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We report a new calculation of the N+O2 potential energy surface using complete active space self-consistent field internally contracted configuration interaction with the Dunning correlation consistent basis sets. The peroxy isomer of NO2 is found to be a very shallow minimum separated from NO+O by a barrier of only 0.3 kcal/mol (excluding zero-point effects). The entrance channel barrier height is estimated to be 8.6 kcal/mol for ICCI+Q calculations correlating all but the O1s and N1s electrons with a cc-pVQZ basis set. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Two-dimensional potential energy surfaces for the CH+N2→HCN+N(4S) reaction are obtained which describe (i) the C2v HCN2 region of the doublet potential energy surface; (ii) the region of the quartet potential energy surface including the C2v minimum and the dissociation pathway to HCN+N(4S); and (iii) the region of crossing of the doublet and quartet surfaces. These surfaces are fit using simple, physically motivated functional forms. In the first and third regions, the active coordinates are the C–N2 [center-of-mass (c.m.)] distance and the N–N distance. In the second region, the active coordinates are the C–N’ separation, where N’ denotes the departing N atom, and the angle H–C–N. In the following paper, this potential is used to study the dynamics of the title reaction and to compute Boltzmann rate constants. [ABSTRACT FROM AUTHOR]

The potential energy surface for NH2+NO has been characterized using complete active space self-consistent-field (CASSCF)/derivative calculations to determine the stationary point geometries and frequencies followed by internally contracted configuration interaction (ICCI) calculations to determine the energetics. Production of N2+H2O is found to involve a complex mechanism. The initially formed NH2NO undergoes a 1,3-hydrogen shift to give an HNNOH isomer (with the substituents trans about the NN bond and cis about the NO bond) which undergoes subsequent cis–trans isomerizations about the NN and NO bonds before decomposing to N2+H2O. The saddle point for production of N2+H2O has an approximately rectangular arrangement of one H atom, the two N atoms, and the O atom. This process does not involve a barrier with respect to NH2+NO. Formation of HN2+OH can occur from any of the isomers of HNNOH with no barrier, but the overall process is endothermic by 0.7 kcal/mol [based on the computed Δ Hf0 (0 K) of HN2]. The results obtained in this work are qualitatively the same as previous work, but both the stationary point geometries and energies should be more reliable due to the use of larger basis sets and more extensive inclusion of electron correlation effects. [ABSTRACT FROM AUTHOR]

We describe an analytic representation of the ArH2 potential energy surface which well reproduces the results of extensive ab initio electronic structure calculations. We also give an empirical modification of the function designed to improve agreement with experimental estimates of the van der Waals minimum. The analytic representation smoothly interpolates between the H+H and strong bonding H2 limits. In the fitting process, an accurate reproduction of regions of the potential expected to be important for high temperature (∼3000 K) collision processes is emphasized. Overall, the analytic representation well reproduces the anisotropy and H2 bond length dependence of the input data. [ABSTRACT FROM AUTHOR]

The potential energy surface (PES) for the CH3OH system has been characterized for the 1CH2+H2O, H2+HCOH, and H2+H2CO product channels using complete-active-space self-consistent-field (CASSCF) gradient calculations to determine the stationary point geometries and frequencies followed by CASSCF/internally contracted configuration-interaction (CCI) calculations to refine the energetics. The 1CH2+H2O channel is found to have no barrier. The long range interaction is dominated by the dipole–dipole term, which orients the respective dipole moments parallel to each other but pointing in opposite directions. At shorter separations there is a dative bond structure in which a water lone pair donates into the empty a‘ orbital of CH2. Subsequent insertion of CH2 into an OH bond of water involves a non-least-motion pathway. The H2+HCOH, and H2+H2CO pathways have barriers located at -5.2 and 1.7 kcal/mol, respectively, with respect to CH3+OH. From comparison of the computed energetics of the reactants and products to known thermochemical data it is estimated that the computed PES is accurate to ±2 kcal/mol. [ABSTRACT FROM AUTHOR]

The potential energy surface (PES) for NH+NO has been characterized using complete active space self-consistent field (CASSCF) gradient calculations to determine the stationary point geometries and frequencies followed by CASSCF/internally contracted configuration interaction (CCI) calculations to refine the energetics. The present results are in qualitative accord with the BAC-MP4 calculations of Melius and Binkley, but there are differences as large as 8 kcal/mol in the detailed energetics. Addition of NH to NO on a 2A’ surface, which correlates with N2+OH or H+N2O products, involves barriers of 3.2 kcal/mol (trans) and 6.3 kcal/mol (cis). Experimental evidence for these barriers is found in the work of Böhmer et al. The 2A‘ surface has no barrier to addition, but does not correlate with ground state products. Surface crossings between the barrierless 2A‘ surface and the 2A’ surface may be important. Production of N2+OH products is predicted to occur via a planar saddle point of 2A’ symmetry. This is in accord with the preferential formation of Π(A’) Λ doublet levels of OH in the experiments of Patel-Misra and Dagdigian. Addition of NH 1Δ to NO is found to occur on an excited state surface and is predicted to lead to N2O product as observed by Yamasaki et al. [ABSTRACT FROM AUTHOR]

In a previous paper in this series [J. Chem. Phys. 93, 2384 (1990)] a global potential energy surface (PES) was presented for H+N2→HN2. In this paper, we report additional calculations which characterize PES’s for the excited 2A‘ state, for a bound 2 2A’ state, for HN+2, and for the Rydberg states associated with HN+2. It is anticipated that these excited state PES’s will be important in interpreting and designing experiments to characterize the ground state HN2 species via neutralized ion beam techniques. [ABSTRACT FROM AUTHOR]

Recent ab initio calculations have focused on the minimum energy path region of this surface [J. Chem. Phys. 88, 6273 (1988), Paper I] and on the saddle point region for H atom exchange via a T-shaped HO2 complex (J. Chem. Phys. 91, 2373 (1989), Paper II). In this paper, the results of additional calculations are discussed that, combined with the previously reported results, provide a global representation of the potential energy surface for this reaction. Complete active space SCF/externally contracted configuration interaction calculations (CASSCF/CCI) were carried out using the same wave function employed in II. The new calculations presented here characterize the potential energy surface for a variety of H atom approach angles, ranging from perpendicular to collinear (measured with respect to the O2 bond) and for a variety of H atom to O2 center-of-mass distances. Additionally, a new collinear exchange saddle point is reported. Using the ab initio results from these new calculations, optimal geometries, and harmonic frequencies based on local polynomial representations of the potential energy surface are reported along the constrained energy minimum path, while anharmonic frequencies are calculated at both the O2 and OH asymptotes and at the HO2 intermediate. [ABSTRACT FROM AUTHOR]

We have performed extensive ab initio calculations on the ground state potential energy surface of H2+H2O, using a large contracted Gaussian basis set and a high level of correlation treatment. An analytical representation of the potential energy surface was then obtained which reproduces the calculated energies with an overall root-mean-square error of only 0.64 mEh. The analytic representation explicitly includes all nine internal degrees of freedom and is also well behaved as the H2 dissociates; it thus can be used to study collision-induced dissociation or recombination of H2. The strategy used to minimize the number of energy calculations is discussed as well as other advantages of the present method for determining the analytical representation. [ABSTRACT FROM AUTHOR]

The reactant, product, and saddle point regions of the 5Π and 3Π potential energy surfaces for the reaction NH+O→N+OH have been characterized using complete active space self consistent field/externally contracted configuration interaction (CASSCF/CCI) calculations with large atomic natural orbital (ANO) basis sets. The computed barrier heights are 5.6 and 11.7 kcal/mol on the 5Π and 3Π surfaces, respectively. Transition state theory with an Eckart tunneling correction is used to estimate the rate constant on the 5Π surface. [ABSTRACT FROM AUTHOR]

A previous calculation for H+N2 [Walch, Duchovic, and Rohlfing, J. Chem. Phys. 90, 3230 (1989)] focused on the minimum energy path (MEP) region of the potential energy surface and on estimates of the lifetime of the HN2 species. In this paper, we report energies computed at geometries selected to permit a global representation of the potential energy surface (PES). As in the previous work, the calculations were performed using the complete active space self-consistent field/externally contracted configuration interaction (CASSCF/CCI) method. The surface was characterized using the same basis set as in the previous paper except that an improved contraction of the H s basis is used. Calculations with a larger basis set were carried out along an approximate MEP obtained with the smaller basis set. The new PES exhibits a sharp curvature, which was not present in the previous calculations, and has a slightly narrower and smaller barrier to dissociation. Saddle points for H atom exchange via collinear and T-shaped HN2 complexes are also reported. [ABSTRACT FROM AUTHOR]

The results of complete active space SCF/multireference contracted CI(CASSCF/CCI) calculations with large atomic natural orbital (ANO) basis sets are presented for the H + NO region of the lowest three potential surfaces of HNO (1A’, 3A‘, and 1A‘). The calculations focus on the minimum energy path for H atom addition to the N end of NO and on the equilibrium geometry region of HNO and HON. [ABSTRACT FROM AUTHOR]

The minimum energy path (MEP) for the addition of a hydrogen atom to N2 is characterized in CASSCF/externally contracted CI calculations using a [4s3p2d1f/3s2p1d] ANO basis set, with additional single point calculations at the stationary points of the potential energy surface using a [5s4p3d2f/4s3p2s] ANO basis set. These calculations represent the most extensive set of ab initio calculations on HN2 completed to date, yielding a zero-point corrected barrier for HN2 dissociation of ≊8.5 kcal mol-1. The lifetime of the HN2 species is estimated from the calculated geometries and energetics using both conventional transition state theory and a method which utilizes an Eckart barrier to compute one-dimensional quantum mechanical tunneling effects. This study concludes that the lifetime of the HN2 species is very short, greatly limiting its role in combustion processes. [ABSTRACT FROM AUTHOR]

Several dynamics studies have indicated that the product isotopic distributions and vibrational energy distributions in the reaction O(1D)+H2(HD) are very sensitive to the long range interaction of the reactants. In this paper we report an improved calculation of the reactant region of the potential energy surface. In agreement with previous work we find no barrier to edge-on insertion (1A’ ground state water surface), but find a smaller (<0.2 kcal/mol) barrier to collinear addition (1Σ+ surface) than in previous work. The long range potential obtained in the present work most closely resembles the SL3 surface. [ABSTRACT FROM AUTHOR]

The potential energy surface for the reaction H+O2→HO[ATOTHER]@B|[/ATOTHER]2 →HO+O has been characterized in the vicinity of the minimum energy path using CASSCF/contracted CI calculations with a basis set which is triple zeta valence quality plus three sets of polarization functions. CASSCF/CI calculations were carried out along the CCI minimum energy path. The latter calculation shows essentially no barrier for addition of an H atom to O2, in agreement with predictions made in earlier studies. The potential surface for recombination of OH and O is complicated by a crossing, at rOO approx. 5.5a0, between the surface for electrostatic (OH dipole–O quadrupole) interaction and that for the formation of an O–O chemical bond. This surface crossing results in a small (approx. 0.5 kcal/mol) barrier. [ABSTRACT FROM AUTHOR]

The F+H2→HF+H potential energy hypersurface has been studied in the saddle-point and entrance channel regions. Using a large [5s 5p 3d 2f 1g/4s 3p 2d] atomic natural orbital basis set, we obtain a classical barrier height of 1.86 kcal/mol at the CASSCF/multireference CI level (MRCI) after correcting for basis set superposition error and including a Davidson correction (+Q) for higher excitations. Based upon an analysis of the computed results, the true classical barrier is estimated to be about 1.4 kcal/mol. We also compute the location of the bottleneck on the lowest vibrationally adiabatic potential curve, and determine the translational energy threshold from a one-dimensional tunneling calculation. Using the difference between the calculated and experimental threshold to adjust the classical barrier height on the computed surface yields a classical barrier in the range of 1.0–1.5 kcal/mol. Combining the results of our direct estimates of the classical barrier height with the empirical values obtained from our approximate calculations of the dynamical threshold, we predict that the true classical barrier height is 1.4±0.4 kcal/mol. Arguments are presented in favor of including the relatively large (approx. 1 kcal/mol)+Q correction obtained when nine electrons are correlated at the CASSCF/MRCI level. [ABSTRACT FROM AUTHOR]

SCF/CCI calculations are presented for selected portions of the potential surfaces for six low-lying states of Ni3. The calculations use the effective core potentials developed by Hay and Wadt. For near equilateral triangle geometries, four states were studied, all of which arise from three Ni atoms in the 4s13d9 state. The 4s electron configuration here is 4sa’214se1, which leads to Jahn–Teller distortion analogous to Cu3. All of these states are within 0.04 eV of each other. For linear geometries two states were studied. The first linear state, which arises from three Ni atoms in the 4s13d9 state, is 0.16 eV higher than the corresponding near equilateral triangle state. The second linear state, which has one 4s23d8 center atom and the other two atoms in the 4s13d9 state, is estimated to be nearly degenerate (within 0.01 eV) with the near equilateral triangle structures. [ABSTRACT FROM AUTHOR]

We present a quasiclassical trajectory study of the NH+NO reaction using a global potential energy surface that is capable of describing branching to the H+N2O and OH+N2 products after initial formation of a HNNO intermediate complex. The surface is based on a many-body expansion wherein fragment potentials for the species N2H, HNO, and N2O are incorporated, using either previously developed potentials, or in the case of N2O, a newly developed potential. The three-body parts of these fragment potentials are damped in the four-body region to provide a zeroth order four-body surface, and then additional four-body terms and mapping transformations are applied to make the final four-body potential match the results of ab initio calculations for eight important HNNO stationary points (minima and saddle points) and for several reaction paths. In addition to this ‘‘best fit’’ surface (surface I), a second surface (surface II) is developed in which the ordering of the saddle points leading to formation of H+N2O and OH+N2 is reversed, and the energy release during 1,3 hydrogen migration is modified so that the N–N stretch experiences smaller distortions from N2 equilibrium during the reaction leading to OH+N2. Quasiclassical trajectory results on surface I show generally good correspondence with experiment, with a branching fraction of 13±3% for the formation of OH+N2 at 300 K, and relatively low OH and N2 vibration/rotation excitation. The results on surface II are similar with respect to both branching and energy partitioning, indicating relatively weak sensitivity of the results of key features of the surface. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present global ab initio potential energy surfaces for the three lowest energy 1A’, 3A‘, and 1A‘ surfaces of HNO. These surfaces are the lowest three states of the HNO and HON molecules, and they correlate to the ground electronic states of H+NO and O+NH. In addition, the 3A‘ surface correlates to the ground state of N+OH. The surfaces are based on approximately 800 ab initio calculations that were done using an internally contracted multireference configuration interaction calculation with a large basis set. The ab initio points were fit to a combination of Morse and spline functions in each of the three possible Jacobi coordinates, and the resulting splines were smoothly switched together, and combined with other functions to yield globally defined potentials. Properties of the HNO and HON minima and dissociation energies on these potentials are in good agreement with previous high quality calculations. The N+OH and O+NH reactions are found to have no barriers to formation of HON or HNO, respectively. Isomerization of HON to HNO involves barriers that are higher than the HON dissociation barrier on the singlet surfaces but not on the triplet surface. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a quasiclassical trajectory study of the collisions which occur on the 1A’, 3A‘, and 1A‘ potential surfaces of HNO using recently developed global potential surfaces that were derived from ab initio calculations. Each of these surfaces was assumed to be uncoupled from the other surfaces for the purposes of the calculations, and the appropriate statistical average of electronic states for each process was calculated. For N+OH and O+NH, we specifically studied reactive collisions which give H+NO as products, and we also studied the production of N+OH from O+NH. Overall rate constants calculated for either N+OH or O+NH are in good agreement with most experiments, and in addition, the product NO vibrational distribution from the N+OH reaction is in good correspondence with recent measurements, revealing modest excitation that is close to what would be expected from a statistical distribution. For O+NH, the calculated NO vibrational distributions are much hotter than statistical, in apparent disagreement with recent measurements. However, a careful analysis of limitations on the measurements due to spectral interference and to collisional relaxation indicates that it is not possible to say if theory and experiment are at odds for this reaction. We find a significant cross section for O+NH→N+OH on the 3A‘ surface (roughly 5% of the total reactive cross section, independent of energy), and this leads to rate constants at low temperatures that are orders of magnitude higher at low temperature than estimates made earlier based on H atom abstraction on the 5Π surface.The mechanism of this reaction is found to involve three steps: addition to form HNO, isomerization to HON, and dissociation to produce N+OH. We have also studied nonreactive vibrational and rotational excitation in H+NO collisions, and we obtain distributions that are somewhat closer to experiment than obtained in previous theoretical studies, although... [ABSTRACT FROM AUTHOR]

In this paper values of the unimolecular decay lifetimes of several vibrational states of HN2 based on an accurate coupled channel dynamics study using a global analytical potential surface that was derived from previously reported ab initio calculations are reported. The surface was developed by fitting the N–N stretch part to a Morse function, with parameters that are represented by a two-dimensional spline function in terms of the H to N2 center of mass distance and approach angle. This surface reproduces the ab initio points with a root mean square error of 0.08 kcal/mol for energies below 20 kcal/mol. Modifications to the potential that describe the effect of improving the basis set in the ab initio calculations are also provided. Converged coupled channel calculations have been done for the ground rotational state of HN2 to determine lifetimes of the lowest ten vibrational states. We find that only the ground vibrational state (000) and first excited bend (001) have lifetimes longer than 1 ps. The best estimates of the lifetimes of these states are 3×10-9 and 2×10-10 s, respectively. Variation of these results with quality of the ab initio calculations is less than or equal to a factor of 5. [ABSTRACT FROM AUTHOR]

The results of CASSCF/multireference contracted CI calculations with large ANO basis sets are presented for the exchange region of the HO2 potential energy surface. The saddle point for H atom exchange is ≊13 kcal/mol below the energy of H+O2; therefore, this region of the surface will be accessible during H+O2 recombination and metathesis reactions. [ABSTRACT FROM AUTHOR]

We present the results of an ab initio quantum-chemical investigation of the structure of the anion O[sub 4][sup -] and potential energy surfaces for several electronic states. In addition to ground-state vibrational frequencies, which are in good agreement with other calculations and with matrix isolation experiments, we have investigated both photodissociation of O[sub 4][sup -] into various states of O[sub 2] and O[sub 2][sup -] and dissociative photodetachment leading to a free electron and various states of two O[sub 2] molecules. Our surfaces lead to predictions for both photodissociation and photodetachment processes that are in excellent agreement with experiment. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reports that in the article 'Theoretical characterization of the potential energy surface for H+O[sub2]...HO[sub2]...OH+O. IIII. Computed points to define a global potential energy surface,' from the Journal of Chemical Physics volume 94, p. 7068, the CASSCF/CCI calculations described in the paper were incorrectly identified as [4s3p2d1f/3s*2p] instead of [4s3p2d1f/3s*2p1d].