 2.80 kcal mol–1) are in reasonable agreement with the experimental results. The experimental and theoretical results of the reaction of CH2OO with CH3SH are compared to the reactions of CH2OO with methanol and hydrogen sulfide, and the trends in reactivity are discussed. The results of the present work indicat...

kinetics of the reaction of the simplest criegee intermediate with ammonia a combination of experiment and theory
Physical Chemistry Chemical Physics, 2018CoAuthors: Mica C Smith, Kaito Takahashi, Maodu Chen, Xiaohu Zhou, Chunlei Xiao, Wenrui Dong, Xueming YangAbstract:The kinetics of the reaction of the simplest Criegee intermediate (CH2OO) with ammonia has been measured under pseudofirstorder conditions with two different experimental methods. We investigated the rate coefficients at 283, 298, 308, and 318 K at a pressure of 50 Torr using an OH laserinduced fluorescence (LIF) method. Weak temperature dependence of the rate coefficient was observed, which is consistent with the theoretical activation energy of −0.53 kcal mol−1 predicted by quantum chemistry calculation at the QCISD(T)/CBS//B3LYP/6311+G(2d,2p) level. At 298 K, the rate coefficient at 50 Torr from the OH LIF experiment was (5.64 ± 0.56) × 10−14 cm3 molecule−1 s−1 while at 100 Torr we obtained a slightly larger value of (8.1 ± 1.0) × 10−14 cm3 molecule−1 s−1 using the UV transient absorption method. These experimental values are within the theoretical error bars of the present as well as previous theoretical results. Our experimental results confirmed the previous conclusion that ammonia is negligible in the consumption of CH2OO in the atmosphere. We also note that CH2OO may compete with OH in the oxidation of ammonia under certain circumstances, such as at nighttime, high altitude and winter time.

effect of unsaturated substituents in the reaction of criegee intermediates with water vapor
Physical Chemistry Chemical Physics, 2018CoAuthors: Kaito TakahashiAbstract:Criegee intermediates (CIs), formed in the reactions of unsaturated hydrocarbons with ozone, are very reactive carbonyl oxides and have recently been suggested as important oxidants in the atmosphere. In this work, we studied the substituent effect on the water monomer and dimer reaction with CIs which include up to three carbon atoms at the QCISD(T)/CBS//B3LYP/6311+G(2d,2p) level. Our calculation showed that for saturated CIs with a hydrogen atom on the same side as the terminal oxygen atom, the reaction with water vapor would likely dominate the removal processes of these CIs in the atmosphere. On the other hand, for unsaturated CIs, the reactivity toward water vapor decreases compared to the saturated species allowing them to survive in humid atmospheric environments. We also evaluated the kinetic isotope effect in the reaction between CI and water vapor by performing calculations with deuterated water. We found that tunneling is not important and the kinetic isotope effect mainly comes from the difference in the zero point energy between water and deuterated water.

how does substitution affect the unimolecular reaction rates of criegee intermediates
Physical Chemistry Chemical Physics, 2017CoAuthors: Kaito TakahashiAbstract:To gain an understanding of the substitution effect on the unimolecular reaction rate coefficients for Criegee intermediates (CIs), we performed ab initio calculations for CH2OO, CH3CHOO, (CH3)2COO, CH3CH2CHOO, CH2CHCHOO and CHCCHOO. The energies of the CIs, products and transition states were calculated with QCISD(T)/CBS//B3LYP/6311+G(2d,2p), while the rate coefficients were calculated with anharmonic vibrational correction by using second order vibrational perturbation theory. It was found that for single bonded substitutions, the hydrogen transfer reaction dominates for the synconformers, while the OO bending reaction dominates for the anticonformers. However once a double bond or a triple bond is added, the OO bending reaction dominates for both syn and anticonformers. The rate coefficients for OO bending reaction show a significant increase when adding a methyl group or ethyl group. On the other hand, the addition of unsaturated vinyl and acetylene groups usually results in a slower thermal decomposition compared to the substitution with saturated carbon groups. Interestingly, for syn_SynCH2CHCHOO, a special five member ring closure reaction forming dioxole was calculated to have an extremely fast rate coefficient of 9312 s−1 at room temperature.

theoretical calculation of the oh vibrational overtone spectra of 1 n alkane diols n 2 4 origin of disappearing hydrogen bonded oh peak
Journal of Physical Chemistry A, 2011CoAuthors: Yulung Cheng, Huiyi Chen, Kaito TakahashiAbstract:In this theoretical study, we simulated the vibrational overtone spectrum of ethylene glycol (EG), 1–3 propanediol (PD), and 1–4 butanediol (BD). Using the local mode model along with the potential energy curve and dipole moment function calculated by B3LYP/631+G(d,p) and QCISD/6311++G(3df,3pd), we obtained the theoretical peak position and integrated absorption coefficient. Furthermore, the vibrational spectra was simulated using a Voigt function using homogeneous and inhomogenous width obtained from quantum chemical calculation methods. Previously, Howard and Kjaergaard recorded the second and third overtone photoacoustic spectra of the three aforementioned alkane diols in the gas phase and observed that the intramolecular hydrogen bonded OH peak becomes difficult to observe as the intramolecular hydrogen bonding strength increased, that is, as the chain length was increased. In this paper we show that the disappearance of the hydrogenbonded OH peak for the OH stretching overtone excitation for BD is...
J S Francisco  One of the best experts on this subject based on the ideXlab platform.

theoretical studies of the structure and thermochemistry of fo2 radical comparison of mo ller plesset perturbation complete active space self consistent field and quadratic configuration interaction methods
Journal of Chemical Physics, 1992CoAuthors: J S Francisco, William A. Lester, Yao Zhao, Ian H. WilliamsAbstract:The structure of FO2 has been calculated for the X 2A‘ ground state using Mo/ller–Plesset (MP) perturbation, complete‐active‐space self‐consistent‐field (CASSCF), and quadratic configuration interaction (QCI) ab initio molecular orbital methods. Basis sets with polarization and diffuse functions were used. Compared with the experimental structure, bond lengths obtained with MP perturbation methods are found to be consistently too short. CASSCF calculations yield a structure which varies considerably with the size of the active space and basis set used. Calculations using the single‐configuration‐based QCI in the single and double‐space with perturbative inclusion of triple substitutions, denoted by QCISD (T), yield structures very close to the experimental structure of FO2. The thermochemistry of FO2 radical has been calculated using the MP, QCI, and gaussian‐1 (G1) methods. The QCI method using isodesmic and isogyric schemes has predicted the heat of formation for FO2 at 0 K to be 8.9±3 kcal mol−1.