Unlinked cluster effects in molecular electronic structure. II. Pair correlations in the molecules HCN and HNC

Taylor, Peter R.; Bacskay, George B.; Hush, Noel S.; Hurley, A. C.

doi:10.1063/1.436420

Cited by 27 publications

(12 citation statements)

References 36 publications

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“…Of many chemically bound molecules, hydrogen cyanide has been studied extensively, both theoretically and experimentally (1)(2)(3)(4)(5)(6)(7)(8)(9). Both hydrogen cyanide and its isomer isocyanide are abundant in interstellar clouds (1).…”

Section: Introductionmentioning

confidence: 99%

A comparative study of the energetics, structures, and mechanisms of the HCN ↔ HNC and LiCN ↔ LiNC isomerizations

Rao¹,

Vijay²,

Chandra³

1996

Can. J. Chem.

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The potential energy surfaces of the HCN H HNC and LiCN w LiNC isomerization processes were determined by ab initio theory using fully optimized triple-zeta double polarization types of basis sets. Both the MP2 corrections and the QCISD level of calculations were performed to correct for the electron correlation. Results show that electron correlation has a considerable influence on the energetics and structures. Analysis of the intramolecular bond rearrangement processes reveals that, in both cases, H (or Li' ) migrates in an almost elliptic path in the plane of the molecule. In HCN H HNC, the migrating hydrogen interacts with the in-plane a , a * orbitals of CN, leading to a decrease in the C-N bond order. In LiCN H LiNC, Li+ does not interact with the corresponding a , a * orbitals of CN.Key words: potential energy surfaces, intra-molecular bond rearrangement, bond orders, elliptic path, migration of Li' .RCsumC : On a dCterminC les surfaces d'Cnergie potentielle des processus d'isomCrisation HCN H HNC et LiCN H LiNC B l'aide de la thCorie ab initio, en faisant appel B des ensembles de base de type double polarisation, triple zeta, totalement optimisCs. Dans le but de corriger la corrClation Clectronique, on a effectuC des corrections MP2 ainsi que des calculs au niveau QCISD. Les rCsultats montrent que la corrtlation Clectronique a une grande influence sur les energies et les structures. L'analyse des processus de rearrangement intramoltculaire des liaisons rCvkle que, dans chacun des cas, H (ou Li+) migre par une voie pratiquement elliptique dans le plan de la moltcule. Dans le rkarrangement HCN H HNC, l'hydrogkne qui migre interagit avec les orbitales a,ap* dans le plan CN conduisant B une diminution de l'ordre de la liaison C-N. Dans LiCN w LiNC, Li+ n'interagit pas avec les orbitales a , a * correspondantes du CN.

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Section: Introductionmentioning

confidence: 99%

A comparative study of the energetics, structures, and mechanisms of the HCN ↔ HNC and LiCN ↔ LiNC isomerizations

Rao¹,

Vijay²,

Chandra³

1996

Can. J. Chem.

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“…The CIOS results of Taylor et al 6 are somewhat higher, however. Our fourthorder polynomial fit of their SCF surface points reproduced their reported force constants, though there was disagreement of 1% with their k2222 value.…”

Section: Resultsmentioning

confidence: 74%

“…X22 (em-I) -56. 6 -49.4 -52.6 The CPA study of Taylor et al 6 is of particular interest since it compares SCF, cms, and a few approximate schemes with CPA, which is essentially CCD. The number of contracted functions in the basis set they used is identical to what was used in the present study, but their primitive set was smaller.…”

Section: Resultsmentioning

confidence: 99%

“…As a result, these higher-order correlation effects can provide a further refinement in the shape of a potential surface than what is obtained at the cms level. 3,5,6 A promising way of including higher-order electron correlation effects is through the use of a double-substitution coupled cluster (CCD) expansion of the wave function, also called coupled-pair many-electron theory (CPMET).7-12 This wave function differs from a usual double-substitution configuration interaction (Cm) wave function in that the linear double-substitution operator in cm is replaced by an exponential operator in CCD. A power series expansion of the exponential produces not only double-substituted configurations when applied to a reference configuration, but also unlinked products of double substitutions, e. g., quadruples, hextuples, and so on.…”

Section: Introductionmentioning

confidence: 99%

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Higher-order electron correlation effects in triatomic potential energy surfaces. Stretching vibrations of HCN and HNC

Dykstra

Secrest

1981

The Journal of Chemical Physics

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An assessment of the importance of higher-order electron correlation effects on harmonic vibrational frequencies, anharmonicities. and equilibrium structures is made for the simple triatomics HCN and HNC. Potential energy surfaces for bond stretching were generated with a large. polarized basis set of contracted Gaussian functions and using self-consistent field (SCF). correlated self-consistent electron pair (SCEP). and approximate double substitution coupled cluster (ACCD) wave functions. SCEP includes correlating configurations that mix directly with the reference wave function. while ACCD includes these configurations and also indirectly mixing configurations that arise from a cluster model of the wave function. The surfaces are compared in the prediction of spectroscopic parameters by using a low-order perturbational treatment of the vibrational wave functions in a basis of normal mode functions and with several polynomial surface fits. The neglect of correlation effects at the SCF level leads to overestimates of the vibrational frequencies.Higher-order correlation effects correct for roughly one third of this overestimation. For all parameters. the ACCD treatment yields the best predictions.

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“…22 From this basis of 37 orbitals it is possible to generate from the parent SCF-CF a total of 22,366 (single and double) CFS. Our largest wavefunction includes 1640 interacting CFS and produces an energy lowering of -0.2582 hartree from an SCF energy of -92.8890 hartrees.…”

Section: Resultsmentioning

confidence: 99%

A systematic CI procedure with modified virtual orbitals

Cooper

Pounder

1980

J Comput Chem

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The use of modified virtual orbitals is studied in a systematic conventional CI procedure which offers considerable potential in regard to convergence and extension to larger systems. The method is applied to the HCN molecule by using 37 basis functions, and analysis of energy expectation values, together with the one-electron density, yields some insight into the physical content of CI wavefunctions. IN T R 0 DUCT I 0 NIn a recent article1 we introduced a simple scheme for the construction of a set of modified virtual orbitals (MVOs) with convergence properties superior to those of the familiar canonical virtual orbital set. The MVO set is constructed so that the virtual orbitals are drawn toward the nuclear centers, and can interact more strongly with the nuclei. This is achieved by solving a modified SCF eigenvalue problem, solely in the SCF virtual orbital subspace, in the presence of the extra potential term $, ,. The operator V,, represents the attractive potential between a single electron and all the nuclei, z is the sum of nuclear charges, and X is a parameter. For reasons of brevity details of the method and relevant references can be found in ref. 1.The traditional CI method has been described as the worst way of constructing correlated wavefunctions, except for any other known method.2 The underlying reason is that CI methods are plagued by slow convergence and involve lengthy transformations, diagonalizations, and configuration selection procedures. A comprehensive and thorough review of the CI procedure can be found elsewhere."The problem of scale is a formidable one in the traditional CI method which involves spin-adapted configuration functions (CFS). The number of possible double-excitation CFs that can be generated by promotion from the parent SCF-CF alone is of the order 1/2(nmc)2(nviA)2. Hence, if an increase in basis set size is contemplated, the number of such CFs increases as (nvirJ2, whereas extension to larger systems means an increase in candidate CFS of at least an order (nm,)2; in general, nvirt will also increase. This is one reason why CI is generally restricted to a range of relatively small molecules with "good" basis sets, for the performance of molecular CI calculations with "relatively poor" basis sets has produced results "worse" than that of the SCF itself.4As an alternative we can try to reduce the number of contributing orbitals by constraining some to be doubly occupied (frozen-core approxi m a t i~n ) .~!~ This, however, has led to deficiencies in certain expectation values7 and to a total lack of correlation between the motions of unlike spin electrons in the high-density regions around the heavy-atom centers.8A reduction in the number of virtual orbitals can, of course, be achieved by truncating the CI at an arbitrary but convenient point.g Both this and the frozen-core approximation, however, can ignore certain single and direct double promotions that have proved to be important in obtaining reliable expectation values.1° All these methods attempt to reduce the scale o...

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Unlinked cluster effects in molecular electronic structure. II. Pair correlations in the molecules HCN and HNC

Cited by 27 publications

References 36 publications

A comparative study of the energetics, structures, and mechanisms of the HCN ↔ HNC and LiCN ↔ LiNC isomerizations

A comparative study of the energetics, structures, and mechanisms of the HCN ↔ HNC and LiCN ↔ LiNC isomerizations

Higher-order electron correlation effects in triatomic potential energy surfaces. Stretching vibrations of HCN and HNC

A systematic CI procedure with modified virtual orbitals

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