Quadratic Configuration Interaction

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Joseph S Francisco - One of the best experts on this subject based on the ideXlab platform.

  • An ab initio study of the reaction of HOCO radicals with NO2: addition/elimination mechanism.
    The Journal of chemical physics, 2009
    Co-Authors: Gabriella Poggi, Joseph S Francisco
    Abstract:

    The reaction between HOCO and NO2 has been examined using the Quadratic Configuration Interaction method to locate the transition states and intermediates involved in the reaction. Analysis of the potential energy surface suggest that two possible intermediates are involved: HOC(O)ONO and HOC(O)NO2. These new species proceed to form the products, HONO+CO2 and HNO2+CO2. Reaction channels leading to the formation of CO are energetically noncompetitive with those yielding CO2. The lower energy pathways produce HONO+CO2 representing new mechanisms for the formation of HONO, through an exchange of carbon and nitrogen centers.

  • An ab initio study of the pathways for the reaction between CH3O2 and BrO radicals
    The Journal of Chemical Physics, 2003
    Co-Authors: Sujata Guha, Joseph S Francisco
    Abstract:

    The pathways for the reaction between methylperoxy (CH3O2) and bromine monoxide (BrO) radicals have been examined by using the Quadratic Configuration Interaction method. It is found that the most feasible pathway of the CH3O2+BrO reaction is the formation of CH3OOOBr as an intermediate during the reaction and its dissociation into CH2O and HOOBr. This study finds that besides CH3OOOBr, CH3OOBrO may also exist as an intermediate complex during the CH3O2+BrO reaction.

  • A Quadratic Configuration Interaction study of the proton affinity of acetic acid
    Chemical Physics Letters, 2002
    Co-Authors: Charles E. Miller, Joseph S Francisco
    Abstract:

    Abstract Optimized geometries and harmonic vibrational frequencies have been obtained for neutral and protonated acetic acid using Quadratic Configuration Interaction methods. The proton affinity of acetic acid calculated at the QCISD(T)/6-311++G(3df,3pd)//QCISD/6-311+G(2d,2p) level of theory is 187.9±0.7 kcal mol −1 , in excellent agreement with the experimental value of 187.3 kcal mol −1 . Thermochemical calculations using the CBS-Q, CBS-QB3, CBS-APNO, G2 and G2(MP2) model chemistries also display excellent agreement with the experimental proton affinity, suggesting that these model chemistries will provide accurate proton affinities for carboxylic acids that are too large to treat with high level ab initio methods.

  • An ab initio study of the hydrogen abstraction reaction of methane by bromine atoms and bromine monoxide radicals
    Journal of Molecular Structure-theochem, 2001
    Co-Authors: Sujata Guha, Joseph S Francisco
    Abstract:

    Abstract The structures, vibrational spectra, and relative energetics for the transition states of the hydrogen abstraction reactions of methane by bromine (Br) atoms and bromine monoxide (BrO) radicals have been examined by using the Quadratic Configuration Interaction method in conjunction with different basis sets. From the calculated energetics of the CH4+Br→CH3+HBr and CH4+BrO→CH3+HOBr reactions, we find that the energy barrier for the formation of HOBr is higher than that for the formation of HBr. The endothermicity of the CH4+Br→CH3+HBr reaction is 7.9 kcal mol−1 higher than that for the CH4+BrO→CH3+HOBr reaction.

  • protonated nitrous acid h2ono molecular structure vibrational frequencies and proton affinity
    Journal of Chemical Physics, 2001
    Co-Authors: Joseph S Francisco
    Abstract:

    The equilibrium structures, harmonic vibrational frequencies of HONO, and protonated HONO have been investigated using ab initio methods. The ab initio methods include second-order Moller-Plesset (MP2) perturbation theory, single-and double-Quadratic Configuration Interaction (QCISD) theory, and the QCIDS(T) method, which incorporates a perturbational estimate of the effects of connected triple excitations. The QCISD(T) equilibrium geometry and vibrational frequencies for nitrous acid are in good agreement with experiment. The lowest energy form of protonated HONO is a complex between water and NO+. The next lowest isomer is 35.1 kcal mol−1 higher in energy. The ab initio proton affinity (PA) of HONO is predicted to be 191.5 kcal mol−1 at the QCISD(T)/6-311++G(3df,3pd) level of theory.

John A. Pople - One of the best experts on this subject based on the ideXlab platform.

  • Reprint of: A fifth-order perturbation comparison of electron correlation theories
    Chemical Physics Letters, 2013
    Co-Authors: Krishnan Raghavachari, John A. Pople, Gary W. Trucks, Martin Head-gordon
    Abstract:

    Abstract Electron correlation theories such as Configuration Interaction (CI), coupled-cluster theory (CC), and Quadratic Configuration Interaction (QCI) are assessed by means of a Moller–Plesset perturbation expansion of the correlation energy up to fifth order. The computational efficiencies and relative merits of the different techniques are outlined. A new augmented version of coupled-cluster theory, denoted as CCSD (T), is proposed to remedy some of the deficiencies of previous augmented coupled-cluster models.

  • Gaussian-3X (G3X) theory using coupled cluster and Brueckner energies
    Chemical Physics Letters, 2002
    Co-Authors: Larry A. Curtiss, Krishnan Raghavachari, Paul C. Redfern, John A. Pople
    Abstract:

    Variations of Gaussian-3X (G3X) theory are presented having the Quadratic Configuration Interaction (QCISD(T)) energy calculation replaced by either a coupled cluster (CCSD(T)) or Brueckner (BD(T)) energy. This modification is reported for several G3X methods including G3X, G3X(MP3), G3X(MP2), G3SX, and G3SX(MP3). In most cases the replacement of the QCISD(T) energy by one of these alternative energies results in a slight improvement in the accuracy of these methods as assessed on the G3/99 test set of molecules. These new G3X methods are useful alternatives to the G3X methods based on Quadratic Configuration Interaction.

  • Gaussian-3 theory using coupled cluster energies
    Chemical Physics Letters, 1999
    Co-Authors: Larry A. Curtiss, Krishnan Raghavachari, Paul C. Redfern, Anwar G. Baboul, John A. Pople
    Abstract:

    Variations of Gaussian-3 (G3) theory are presented in which the Quadratic Configuration Interaction (QCISD(T)) energy calculation is replaced by a coupled cluster (CCSD(T)) energy calculation. This modification is made to four G3 methods that have been previously introduced. The replacement of the QCISD(T) energy by the CCSD(T) energy results in little change in the accuracy of the methods as assessed on the G2/97 test set, although the maximum deviations decrease slightly. These new G3 methods based on the coupled cluster technique are a useful alternative to the G3 methods based on Quadratic Configuration Interaction.

  • gaussian 2 theory use of higher level correlation methods Quadratic Configuration Interaction geometries and second order mo ller plesset zero point energies
    Journal of Chemical Physics, 1995
    Co-Authors: Larry A. Curtiss, Krishnan Raghavachari, John A. Pople
    Abstract:

    The performance of Gaussian‐2 theory is investigated when higher level theoretical methods are included for correlation effects, geometries, and zero‐point energies. A higher level of correlation treatment is examined using Brueckner doubles [BD(T)] and coupled cluster [CCSD(T)] methods rather than Quadratic Configuration Interaction [QCISD(T)]. The use of geometries optimized at the QCISD level rather than the second‐order Mo/ller–Plesset level (MP2) and the use of scaled MP2 zero‐point energies rather than scaled Hartree–Fock (HF) zero‐point energies have also been examined. The set of 125 energies used for validation of G2 theory [J. Chem. Phys. 94, 7221 (1991)] is used to test out these variations of G2 theory. Inclusion of higher levels of correlation treatment has little effect except in the cases of multiply‐bonded systems. In these cases better agreement is obtained in some cases and poorer agreement in others so that there is no improvement in overall performance. The use of QCISD geometries yiel...

  • gaussian 2 theory use of higher level correlation methods Quadratic Configuration Interaction geometries and second order mo ller plesset zero point energies
    Journal of Chemical Physics, 1995
    Co-Authors: Larry A. Curtiss, Krishnan Raghavachari, John A. Pople
    Abstract:

    The performance of Gaussian‐2 theory is investigated when higher level theoretical methods are included for correlation effects, geometries, and zero‐point energies. A higher level of correlation treatment is examined using Brueckner doubles [BD(T)] and coupled cluster [CCSD(T)] methods rather than Quadratic Configuration Interaction [QCISD(T)]. The use of geometries optimized at the QCISD level rather than the second‐order Mo/ller–Plesset level (MP2) and the use of scaled MP2 zero‐point energies rather than scaled Hartree–Fock (HF) zero‐point energies have also been examined. The set of 125 energies used for validation of G2 theory [J. Chem. Phys. 94, 7221 (1991)] is used to test out these variations of G2 theory. Inclusion of higher levels of correlation treatment has little effect except in the cases of multiply‐bonded systems. In these cases better agreement is obtained in some cases and poorer agreement in others so that there is no improvement in overall performance. The use of QCISD geometries yields significantly better agreement with experiment for several cases including the ionization potentials of CS and O2, electron affinity of CN, and dissociation energies of N2, O2, CN, and SO2. This leads to a slightly better agreement with experiment overall. The MP2 zero‐point energies gives no overall improvement. These methods may be useful for specific systems.

Ian Carmichael - One of the best experts on this subject based on the ideXlab platform.

  • ATOMIC SPIN DENSITIES FROM CORRELATION-CONSISTENT BASIS SETS
    Journal of Physical Chemistry A, 1997
    Co-Authors: Ian Carmichael
    Abstract:

    Recently introduced correlation-consistent (cc-) basis sets are used together with the Quadratic Configuration Interaction (QCISD) correlation recovery technique and a hybrid density functional met...

  • Ab initio Quadratic Configuration Interaction calculation of indirect NMR spin-spin coupling constants
    The Journal of Physical Chemistry, 1993
    Co-Authors: Ian Carmichael
    Abstract:

    Finite-field perturbation theory is combined with the Quadratic Configuration Interaction method of ab initio electronic structure theory to calculate the Fermi-contact component of the indirect spin-spin coupling constant between adjacent carbon nuclei in a range of bonding environments. Basis set requirements for accuracy are first investigated in ethane and a small modification of the outer-core, inner-valence region of the standard Dunning double-ζ contraction is shown to be satisfactory

  • ab initio Quadratic Configuration Interaction calculation of the isotropic hyperfine coupling constants in the ethyl radical
    The Journal of Physical Chemistry, 1991
    Co-Authors: Ian Carmichael
    Abstract:

    The isotropic hyperfine coupling constraints in the electronic ground state of the ethyl radical have been determined by means of ab initio molecular orbital theory. Extensive inclusion of electron correlation in a single-determinant unrestricted Hartree-Fock (UHF) description is coupled with finite (Fermi contact) field perturbation theory to derive the required spin density distribution. Results obtained with a modest polarized double-{zeta} basis set are already in quantitative accord with experiment. Augmentation of this basis set improves the level of agreement still further. An analysis is made of the correlation contributions to the magnetic coupling at each nucleus. At the radical center, {sup 13}C{sub {alpha}}, a large ({approximately}245 MHz) positive spin polarization estimated by the UHF method to be developed in the valence shell is seen to be sharply reduced (to {approximately} 155 MHz) by the inclusion of electron correlation. Taken together with the computed core polarization ({approximately}-75 MHz), the direct spin density ({approximately}20 MHz) from the highest occupied {alpha} molecular orbital, and a vibrational correction ({approximately}15 MHz) due to zero-point out-of-plant wagging of the methylenic hydrogens, the predicted carbon-13 splitting is within 1% of observation. At the adjoining methylenic hydrogens almost the entire coupling constant derives from spin polarization which ismore » overestimated at the UHF level and which again is moderated by the inclusion of electron correlation. A small (<2 MHz) vibrational correction brings the computed value into essentially exact agreement with the observed splitting.« less

  • Ab initio Quadratic Configuration Interaction calculations of isotropic hyperfine coupling constants
    The Journal of Physical Chemistry, 1991
    Co-Authors: Ian Carmichael
    Abstract:

    Results are presented from Quadratic Configuration Interaction (QCISD) calculations of the isotropic component of the magnetic hyperfine coupling in the electronic ground states of the atoms B-F and of some radicals and radical cations derived from associated diatomic hydrides. Moderately large basis sets of contracted Gaussian functions are employed. Starting from unrestricted Hartree-Fock (UHF) reference spaces the unpaired spin density is determined by finite Fermi contact field perturbation theory. Comparison is made with previous values obtained by an approximate augmented coupled-cluster procedure in which both single and triple excitations are handled perturbatively. In all cases the QCISD predictions lie closer to the available experimentally observed hyperfine splittings.

  • Ab initio Quadratic Configuration Interaction calculation of the isotropic hyperfine coupling constants in the ethyl radical
    The Journal of Physical Chemistry, 1991
    Co-Authors: Ian Carmichael
    Abstract:

    The isotropic hyperfine coupling constraints in the electronic ground state of the ethyl radical have been determined by means of ab initio molecular orbital theory. Extensive inclusion of electron correlation in a single-determinant unrestricted Hartree-Fock (UHF) description is coupled with finite (Fermi contact) field perturbation theory to derive the required spin density distribution. Results obtained with a modest polarized double-{zeta} basis set are already in quantitative accord with experiment. Augmentation of this basis set improves the level of agreement still further. An analysis is made of the correlation contributions to the magnetic coupling at each nucleus. At the radical center, {sup 13}C{sub {alpha}}, a large ({approximately}245 MHz) positive spin polarization estimated by the UHF method to be developed in the valence shell is seen to be sharply reduced (to {approximately} 155 MHz) by the inclusion of electron correlation. Taken together with the computed core polarization ({approximately}-75 MHz), the direct spin density ({approximately}20 MHz) from the highest occupied {alpha} molecular orbital, and a vibrational correction ({approximately}15 MHz) due to zero-point out-of-plant wagging of the methylenic hydrogens, the predicted carbon-13 splitting is within 1% of observation. At the adjoining methylenic hydrogens almost the entire coupling constant derives from spin polarization which ismore » overestimated at the UHF level and which again is moderated by the inclusion of electron correlation. A small (

John A Montgomery - One of the best experts on this subject based on the ideXlab platform.

  • a complete basis set model chemistry iv an improved atomic pair natural orbital method
    Journal of Chemical Physics, 1994
    Co-Authors: John A Montgomery, Joseph W Ochterski, George A Petersson
    Abstract:

    An improved complete basis set‐Quadratic Configuration Interaction/atomic pair natural orbital (CBS‐QCI/APNO) model is described in this paper. It provides chemical energy differences (i.e., D0 I.P., and E.A.) with a mean absolute error of 0.53 kcal/mol for the 64 first‐row examples from the G2 test set, and is computationally feasible for species with up to three first‐row atoms. A set of 20 CBS‐QCI/APNO bond dissociation energies of hydrocarbons also agree with known experimental values to within less than 1 kcal/mol. Calculations on the cyclopropenyl radical and cyclopropenylidene provide new dissociation energies which are in accord with an interpretation of the thermochemistry emphasizing ring strain and aromaticity.An improved complete basis set‐Quadratic Configuration Interaction/atomic pair natural orbital (CBS‐QCI/APNO) model is described in this paper. It provides chemical energy differences (i.e., D0 I.P., and E.A.) with a mean absolute error of 0.53 kcal/mol for the 64 first‐row examples from the G2 test set, and is computationally feasible for species with up to three first‐row atoms. A set of 20 CBS‐QCI/APNO bond dissociation energies of hydrocarbons also agree with known experimental values to within less than 1 kcal/mol. Calculations on the cyclopropenyl radical and cyclopropenylidene provide new dissociation energies which are in accord with an interpretation of the thermochemistry emphasizing ring strain and aromaticity.

  • a complete basis set model chemistry iv an improved atomic pair natural orbital method
    Journal of Chemical Physics, 1994
    Co-Authors: John A Montgomery, Joseph W Ochterski, George A Petersson
    Abstract:

    An improved complete basis set‐Quadratic Configuration Interaction/atomic pair natural orbital (CBS‐QCI/APNO) model is described in this paper. It provides chemical energy differences (i.e., D0 I.P., and E.A.) with a mean absolute error of 0.53 kcal/mol for the 64 first‐row examples from the G2 test set, and is computationally feasible for species with up to three first‐row atoms. A set of 20 CBS‐QCI/APNO bond dissociation energies of hydrocarbons also agree with known experimental values to within less than 1 kcal/mol. Calculations on the cyclopropenyl radical and cyclopropenylidene provide new dissociation energies which are in accord with an interpretation of the thermochemistry emphasizing ring strain and aromaticity.

  • Vinylidene and the Hammond postulate
    Journal of the American Chemical Society, 1992
    Co-Authors: George A Petersson, Thomas G. Tensfeldt, John A Montgomery
    Abstract:

    The potential energy barrier for the isomerization of vinylidene, :C=CH 2 , to acetylene, HC≡CH, has been calcuted by the Quadratic Configuration Interaction method using a 6s6p3d2f,4s2p1d bas of atomic pair natural orbitals with extrapolation to the complete basis set limit (CBS-QCI/[...3d2f]APNO model). The calculated barrier (ΔE e =2.2±0.5 kcal/mol) and energy change from vinylidene to acetylene (ΔE e =-43.9±0.5 kcal/mol) are in excellent agreement with recent experimental values (ΔE e =2 kcal/mol, and ΔE 0 =-44.1±0.7 kcal/mol)

  • a complete basis set model chemistry iii the complete basis set Quadratic Configuration Interaction family of methods
    Journal of Chemical Physics, 1991
    Co-Authors: George A Petersson, Thomas G. Tensfeldt, John A Montgomery
    Abstract:

    The major source of error in most ab initio calculations of molecular energies is the truncation of the one‐electron basis set. A family of complete basis set (CBS) Quadratic CI (QCI) model chemistries is defined to include corrections for basis set truncation errors. These models use basis sets ranging from the small 6‐31 G°° double zeta plus polarization (DZ+P) size basis set to the very large (14s9p4d2f,6s3p1d)/[6s6p3d2f,4s2p1d] atomic pair natural orbital basis set. When the calculated energies are compared with the experimental energies of the first‐row atoms and ions and the first‐row diatomics and hydrides H2, LiH, Li2, CH4, NH3, H2O, HF, LiF, N2, CO, NO, O2, and F2, two very promising new model chemistries emerge. The first is of comparable accuracy, but more than ten times the speed of the G1 model of Pople and co‐workers. The second is less than one‐tenth the speed of the G1 model, but reduces the root‐mean‐square (rms) errors in ionization potentials (IPs), electron affinities (EAs), and D0’s t...

  • a complete basis set model chemistry iii the complete basis set Quadratic Configuration Interaction family of methods
    Journal of Chemical Physics, 1991
    Co-Authors: George A Petersson, Thomas G. Tensfeldt, John A Montgomery
    Abstract:

    The major source of error in most ab initio calculations of molecular energies is the truncation of the one‐electron basis set. A family of complete basis set (CBS) Quadratic CI (QCI) model chemistries is defined to include corrections for basis set truncation errors. These models use basis sets ranging from the small 6‐31 G°° double zeta plus polarization (DZ+P) size basis set to the very large (14s9p4d2f,6s3p1d)/[6s6p3d2f,4s2p1d] atomic pair natural orbital basis set. When the calculated energies are compared with the experimental energies of the first‐row atoms and ions and the first‐row diatomics and hydrides H2, LiH, Li2, CH4, NH3, H2O, HF, LiF, N2, CO, NO, O2, and F2, two very promising new model chemistries emerge. The first is of comparable accuracy, but more than ten times the speed of the G1 model of Pople and co‐workers. The second is less than one‐tenth the speed of the G1 model, but reduces the root‐mean‐square (rms) errors in ionization potentials (IPs), electron affinities (EAs), and D0’s t...

Leo Radom - One of the best experts on this subject based on the ideXlab platform.