Large Atomic Natural Orbital Basis Research Articles

Examining the ground and first excited states of methyl peroxy radical with high-level coupled-cluster theory

Peroxy radicals (RO2) are intermediates in fuel combustion, where they engage in efficiency-limiting autoignition reactions. They also participate in atmospheric chemistry leading to the formation of unwanted tropospheric ozone. Advances in spectroscopic techniques have allowed for the possibility of employing the lowest () electronic transition of RO2 as a tool to selectively monitor these species, enabling accurate kinetic values to be obtained. Herein, high-level ab initio methods are employed to systematically refine spectroscopic predictions for the methyl peroxy radical (CH3O2), one of the most abundant peroxy radicals in the atmosphere. In particular, vibrationally corrected geometries and anharmonic vibrational frequencies for both the ground () and first excited () state are predicted using coupled-cluster theory with up to perturbative triples [CCSD(T)] and large atomic natural orbital basis sets. Equation-of-motion coupled-cluster theory is utilised to compute vertical transition properties; a radiative lifetime of 4.7 ms is suggested for the excited state. Finally, we predict the adiabatic excitation energy (T0) via systematic extrapolation to the complete basis limit of coupled-cluster with up to full quadruples (CCSDTQ). After accounting for several approximations, and including an anharmonic zero-point vibrational energy correction, we match experiment for this transition to within 9 cm−1.

The valence and Rydberg excited states of CH2: A theoretical exploration

Using the completed active space second-order perturbation (CASPT2) method, valence and Rydberg excited states of CH(2) molecule are probed with the large atomic natural orbital (ANO-L) basis set. Five states are optimized and the geometric parameters are in good agreement with the available data in literatures, furthermore, the state of 2(1)B(1) is obtained for the first time. Valence and Rydberg excited states of CH(2) are also calculated for the vertical transitions with the ANO-L+ basis set that is constructed by adding a set of 1s1p1d Rydberg orbitals into the ANO-L basis set. Two Rydberg states of the p(~3)A(2) and r(~3) B(1) at 9.88 and 10.50 eV are obtained for the first time, and the 3a(1) → 3d(yz) nature of the state p(~3)A(2) and the 3a(1) → d(x2-y2) nature of the state r(~3)B(1) are confirmed.

Low-Energy States of Manganese–Oxo Corrole and Corrolazine: Multiconfiguration Reference ab Initio Calculations

Manganese(V)-oxo corrole and corrolazine have been studied with ab initio multiconfiguration reference methods (CASPT2 and RASPT2) and large atomic natural orbital (ANO) basis sets. The calculations confirm the expected singlet d(δ)(2) ground states for both complexes and rule out excited states within 0.5 eV of the ground states. The lowest excited states are a pair of Mn(V) triplet states with d(δ)(1)d(π)(1) configurations 0.5-0.75 eV above the ground state. Manganese(IV)-oxo macrocycle radical states are much higher in energy, ≥1.0 eV relative to the ground state. The macrocyclic ligands in the ground states of the complexes are thus unambiguously 'innocent'. The approximate similarity of the spin state energetics of the corrole and corrolazine complexes suggests that the latter macrocycle on its own does not afford any special stabilization for the Mn(V)O center. The remarkable stability of an Mn(V)O octaarylcorrolazine thus appears to be ascribable to the steric protection afforded by the β-aryl groups.

A theoretical investigation of the excited states of OCLO radical, cation, and anion using the CASSCF/CASPT2 method

Using the complete active space self-consistent field method with a large atomic natural orbital basis set, 10, 13, and 9 electronic states of the OClO radical, OClO(+) cation, and OClO(-) anion were calculated, respectively. Taking the further correlation effects into account, the second-order perturbation (CASPT2) calculations were carried out for the energetic calibration. The photoelectron spectroscopy of the OClO radical and OClO(-) anion were extensively studied in the both case of the adiabatic and vertical ionization energies. The calculated results presented the relatively complete assignment of the photoelectron bands of the experiments for OClO and its anion. Furthermore, the Rydberg states of the OClO radical were investigated by using multiconfigurational CASPT2 (MS-CASPT2) theory under the basis set of large atomic natural orbital functions augmented with an adapted 1s1p1d Rydberg functions that have specially been built for this study. Sixteen Rydberg states were obtained and the results were consistent with the experimental results.

Theoretical Studies on the Low-Lying Electronic States of the HSO Neutral Radical and Its Cation

Using the complete active space self-consistent field (CASSCF) method with large atomic natural orbital (ANO-L) basis set, four electronic states of the HSO neutral radical are optimized. The vertical transitions of the HSO neutral radical are investigated by using the same method under the basis set of ANO-L functions augmented with a series of adapted 1s1p1d Rydberg functions, through which eight valence states and eight Rydberg states are probed. Ionic states of the HSO neutral radical are extensively studied in both cases of the adiabatic and vertical ionization, from which the relatively complete understanding of ionization energies is given. To include further correlation effects, the second-order perturbation method (CASPT2) is implemented, and the comparison between CASSCF and CASPT2 methods is performed.

The electronic structure of vanadium carbide, VC

Within an energy range of 2.4 eV, we have explored 29 of the 36 states of the diatomic molecule VC that arise from the atoms in their ground state, V(4s23d3;4F)+C(2s2 2p2;3P). We use multireference methods with large atomic natural orbital basis sets. The ground state is of 2Delta symmetry with the first two excited states, 4Delta and 2Sigma+, located 4.2 and 7.0 kcal/mol above the X state. All the states examined in this work are relatively strongly bound and show significant charge transfer from V to C. The binding energy of the X 2Delta state is estimated to be 95.3 kcal/mol in good agreement with the experimental value.

Theoretical Study of Chlorine Nitrates: Implications for Stratospheric Chlorine Chemistry.

Reported here is a theoretical study of possible stratospheric chlorine reservoir species including isomers of chemical formula ClNO(4) and ClNO(5), in addition to the well-known ClONO(2) reservoir species. Density functional theory (DFT) in conjunction with large one-particle basis sets has been used to determine equilibrium structures, dipole moments, rotational constants, harmonic vibrational frequencies, and infrared intensities. The B3LYP functional was used since it has previously been shown to perform well for similar compounds. The equilibrium geometry and vibrational spectra of ClONO(2) are shown to be in good agreement with the experimental data and also with high-level coupled-cluster calculations reported previously. Three stable isomers have been identified for each ClNO(4) and ClNO(5). The vibrational spectrum of O(2)ClONO(2) has been compared with the available experimental data and found to be in good agreement. The relative energetics of the ClNO(4) and ClNO(5) isomers have been determined using large atomic natural orbital (ANO) basis sets in conjunction with the singles and doubles coupled-cluster method that includes a perturbational correction for triple excitations, denoted CCSD(T). Accurate heats of formation have been evaluated by computing energies for isodesmic reactions involving the ClNO(4) and ClNO(5) isomers. The stability of these molecules with respect to thermal dissociation is examined. The present study suggests that isomers of ClNO(4) and ClNO(5) may have no atmospheric chemical relevance because the atmospheric concentrations of the necessary reactants are insufficient, but it is also found that under laboratory conditions the formation of O(2)ClONO(2) cannot be ignored.

Theoretical study of chlorine nitrates: implications for stratospheric chlorine chemistry.

Reported here is a theoretical study of possible stratospheric chlorine reservoir species including isomers of chemical formula ClNO(4) and ClNO(5), in addition to the well-known ClONO(2) reservoir species. Density functional theory (DFT) in conjunction with large one-particle basis sets has been used to determine equilibrium structures, dipole moments, rotational constants, harmonic vibrational frequencies, and infrared intensities. The B3LYP functional was used since it has previously been shown to perform well for similar compounds. The equilibrium geometry and vibrational spectra of ClONO(2) are shown to be in good agreement with the experimental data and also with high-level coupled-cluster calculations reported previously. Three stable isomers have been identified for each ClNO(4) and ClNO(5). The vibrational spectrum of O(2)ClONO(2) has been compared with the available experimental data and found to be in good agreement. The relative energetics of the ClNO(4) and ClNO(5) isomers have been determined using large atomic natural orbital (ANO) basis sets in conjunction with the singles and doubles coupled-cluster method that includes a perturbational correction for triple excitations, denoted CCSD(T). Accurate heats of formation have been evaluated by computing energies for isodesmic reactions involving the ClNO(4) and ClNO(5) isomers. The stability of these molecules with respect to thermal dissociation is examined. The present study suggests that isomers of ClNO(4) and ClNO(5) may have no atmospheric chemical relevance because the atmospheric concentrations of the necessary reactants are insufficient, but it is also found that under laboratory conditions the formation of O(2)ClONO(2) cannot be ignored.

Unitary group based coupled cluster method for open-shell singlets: application to the a 1Δ state of OH +

The fully spin-adapted coupled cluster method with singles and doubles (CCSD) that is based on the unitary group approach (UGA) is applied to the metastable a 1Δ state of the OH + ion. Very few experimental data are available for this state, which is assumed to play a significant role in recent theories of the dissociative recombination of OH + with an electron, which in turn plays an important role in modeling of interstellar clouds and planetary atmospheres. The appropriateness of our approach is first tested using a double-zeta plus polarization (DZP) model of this cation, in which case the comparison with the exact FCI results is possible. The actual calculations are then carried out with a rather large atomic natural orbital (ANO) basis set. An excellent agreement is found between the computed and a few observed quantities, indicating the reliability and the accuracy of other predicted spectroscopic data.

Theoretical study of XONO2 (X=Br, OBr, O2Br): Implications for stratospheric bromine chemistry

The equilibrium structure, dipole moment, harmonic vibrational frequencies, and infrared intensities of XONO2 (X=Br, OBr, O2Br) are determined using density functional theory in conjunction with a TZ2P (triple zeta double polarized) basis set. The B3LYP functional was used since this has previously been shown to perform well for similar bromine compounds. The equilibrium geometry and vibrational spectra of BrONO2 are shown to be in good agreement with the experimental data and also with high-level coupled-cluster calculations. The vibrational spectrum of O2BrONO2 has been compared with that of the chlorine analog, O2ClONO2, for which some experimental data exist. The bonding in OBrONO2 is shown to be more similar to that in BrONO2. Using large atomic natural orbital basis sets, the singles and doubles coupled-cluster method that includes a perturbative correction for triple excitations, denoted CCSD(T), was employed to compute energies for three isodesmic reactions in order to determine heats of formation for OBrONO2 and O2BrONO2. Our best estimates are 36.7 and 38.7 kcal/mol, respectively. Finally, the possible formation of O2BrONO2 in the stratosphere by adduct formation and oxidation of OBrONO2 and the implications for stratospheric bromine chemistry are discussed.

The electronic structure of the low lying sextet and quartet states of CrF and CrCl

The electronic structure of CrF and CrCl in X 6Σ+, 6Π, 6Δ, A6Σ+, 4Σ+, 4Π, and 4Δ states that correlate with the low lying 6S, 6D, and 4D states of Cr+ have been studied, using large atomic natural orbital (ANO) basis sets and a variety of ab initio methods, including multi-reference configuration interaction (MRCI) and coupled cluster with perturbative triples (RCCSD(T)). We include scalar relativistic effects perturbatively and also explore the consequence of correlating the 3s and 3p electrons on the transition metal. We report T e, R e,ωe, as well as dipole moments, bond energies, and charge distributions and compare with the available experimental data as well as previous theoretical results.

Structure and stability of the AlX and AlX− species

The electronic and geometrical structures of the ground and low-lying excited states of the diatomic AlX and AlX− series (X=H, Li, Be, B, C, N, O, and F) are calculated by the coupled-cluster method with all singles and doubles and noniterative inclusion of triples using a large atomic natural orbital basis. All the ground-state AlX molecules except for AlF can attach an additional electron and form ground-state AlX− anions. The ground-state AlBe−, AlB−, AlC−, AlN−, and AlO− anions possess excited states that are stable toward autodetachment of an extra electron; AlBe− also has a second excited state. Low-lying excited states of all AlX but AlN can attach an extra electron and form anionic states that are stable with respect to their neutral (excited) parent states. The ground-state AlLi−, AlBe−, AlB−, AlN−, and AlO− anions are found to be thermodynamically more stable than their neutral parents. The most stable is AlO−, whose dissociation energy to Al+O− is 6.4 eV. Correspondingly, AlO possesses the largest electron affinity (2.65 eV) in the series.

Energies and structures of isomers of Cl 2O 2 calculated by density functional methods

The geometries and the relative energies of three low-lying (ClO) 2 isomers in the ground state have been calculated using density functional theory (DFT) methods. The DFT relative energies using extended basis sets vary by as much as 4.5 cal/mol, depending upon the functionals employed, but differ by less than 2.5 cal/mol from the available CCSD(T) results with large atomic natural orbital basis sets. DFT calculated geometries of the isomers are also in reasonable agreement with experiment. The present study shows that the DFT methods could be used to investigate relative energetics of (ClO) 2 isomers, for which large basis sets are essential even for modest accuracy.

Valence and excited states ofLiH−

Valence and excited dipole-bound states of the ${\mathrm{LiH}}^{\mathrm{\ensuremath{-}}}$ anion are calculated with the recently developed electron-attachment equation-of-motion coupled-cluster technique. It is found that the first dipole-bound state of ${\mathrm{LiH}}^{\mathrm{\ensuremath{-}}}$ corresponds to the second dissociation channel ${\mathrm{LiH}}^{\mathrm{\ensuremath{-}}}{\ensuremath{\rightarrow}\mathrm{Li}}^{\mathrm{\ensuremath{-}}}{(}^{1}S)+\mathrm{H}{(}^{2}S).$ The second (excited) dipole-bound state of ${\mathrm{LiH}}^{\mathrm{\ensuremath{-}}}$ is below the neutral ground-state potential energy curve only for some range of the Li-H internuclear distance. This state appears at bond lengths larger than $\ensuremath{\approx}2.0\AA{}$ and decays at Li-H distances longer than \ensuremath{\approx}4.2 \AA{}, where the dipole moment of LiH becomes smaller than the critical value of 2.5 D. The adiabatic electron affinity of LiH calculated at the coupled-cluster level with the iterative inclusion of all single, double, and triple excitations and a large atomic natural orbital basis set is 0.327 eV, almost matching the recently obtained experimental value of $0.342\ifmmode\pm\else\textpm\fi{}0.012\mathrm{eV}.$

Singlet-Triplet Splitting in Methylene: An Accurate Description of Dynamic and Nondynamic Correlation by Reduced Multireference Coupled Cluster Method

A classical multireference problem - the singlet-triplet separation in methylene - is examined by the recently introduced reduced multireference (RMR) singles and doubles coupled cluster (CCSD) method, using both double zeta plus polarization (DZP) and large atomic natural orbital (ANO) basis sets. In the former case, the performance of the RMR CCSD as well as of other approaches is assessed by a comparison with the full configuration interaction (FCI) result that represents the exact solution for this basis, while in the latter case a comparison is made with the experiment. It is shown that using a minimal two-configuration reference space, the RMR CCSD result compares well with either FCI or experiment; and is of the same quality as that provided by the two-reference state universal MR CCSD theory. Both MR CCSD approaches give a balanced description for the singlet and triplet states involved and correct the shortcomings of the single reference CCSD approach that is lacking in the presence of nondynamical correlation effects.

Adiabatic electron affinities of small superhalogens: LiF2, LiCl2, NaF2, and NaCl2

Geometries and frequencies for the neutral MX2 and ionic MX2− species (M=Li, Na, and X=F, Cl) are studied by several theoretical methods: density functional theory (Becke-3-Lee-Yang-Parr) [DFT(B3LYP)], second-order many-body perturbation theory [MBPT(2)], and coupled-cluster with singles and doubles (CCSD). The geometries optimized at the CCSD/6-311+G(d) level are used in CCSD(T) calculations with a large atomic natural orbital basis to compute adiabatic electron affinities (EAad), which are found for LiF2, LiCl2, NaF2, and NaCl2 to be 5.45, 4.97, 5.12, and 4.69 eV, respectively. The highest EAs among all the atoms of the periodic table occur in the halogen atoms (fluorine, 3.40 eV; chlorine, 3.62 eV); therefore all four of these triatomic radicals are properly termed superhalogens. LiF2, LiCl2, NaF2, and NaCl2 are thermodynamically stable, and their dissociation energies computed at the CCSD with the noniterative inclusion of triples [CCSD(T)] level are 20.5, 24.9, 19.3, and 25.2 kcal/mol, respectively. LiF2−, LiCl2−, NaF2−, and NaCl2− are more stable than their neutral parents with CCSD(T) dissociation energies of 69.5, 58.7, 49.0, and 52.5 kcal/mol, respectively. The computed vertical electron detachment energies of LiF2−, LiCl2−, NaF2−, and NaCl2− are 6.51, 5.88, 6.18, and 5.77 eV, respectively, which are in nice agreement with the values calculated by Scheller and Cederbaum by the Green–Function method.

The varying nature of fluorine oxygen bonds

The singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, CCSD(T), together with a triple zeta double polarized (TZ2P) one-particle basis set is used to determine the geometries, harmonic frequencies, infrared intensities, and dipole moments of HOF, F2O, HOOF, FOOF, C1OOF. Agreement with experiment is very good, with the exception that the currently accepted experimental assignment of the symmetric and antisymmetric O–F stretches in FOOF is shown to be reversed (and to be consistent with an earlier experimental study). Very accurate heats of formation of HOOF, FOOF and C1OOF are also computed using the CCSD(T) method in conjunction with large atomic natural orbital basis sets. The F–O bond distances, quadratic force constants, bond energies, and fluorine and oxygen atomic charges from the above five molecules and six previously studied molecules (FONO2, trans-FONO cis-FONO, FOC1, FOBr and FON) are compared and used to deduce a simple model of F–O bonding. The unusual relationship between the F–O bond distance and quadratic force constant shows that F–O bonding is a function of at least three effects, which are degree of covalent character, degree of ionic character, and extent of lone electron-pair repulsions. All of the data are qualitatively consistent with this simple model The bonding in cis-FONO is even more complicated, involving also dispersion interactions between fluorine and the terminal oxygen. It is suggested that the general importance of lone pair repulsions in F–O bonding and the additional importance of intra-molecular dispersion interactions explains why many density functionals have difficulty in describing the geometry of cis-FONO.

The proton affinity of HOBr

The proton affinity (PA) of HOBr is determined to be 161.5 ± 1.0 kcal/mol (298.15 K). The single and double coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, denoted CCSD(T), was used in conjunction with large spdfg atomic natural orbital basis sets to arrive at this value. The PA of HOBr is 7.0 kcal/mol larger than a recent prediction for HOCl. The most stable form of protonated HOBr is H 2OBr + with HOBrH + determined to be 17.4 ± 1.0 kcal/mol (298.15 K) higher in energy.

A CASPT2 calculation of the lowest excited states of H 2Fe(CO) 4

CASPT2 calculations based on CASSCF reference wavefunctions, using large atomic natural orbital basis sets, are reported for the lowest excited states of H 2Fe(CO) 4. The excitation energies obtained with this accurate method are compared to the values from multireference contracted CI calculations based on a unique CASSCF wavefunction. This computational strategy is usually employed for the calculation of the one-electron properties, spin-orbit interaction or costly potential energy surfaces describing the photochemistry of organometallic molecules. The relative order of the excited states is not affected by this more refined treatment and the largest excitation energy differences are of the order of 0.25 eV for the lowest excited states. However, the computational strategy seems to have a dramatic influence on the results obtained for the highest excited states pointing out the limit of less accurate methods. The theoretical spectra obtained for this family of molecules must be compared with poorly resolved experimental spectra.

Large atomic natural orbital basis sets for the first transition row atoms

Large atomic natural orbital (ANO) basis sets are tabulated for the Sc to Cu atoms. The primitive sets are taken from the large sets optimized by Partridge, namely (21s13p8d) for Sc and Ti and (20s12p9d) for V to Cu. These primitive sets are supplemented with threep, oned, sixf, and fourg functions. The ANO sets are derived from configuration interaction density matrices constructed as the average of the lowest states derived from the 3d n 4s2 and 3dn+14s1 occupations. For Ni, the1S(3d10) state is included in the averaging. The choice of basis sets for molecular calculations is discussed.