Sets Of Polarization Functions Research Articles

Air-Stable Dy(III)-Macrocycle Enantiomers: From Chiral to Polar Space Group

Air-Stable Dy(III)-Macrocycle Enantiomers: From Chiral to Polar Space Group

A comparison between plane wave and Gaussian-type orbital basis sets for hydrogen bonded systems: Formic acid as a test case

The formic acid molecule, its dimers, and its molecular crystal are adopted as test systems to compare results obtained with plane wave (PW) basis sets and norm-conserving pseudopotentials to all-electron Gaussian-type orbital (GTO) calculations. The CPMD and CRYSTAL06 codes, respectively, are applied with the PBE, PW91, and BLYP density functionals. Hydrogen bonding is the leading interaction in the dimers and the crystal. In the latter, dispersive and weak C-H...O interactions are also relevant. Irrespective of the adopted functional, for all considered structures PW and GTO results converge smoothly as a function of the quality of the adopted basis sets to the same values for structures, energies of interaction, and harmonic vibrational features. To achieve a high level of mutual agreement the use of GTO basis sets of at least of triple-zeta quality including one set of polarization functions and PW basis sets with a kinetic energy cutoff higher than 110 Ry is recommended. Pros and cons of both approaches for studying molecular crystals are also discussed.

Tetra-hydrides of the third-row transition elements: spin–orbit coupling effects on geometrical deformation in WH4 and OsH4

The dissociation energies of MH4 (M = La, Hf–Hg) were computed using full optimized reaction space (FORS) multi-configuration self-consistent field (MCSCF) and second-order multi-reference Moller–Plesset perturbation methods with the SBKJC basis sets augmented by a set of polarization functions (SBKJC(f,p)). It was shown that of the molecules examined, only four tetra-hydrides HfH4, TaH4, WH4, and OsH4 with Td symmetry are lower in energy than the corresponding dissociation limits. For WH4 and OsH4, the potential energy surfaces from the D4h to the Td structure were explored from both theoretical calculations and symmetry arguments based on the pseudo-Jahn- Teller effect. As for WH4, it is found that the ground state could be 3Eg, 3A2g, or 3B2g at the D4h structure. The present calculations suggest that the ground state is 3Eg, and that this state is stabilized by the eu deformation into a C2v structure (3B1) and then sequentially to the most stable Td structure (3A2). If the molecular system is promoted to the lowest 3B2g state, the D4h structure can directly deform into the most stable Td structure along the b2u vibrational mode. For OsH4, the ground state (5B1g) at the D4h structure deforms into a D2d structure and the resulting 5B2 state strongly interacts with the lowest 3E and 1A1 states due to the spin-orbit couplings (SOCs). As a result, it was shown that the relativistic potential energy of the lowest spin-mixed state (ground state) monotonically decreases along the D2d deformation path from the D4h to the Td structure.

A vibrational circular dichroism implementation within a Slater-type-orbital based density functional framework and its application to hexa- and hepta-helicenes

We describe the implementation of the rotational strengths for vibrational circular dichroism (VCD) in the Slater-type orbital based Amsterdam Density Functional (ADF) package. We show that our implementation, which makes use of analytical derivative techniques and London atomic orbitals, yields origin independent rotational strengths. The basis set dependence in the particular case of Slater-type basis functions is also discussed. It turns out that the triple zeta STO basis sets with one set of polarization functions (TZP) are adequate for VCD calculations. The origin- dependence of the atomic axial tensors is checked by a distributed origin gauge implementation. The distributed and common origin gauge implementations yield virtually identical atomic axial tensors with the Slater-type basis sets employed here, proving that our implementation yields origin independent rotational strengths. We verify the implementation for a set of benchmark molecules, for which the dependence of the VCD spectra on the particular choice of the exchange–correlation functional is studied. The pure functionals BP86 and OLYP show a particularly good performance. Then, we apply this approach to study the VCD spectra of hexa- and hepta- helicenes. In particular we focus on relationships between the sign of the rotational strengths of the two helicenes.

Dissociation Potential Curves of Low-Lying States in Transition Metal Hydrides. 3. Hydrides of Groups 6 and 7

The dissociation curves of low-lying spin-mixed states in monohydrides of groups 6 and 7 were calculated by using an effective core potential (ECP) approach. This approach is based on the multiconfiguration self-consistent field (MCSCF) method, followed by first-order configuration interaction (FOCI) calculations, in which the method employs an ECP basis set proposed by Stevens and co-workers (SBKJC) augmented by a set of polarization functions. Spin-orbit coupling (SOC) effects are estimated within the one-electron approximation by using effective nuclear charges, since SOC splittings obtained with the full Breit-Pauli Hamitonian are underestimated when ECP basis sets are used. The ground states of group 6 hydrides have Omega = (1)/(2)(X(6)Sigma(+)(1/2)), where Omega is the z component of the total angular momentum quantum number. Although the ground states of group 7 hydrides have Omega = 0(+), their main adiabatic components are different; the ground state in MnH originates from the lowest (7)Sigma(+), while in TcH and ReH the main component of the ground state is the lowest (5)Sigma(+). The present paper reports a comprehensive set of theoretical results including the dissociation energies, equilibrium distances, electronic transition energies, harmonic frequencies, anharmonicities, and rotational constants for several low-lying spin-mixed states in these hydrides. Transition dipole moments were also computed among the spin-mixed states and large peak positions of electronic transitions are suggested theoretically for these hydrides. The periodic trends of physical properties of metal hydrides are discussed, based on the results reported in this and other recent studies.

Density-Functional-Theory Studies of the Infrared Spectra of Titanium Carbide Nanocrystals

The infrared (IR) spectra of cuboidic titanium carbide (TiC) nanocrystals have been studied at the density-functional-theory (DFT) level using the Becke-Perdew (BP) functional and triple-zeta quality basis sets augmented by one set of polarization functions (TZVP). The accuracy of the calculations was checked by DFT calculations using the Perdew-Burke-Ernzerhof hybrid functional (PBE0) and up to quadruple-zeta quality basis sets augmented by one set of polarization functions (QZVP). The calculated IR spectrum for Ti(14)C(13) (3 x 3 x 3) is found to be in fair agreement with the experimental IR spectrum obtained by infrared resonance-enhanced multiphoton ionization (IR-REMPI) measurements, whereas, for Ti(18)C(18) (4 x 3 x 3) and Ti(32)C(32) (4 x 4 x 4), the calculated IR spectra differ significantly from the experimental ones. The smallest TiC cluster (Ti(4)C(4), 2 x 2 x 2) considered has not been reported in any mass-spectrometer studies. The present DFT calculations show that the vibrational modes related to the in-plane vibrations of solid TiC are not observed in the IR-REMPI spectra of nanocrystals larger than Ti(14)C(13). Contrary to solid TiC, the studied TiC nanocrystals are nonmetallic with optical gaps of 0.62 eV (0.55 eV) and 0.028 eV (0.027 eV) for Ti(32)C(32) and Ti(108)C(108) (6 x 6 x 6), calculated at the time-dependent density-functional-theory (TDDFT) level using the BP functional. The HOMO-LUMO gaps obtained in the BP DFT calculations are given within parentheses. At the PBE0 DFT level, the HOMO-LUMO gaps for Ti(32)C(32) and Ti(108)C(108) are 1.74 and 0.32 eV, respectively.

Calculation of the vibration frequencies of α‐quartz: The effect of Hamiltonian and basis set

The central-zone vibrational spectrum of alpha-quartz (SiO2) is calculated by building the Hessian matrix numerically from the analytical gradients of the energy with respect to the atomic coordinates. The nonanalytical part is obtained with a finite field supercell approach for the high-frequency dielectric constant and a Wannier function scheme for the evaluation of Born charges. The results obtained with four different Hamiltonians, namely Hartree-Fock, DFT in its local (LDA) and nonlocal gradient corrected (PBE) approximation, and hybrid B3LYP, are discussed, showing that B3LYP performs far better than LDA and PBE, which in turn provide better results than HF, as the mean absolute difference from experimental frequencies is 6, 18, 21, and 44 cm(-1), respectively, when a split valence basis set containing two sets of polarization functions is used. For the LDA results, comparison is possible with previous calculations based on the Density Functional Perturbation Theory and usage of a plane-wave basis set. The effects associated with the use of basis sets of increasing size are also investigated. It turns out that a split valence plus a single set of d polarization functions provides frequencies that differ from the ones obtained with a double set of d functions and a set of f functions on all atoms by on average less than 5 cm(-1).

Dissociation Potential Curves of Low-Lying States in Transition Metal Hydrides. 2. Hydrides of Groups 3 and 5

The dissociation energy curves of low-lying spin-mixed states for Group 5 hydrides (VH, NbH, and TaH), as well as Group 3 hydrides (ScH, YH, and LaH), have been calculated by using both effective core potential (ECP) and all-electron (AE) approaches. The two approaches are based on the multiconfiguration self-consistent field (MCSCF) method, followed by second-order configuration interaction (SOCI) calculations: the first method employs an ECP basis set proposed by Stevens and co-workers (SBKJC) augmented by a set of polarization functions, and spin−orbit coupling effects are estimated with a one-electron approximation, using effective nuclear charges. The second method employs a double-ζ basis set developed by Huzinaga (MIDI) and three sets of p functions are added to both transition element and hydrogen and one set of f functions is also added to the transition element. The relativistic elimination of small components (RESC) scheme and full Breit−Pauli Hamiltonian are employed in the AE approaches to incorporate relativistic effects. The present paper reports a comprehensive set of theoretical results including the dissociation energies, equilibrium distances, electronic transition energies, harmonic frequencies, anharmonicities, and rotational constants for several low-lying spin-mixed states in the hydrides, filling a considerable gap in available data for these molecules. Transition moments are also computed among the spin-mixed states, and qualitative agreement is obtained for Group 3 hydrides in comparison with the experimental results reported by Ram and Bernath. Peak positions of emission spectra in Group 5 hydrides are also predicted.

Nucleus independent chemical shift evaluation of the aromaticity of pentafulvene and its exocyclic Si, Ge, and Sn derivatives

Nucleus independent chemical shift (NICS) values, the negative isotropic shielding calculated at the ring centers, were used to evaluate the aromatic character of a series of substituted pentafulvene analogs. NICSs were calculated for pentafulvene 3a, its exocyclic Si, Ge, and Sn analogs 3b– d, and their 7,8-difluoro derivatives 4a– d. Calculations on cyclopentadiene, 1, and the aromatic cyclopentadienyl anion, 2, were also carried out for comparison. Geometries were optimized using the B3-LYP method while NICS values were calculated at both the HF and B3-LYP levels using the gauge invariant atomic orbital (GIAO) approach. All calculations used a triple split basis set with two sets of polarization functions (tsV+2P). Calculated NICS values suggest 3a– d and 4a exhibit little or no aromaticity. However, the 7,8-difluorosilyl- and 7,8-difluorogermyl derivatives ( 4b and 4c) appear to exhibit a degree of aromaticity, which might be due to π-donation from fluorine to silicon (germanium) permitting some net π-donation from Si and Ge to the cyclopentadiene ring. The SnF 2 derivative 4d, in contrast, does not appear to exhibit aromaticity.

Calculations for XeOn (n = 2−4): Could the Xenon Dioxide Molecule Exist?

the inclusion of core electrons increases the size of the computational system without significant increase in accuracy. The Nicklass basis set for xenon was augmented with three d and one f set of polarization functions, with the exponents 0.4460, 0.2322, and 0.1208 for the d and 0.5157 for f, respectively. 12 For the oxygen atoms, the 6-311G* and the aug- ccPVTZ basis sets were tried, and the latter gave much better accuracy for heats of formation of XeO3 and XeO4. Because of computational limitations, the geometries and vibrational fre- quencies of XeO3 and XeO4 were calculated using the 6-311G* basis set, followed by single-point calculations with the aug- ccPVTZ basis to obtain more reliable energetic values for the tri- and tetroxide.

Ab Initio Molecular Orbital Study of XO2+(X = F, Cl, Br, I) Systems

The depletion of stratospheric ozone has resulted in an increasing interest in the study of the possible reaction mechanisms responsible for its depletion. The structures and relative stabilities of the cationic forms of the halogen dioxides have been studied by means of ab initio molecular orbital calculations. For fluorine- and chlorine-containing compounds the geometries and the harmonic vibrational frequencies of all possible isomers were calculated at the QCISD/6-311+G(2d) level of theory. For bromine- and iodine-containing compounds the effective core-potential basis sets of Hay and Wadt, modified to include a set of diffuse functions and two sets of polarization functions, were employed. For all systems the final energies were obtained at the QCISD(T)/6-311+G(3df) level of theory. In addition, multiconfiguration-based methods have also been used. The relative stabilities of structures XOO{sup +} and OXO{sup +} are greatly reduced relative to those observed for the corresponding neutral species. In fact, for Cl and I derivatives, the lowest energy isomer corresponds to the symmetric OXO{sup +} open-chain species. The corresponding cyclic structures arise as local minima on the respective potential energy surfaces, but they lie much higher in energy than the OXO{sup +} open-chain form or the XOO{sup +} isomer. There are significant differencesmore » in bonding between XOO{sup +} and OXO{sup +}, the X-O interaction in OXO{sup +} being more covalent than in XOO{sup +}. There are also trends along the series that reflect the pronounced disparity between the electron affinity of F{sup +} and those of the heavier atoms of the group. FOO{sup +} species can be viewed as F({sup 2}P)-O{sub 2}{sup +} complexes, whereas XOO{sup +}(X = Br, I) species can be regarded as X{sup +}({sup 3}P)-O{sub 2} complexes. The OXO{sup +} open-chain species have an electron charge distribution similar to that of the ozone molecule, reflecting the same number of valence electrons in each case.« less

X̃ 3B1, ã 1A1, b̃ 1B1, and c̃ 1 Electronic States of

Ab initio molecular electronic structure theory has been employed in order to investigate systematically the X̃ 3B1, ã 1A1, b̃ 1B1, and c̃ 1 states of N , with emphasis placed on the b̃ 1B1 and c̃ 1 states. The self-consistent-field (SCF), configuration interaction with single and double excitations (CISD), complete active space (CAS) SCF, and CASSCF second-order configuration interaction (SOCI) wave functions with nine basis sets, the largest being a triple-ζ basis set with three sets of polarization functions and two additional sets of higher angular momentum and diffuse functions [TZ3P(2f,2d)+2diff], were used to determine equilibrium geometries, harmonic vibrational frequencies, infrared (IR) intensities, and dipole moments. The ground, first, and second excited states are confirmed to be bent, while the third excited state is predicted to be linear. The bond angles of N are shown to be larger than those of the corresponding isoelectronic CH2 molecule. At the highest level of theory, TZ3P(2f,2d)+2diff CASSCF-SOCI, the triplet−singlet splitting is predicted to be 29.4 kcal/mol (1.28 eV, 10 300 cm-1), which is in good agreement with the experimental observation of 30.1 kcal/mol (1.305 eV, 10 530 cm-1). With the same method, the second excited state (b̃ 1B1) lies 43.7 kcal/mol (1.89 eV, 15 300 cm-1) above the ground state, which is significantly lower than the experimentally proposed value of 2.54 eV. The third excited state (c̃ 1 ) is predicted to lie 77.0 kcal/mol (3.34 eV, 26 900 cm-1) above the ground state. The equilibrium geometry of this c̃ 1 state is determined to be re = 1.030 Å at the TCSCF-CISD level with the largest basis set. Since the IR intensities of all active vibrational modes are predicted to be substantial, IR spectroscopic studies of the four states are feasible. However, of the six fundamentals experimentally assigned to date, two appear to be incorrect. The energy separations among the four lowest-lying states of N are found to be larger than the corresponding states of CH2.

Nuclear quadrupole coupling constants in alkali halide molecules: an ab initio quantum chemical study

The electric field gradients, and hence nuclear quadrupole coupling constants, plus the dipole moments of the alkali halide molecules LiF, LiCl, LiBr, NaF, NaCl, NaBr, KF, KCl and KBr have been calculated using ab initio quantum chemical methods that range from SCF to coupled cluster techniques with perturbative corrections for triple excitations [CCSD(T)], employing basis sets of quadruple zeta quality with multiple sets of polarization functions. Zero-level vibrational corrections to these quantities and spectroscopic constants also were computed. The level of agreement with experimental data is generally good, especially in the case of the light molecules. The field gradients at the Na and Cl nuclei in NaCl were analysed using the constrained spatial orbital variation technique in order to quantify the importance of polarization, charge transfer and Pauli repulsion contributions, and to understand why a simple electrostatic interpretation of the field gradients fails in the case of alkali halides. The dipole polarizabilities, Sternheimer antishielding constants γ∞ and electric field gradient–electric field hyperpolarizabilities ϵ of the alkali and halide ions also were calculated at a correlated level of theory (ACPF).

Molecular Structure of the GeH2···OH2 Complex

Hartree−Fock and post-Hartree−Fock (density functional DFT, many-body perturbation theory MP2 and MP4(SDTQ), and coupled-cluster CCSD, and CCSD[T]) quantum-mechanical methods were applied to establish molecular structures and interaction energies of the GeH2···OH2 complex. Molecular geometries of nonplanar and planar conformers were optimized at the Hartree−Fock, DFT, and MP2 levels of theory using triple-ζ-quality (TZP) augmented by sets of polarization functions (TZP(2d,2p) and TZP(2df,2pd)) basis sets. The optimized nonplanar structures correspond to minimum-energy species at all applied levels of theory. Calculated interaction energies (corrected for the basis set superposition error, CP) are relatively large and amount to −9.43 kcal/mol (MP4(CP)(SDTQ)/TZP(2df,2pd)//MP2/TZP(2df,2pd)), −9.77 kcal/mol (DFT(CP)/TZP(2df,2pd)//DFT/TZP(2df,2pd)), −8.16 kcal/mol (CCSD(CP)/TZP(2df,2pd)//MP2/TZP(2df,2pd)), and −8.90 kcal/mol at the CCSD[T](CP)/TZP(2df,2pd) levels. The optimized planar molecular structure of the studied complex (MP2/TZP(2df,2pd) level) corresponds to the transition-state form (one imaginary, 66i cm-1 harmonic vibrational frequency). A repulsive (2.07 kcal/mol) but minimum-energy planar structure of GeH2···OH2 is predicted at the DFT/TZP(2df,2pd) level of approximation. However, single-point calculations at the planar DFT-optimized geometry reveal this complex to be slightly nonbounded (0.01 kcal/mol, CCSD(CP)) or very weakly bounded (−0.17 kcal/mol, CCSD[T](CP)).

Structure, energetics and vibrational spectra of dimers, trimers, and tetramers of HX (X = Cl, Br, I)

The title complexes are studied by correlated ab initio methods using a pseudopotential double-ζ basis set, augmented by diffuse sp and two sets of polarization functions. The binding energies of the complexes decrease in the order HCl > HBr > HI. In the mixed HX…HX' dimers, the nature of the proton-donor molecule is more important than is the proton-acceptor with respect to the strength of the interaction. Only one minimum is found on the potential energy surface of the trimers and tetramers, which corresponds to the C nh cyclic structure. Enlargement of the complex leads to progressively greater individual H-bond energy and HX bond stretch, coupled with reduced intermolecular separation and smaller nonlinearity of each H-bond. Electron correlation makes a larger contribution as the atomic number of X increases. The highest degree of cooperativity is noted for oligomers of HCl and HBr, as compared to HI. The nonadditivity is dominated by terms present at the SCF level. The vibrational frequencies exhibit trends that generally parallel the energetics and geometry patterns, particularly the red shifts of the HX stretches and the intermolecular modes.

1,3,5-Trisilabenzene: has it been synthesized?

The molecules 1,3,5-trisilabenzene and 1,3,5-trisilylene cyclohexane are two isomers of Si 3C 3H 6 which are possible products of the triple-dehydrogenation reaction of 1,3,5-trisilacyclohexane. In order to determine the geometries and relative energies of these potential products, ab initio quantum mechanical techniques have been employed on planar I,3,5-trisilabenzene and the planar, boat and chair conformations of 1,3,5-trisilylene cyclohexane. Basis sets as large as triple-ζ plus two sets of polarization functions (TZ2P) have been utilized for all of the molecules with the Hartree-Fock self-consistent field method, while a double-ζ plus polarization (DZP) basis set has been utilized with correlated methods as sophisticated as the coupled cluster approach including all single, double, and perturbative triple excitations (CCSD(T)). The molecules 1,3,5-trisilabenzene and the chair and boat conformations of 1,3,5-trisilylene cyclohexane have been characterized as minima by the evaluation of their vibrational frequencies. Of these the trisilabenzene is the most stable, with the chair and boat conformations of the silylene lying 35.0 and 36.5 kcal mol −1 higher in energy with the DZP CCSD(T) level of theory at the singles and doubles excitation configuration interaction (CISD) optimized geometry points and including zero-point vibrational energies. In a further attempt to measure quantitatively the aromatic stabilization of 1,3,5-trisilabenzene, the heat of bond separation for this compound was determined. Results show 1,3,5-trisilabenzene to have a bond separation energy of 53.1 kcal mol −1, which is 83% of that of benzene at the best level of theory.

The induced polarization of the water molecule

The molecular polarization potential map (MPP) of the water molecule computed at the SCF level with a double zeta basis set with two sets of polarization functions was used as reference to assess the accuracy of the induced polarization description of various force fields. The MPP was computed on grids of points defined on shells ranging from 1.0 to 4.0 times the van der Waals radius. The comparison between the reference MPP and the one computed with different parameterizations shows large average deviations for some of them, while others exhibit similar behaviour to that of the ab initio map. The MPP has been used to derive atom polarizabilities by a least-squares fitting. Two proposed sets of parameters were developed. First, using all the points calculated, the procedure yields 0.911 Å 3 for a single polarizability centered on the oxygen atom and 0.702 and 0.106 Å 3 for a two-parameter fit centered on the oxygen and hydrogens, respectively. Second, if only layers between 1.2 and 4.0 times the van der Waals radius are considered, the one-parameter fit yields a polarizability of 1.041 Å 3, whereas the two-parameter fit yields 0.817 and 0.112 Å 3 for the oxygen and hydrogen.

Electronic structure calculations of 1,3‐dipolar cycloadditions using density functional and Hartree–Fock methods

AbstractLocal spin density (LSD) methods were used to study the concerted 1,3‐dipolar cycloadditions for fulminic acid plus acetylene, fulminic acid plus ethylene, and nitrone plus ethylene. Cartesian Gaussian double‐zeta split‐valence basis sets augmented with one set of polarization functions (DZVP) were used for the LSD calculations. The LSD calculations were performed with the LSD exchange functional (Dirac) and with the Vosko, Wilk, and Nusair correlation energy functional (VWN). Nonlocal spin‐density corrections (NLSD) were estimated with the exchange functional of Becke and the correlation energy functional of Perdew (VWN + BP) and Becke, and the correlation energy functional of Lee, Yang, and Parr (B‐LYP). Vibrational frequencies were computed at the VWN and B‐LYP levels by numerical differentiation of the analytical first derivatives of the energy. Each of these reactions was examined using Hartree–Fock and Møller–Plesset perturbation theory for comparison. Geometry optimizations were carried out at the Hartree–Fock level with the 6‐311G(d,p) basis set, and correlation energies were computed up to the MP4SDTQ/6‐311G(d,p) level of theory. For the reactions of fulminic acid plus acetylene, fulminic acid plus ethylene, and nitrone plus ethylene, our best estimated density functional barrier heights are 7.8 ± 1.5, 8.9 ± 0.3, and 11.05 ± 1.9 kcal/mol, respectively. These results are in reasonable agreement with the correlated wave‐function calculations and experimental estimates. © 1994 John Wiley & Sons, Inc.

Large basis set calculations using Brueckner theory

Calculations are reported using the Brueckner doubles method, with and without a perturbative estimate of the effects of triple excitations, and using basis sets which are at least triple zeta with two sets of polarization functions in quality. Equilibrium geometries and harmonic vibrational frequencies are calculated for HF, N2, H2O, NH3, CH4, HCN, H2CO, NNO, and O3. Comparison with experimental data confirm the high accuracy of Brueckner methods.

The vibrational spectrum of H2O3: An ab initio investigation

Theoretically determined frequencies and absorption intensities are reported for the vibrational spectra of the covalent HOOOH and hydrogen bonded HO...HOO intermediates that may form in the reaction of the hydroxyl and hydroperoxyl radicals. Basis sets of double-zeta-polarized quality, augmented by diffuse and second sets of polarization functions have been used with complete-active-space, self-consistent-field wave functions. The calculated harmonic vibrational frequencies of HOOOH have been corrected with empirical factors and presented in the form of a ‘‘stick’’ spectrum. The oxygen backbone vibrations, predicted to occur at 519, 760, and 870 cm−1, are well separated from most interferences, and may be the most useful for the species’ identification. In the case of the hydrogen bonded isomer, emphasis has been placed upon prediction of shifts in the intramolecular vibrational frequencies that take place upon formation of the complex. In particular, the HO stretch and HOO bend of HO2 are predicted to have shifts of −59 and 53 cm−1, respectively, which should facilitate their identification. It is also noted that the antisymmetric stretching frequency of the oxygen backbone in HOOOH exhibits a strong sensitivity to the degree of electron correlation, similar to that which has been previously observed for ozone.