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Sudip Chattopadhyay - One of the best experts on this subject based on the ideXlab platform.

  • Applications of linear response theories to compute the low-lying potential energy surfaces: state-specific MRCEPA-based approach
    Journal of Physics B: Atomic Molecular and Optical Physics, 2007
    Co-Authors: Sudip Chattopadhyay, Debasis Mukhopadhyay
    Abstract:

    Linear response theories based on the state specific multi-reference coupled Electron-Pair Approximation-like methods (SS-MRCEPA) (Chattopadhyay and Mahapatra 2004 J. Phys. Chem. A 108 11664) (MRCEPA-LRT) have been amply utilized for computing excited-state potential energy surfaces (PES). Our MRCEPA-LRT methods provide energies in a size-intensive manner. As a pilot application, the effectiveness of the method is demonstrated by computing the PES of some low-lying excited states including the ground state of the BeH2 molecule. This system is very effective and widely used to judge the potentiality of any state-specific theory since its ground state possesses degeneracy/quasi-degeneracy and also faces intruders at different regions of the PES.

  • Towards the Development and Applications of Manifestly Spin-free Multi-reference Coupled Electron-Pair Approximation-like Methods: A State Specific Approach
    Theoretical Chemistry Accounts, 2006
    Co-Authors: Dola Pahari, Pradipta Ghosh, Debashis Mukherjee, Sudip Chattopadhyay
    Abstract:

    As a practical tool of being applicable to bigger molecules, a full-blown state-specific multi-reference coupled cluster formalism developed by us (Mahapatra et al. in J Chem Phys 110:6171, 1999) would be rather demanding computationally, and it is worthwhile to look for physically motivated Approximation schemes which capture a substantial portion of the correlation of the full-blown theory. In this spirit, we have recently proposed coupled Electron-Pair Approximation (CEPA)-like various approximants to the parent spin-adapted state-specific multi-reference coupled cluster (SS-MRCC) theory which depend on the inclusion of EPV terms to various degree. Here, the space of excitations is confined to the first order interactive virtual space generated by the cluster operator, but the EPV terms are included exactly. We call them spin-free state specific multi-reference CERA (SS-MRCEPA) theories. They work within the complete active space (CAS) and have been found to be very effective in bypassing the intruders, similar in performance to that of the parent SS-MRCC theory. The spin-adaptation of the working equations of both the SS-MRCC and the CEPA-like approximants is a non-trivial exercise. In this paper, we delineate briefly the essentials of a spin-free formulation of the SS-MRCC and SS-MRCEPA theories. This allows us to include open-shell configuration state functions (CSF) in the CAS. We consider three variants of SS-MRCEPA method. Two are explicitly orbital invariant: (1) SS-MRCEPA(0), a purely lineralized version of the SS-MRCC theory, (2) SS-MRCEPA(I), which includes all the EPV terms explicitly and exactly in an orbital invariant manner and (3) the SS-MRCEPA(D), which emerges when we keep only the diagonal terms of a set of dressed operators in the working equations. Unlike the first two, the third version is not invariant under the orbital transformation within the set of doubly occupied core, valence and virtual orbitals. The SS-MRCEPA methods produce very encouraging results as was evidenced in the applications on the computation of potential energy surfaces for the ground states of LiH and HF molecules.

  • molecular applications of a state specific multireference coupled Electron Pair Approximation ss mrcepa like method
    Journal of Physical Chemistry A, 2004
    Co-Authors: Sudip Chattopadhyay, Uttam Sinha Mahapatra
    Abstract:

    In this paper, we present a coupled Electron-Pair (CEPA) type variant of the state-specific multireference coupled cluster (SS-MRCC) method [Mahapatra, U. S., et al. J. Chem. Phys. 1999, 110, 6171]. The method termed as SS-MRCEPA based on complete active space (CAS) can handle quasi-degeneracy of varying degrees over a wide range of potential energy curves (PECs), including regions of real or avoided curve-crossing. The method is size-extensive and avoids the intruder problem in a natural manner. Exploiting a two-dimensional CAS-based SS-MRCEPA method, we consider, in this paper, several demanding molecular systems that benefit from multireference description. The reliability of computational results of the method for PECs of the ground state of P4, H4, H 8 , perpendicular insertion of Be into H 2 , Li 2 , and ground-state energy at the equilibrium point of CH 2 will be discussed with respect to the parent SS-MRCC and full CI/large scale CI results. We have also reported the excitation energies corresponding to the ground states of H 8 and CH 2 systems. The method has also been applied to study the bond breaking in the F 2 molecule which is a challenging task for any ab initio method. In all cases, the comparison is also made with the results obtained from other CC- and CEPA-type methods wherever available.

  • Numerically oriented static response approach based on state-specific multi-reference coupled Electron-Pair Approximation (SS-MRCEPA) like methods
    Journal of Physics B: Atomic Molecular and Optical Physics, 2004
    Co-Authors: Sudip Chattopadhyay
    Abstract:

    In this paper, we have exploited the recently developed state-specific multi-reference coupled Electron-Pair Approximation (SS-MRCEPA) like methods for computing the electrostatic response properties. The SS-MRCEPA methods are formulated on complete active space reference functions and the required energies are computed via the diagonalization of an effective operator within this space. The SS-MRCEPA theories are size-consistent and size-extensive. They are very efficient for treating quasidegeneracy of varying extent and for bypassing the intruder problem. The efficacy of the methods is illustrated via the computation of the static dipole moment and polarizability of the ground state and the corresponding first excited state of the trapezoidal H4 model (H4) as well as the lowest two singlet states of the CH2 system using perturbed orbitals generated via the finite-field strategy, a numerically oriented static response approach.

  • a state specific approach to multireference coupled Electron Pair Approximation like methods development and applications
    Journal of Chemical Physics, 2004
    Co-Authors: Sudip Chattopadhyay, Dola Pahari, Debashis Mukherjee, Uttam Sinha Mahapatra
    Abstract:

    The traditional multireference (MR) coupled-cluster (CC) methods based on the effective Hamiltonian are often beset by the problem of intruder states, and are not suitable for studying potential energy surface (PES) involving real or avoided curve crossing. State-specific MR-based approaches obviate this limitation. The state-specific MRCC (SS-MRCC) method developed some years ago [Mahapatra et al., J. Chem. Phys. 110, 6171 (1999)] can handle quasidegeneracy of varying degrees over a wide range of PES, including regions of real or avoided curve-crossing. Motivated by its success, we have suggested and explored in this paper a suite of physically motivated coupled Electron-Pair Approximations (SS-MRCEPA) like methods, which are designed to capture the essential strength of the parent SS-MRCC method without significant sacrificing its accuracy. These SS-MRCEPA theories, like their CC counterparts, are based on complete active space, treat all the reference functions on the same footing and provide a descrip...

Jean-paul Malrieu - One of the best experts on this subject based on the ideXlab platform.

  • Ability of a coupled Electron Pair Approximation to treat single bond breakings
    Chemical Physics Letters, 2010
    Co-Authors: Jean-paul Malrieu, Hongjiang Zhang
    Abstract:

    The self-consistent size-consistent CI ((SC)2CI), which considers all exclusion principle violating (EPV) diagrams, is able to treat single bond breakings, despite its single-reference character. It is shown here that a CEPA-3 (coupled Electron Pair Approximation) dressing, considering only the EPVs through the occupied molecular orbitals, treats satisfactorily the single bond breaking processes, while the uniform dressings, e.g. CEPA-0, the averaged coupled Pair functional (ACPF) or averaged quadratic coupled cluster (AQCC) lead to non-convergence or spurious energy lowering in such processes. A number of test calculations illustrate this rationalizable feature.

  • Ability of a coupled Electron Pair Approximation to treat single bond breakings
    Chemical Physics Letters, 2010
    Co-Authors: Jean-paul Malrieu, Hongjiang Zhang
    Abstract:

    International audienceThe self-consistent size-consistent CI ((SC)2CI), which considers all exclusion principle violating (EPV) diagrams, is able to treat single bond breakings, despite its single-reference character. It is shown here that a CEPA-3 (coupled Electron Pair Approximation) dressing, considering only the EPVs through the occupied molecular orbitals, treats satisfactorily the single bond breaking processes, while the uniform dressings, e.g. CEPA-0, the averaged coupled Pair functional (ACPF) or averaged quadratic coupled cluster (AQCC) lead to non-convergence or spurious energy lowering in such processes. A number of test calculations illustrate this rationalizable feature

  • size consistent multireference configuration interaction method through the dressing of the norm of determinants
    Molecular Physics, 2003
    Co-Authors: Jaroslaw Meller, Jean-paul Malrieu, Jean-louis Heully
    Abstract:

    A new determinant-specific, effective change (‘dressing’) of the norm of the multireference configuration interaction (MRCI) wavefunction is proposed in order to achieve the size-consistency of the MRCI method. The new approach provides a unifying framework for analysis of size-consistent extensions of the MRCI method that are based on the coupled Pair functional (CPF) strategy and lead to simplified computations of the analytical gradients. Using the new framework, a generalized multireference full coupled Pair functional (MR-FCPF) method is introduced. The MR-FCPF method may be viewed as a functional counterpart of the recently proposed generalized (‘full’) coupled Electron Pair Approximation (CEPA), referred to as the size-consistent self-consistent CI ((SC)2CI) method. A straightforward extension of the MR-FCPF method leads to a pseudo-functional form of the coupled cluster (CC) type formalisms. Therefore, the new approach may be used to introduce a simple alternative to existing CC-type gradient tech...

  • Direct determination of localized Hartree–Fock orbitals as a step toward N scaling procedures
    The Journal of Chemical Physics, 1997
    Co-Authors: Jaime Rubio, Jean-paul Malrieu, Àngels Povill, Peter Reinhardt
    Abstract:

    A method is proposed for the solution of the self-consistent field equations that can lead to localized occupied and virtual molecular orbitals, avoiding the need for solving for the canonical molecular orbitals. The method starts with strongly localized “guess molecular orbitals”, it is nonperturbative and proceeds through the diagonalization of single configuration interaction matrices which may be rendered size-consistent through appropriate coupled Electron Pair Approximation or coupled-cluster-type dressings. We see a potential utility for the method in applications to large systems where localized orbitals will improve the scaling of the computational resources required with increasing system size.

  • Size-consistent self-consistent truncated or selected configuration interaction
    The Journal of Chemical Physics, 1993
    Co-Authors: Jean-pierre Daudey, Jean-louis Heully, Jean-paul Malrieu
    Abstract:

    Based on the principle of intermediate effective Hamiltonians, a simple procedure is proposed in order to eliminate the unlinked contributions of any truncated or selected configuration interaction (CI). The corrections are diagonal energy shifts, easily calculated. A self‐consistent version is proposed, which insures separability if localized molecular orbitals (MO) are used. In the special case of double CI, the present method is an improved version of the coupled Electron Pair Approximation (CEPA), but it may be applied to any selected model space, involving configurations of various degrees of excitation. The efficiency of the proposed algorithms is illustrated on a series of test calculations performed on Be2, F2, N2, and NH3.

Debashis Mukherjee - One of the best experts on this subject based on the ideXlab platform.

  • Towards the Development and Applications of Manifestly Spin-free Multi-reference Coupled Electron-Pair Approximation-like Methods: A State Specific Approach
    Theoretical Chemistry Accounts, 2006
    Co-Authors: Dola Pahari, Pradipta Ghosh, Debashis Mukherjee, Sudip Chattopadhyay
    Abstract:

    As a practical tool of being applicable to bigger molecules, a full-blown state-specific multi-reference coupled cluster formalism developed by us (Mahapatra et al. in J Chem Phys 110:6171, 1999) would be rather demanding computationally, and it is worthwhile to look for physically motivated Approximation schemes which capture a substantial portion of the correlation of the full-blown theory. In this spirit, we have recently proposed coupled Electron-Pair Approximation (CEPA)-like various approximants to the parent spin-adapted state-specific multi-reference coupled cluster (SS-MRCC) theory which depend on the inclusion of EPV terms to various degree. Here, the space of excitations is confined to the first order interactive virtual space generated by the cluster operator, but the EPV terms are included exactly. We call them spin-free state specific multi-reference CERA (SS-MRCEPA) theories. They work within the complete active space (CAS) and have been found to be very effective in bypassing the intruders, similar in performance to that of the parent SS-MRCC theory. The spin-adaptation of the working equations of both the SS-MRCC and the CEPA-like approximants is a non-trivial exercise. In this paper, we delineate briefly the essentials of a spin-free formulation of the SS-MRCC and SS-MRCEPA theories. This allows us to include open-shell configuration state functions (CSF) in the CAS. We consider three variants of SS-MRCEPA method. Two are explicitly orbital invariant: (1) SS-MRCEPA(0), a purely lineralized version of the SS-MRCC theory, (2) SS-MRCEPA(I), which includes all the EPV terms explicitly and exactly in an orbital invariant manner and (3) the SS-MRCEPA(D), which emerges when we keep only the diagonal terms of a set of dressed operators in the working equations. Unlike the first two, the third version is not invariant under the orbital transformation within the set of doubly occupied core, valence and virtual orbitals. The SS-MRCEPA methods produce very encouraging results as was evidenced in the applications on the computation of potential energy surfaces for the ground states of LiH and HF molecules.

  • a state specific approach to multireference coupled Electron Pair Approximation like methods development and applications
    Journal of Chemical Physics, 2004
    Co-Authors: Sudip Chattopadhyay, Dola Pahari, Debashis Mukherjee, Uttam Sinha Mahapatra
    Abstract:

    The traditional multireference (MR) coupled-cluster (CC) methods based on the effective Hamiltonian are often beset by the problem of intruder states, and are not suitable for studying potential energy surface (PES) involving real or avoided curve crossing. State-specific MR-based approaches obviate this limitation. The state-specific MRCC (SS-MRCC) method developed some years ago [Mahapatra et al., J. Chem. Phys. 110, 6171 (1999)] can handle quasidegeneracy of varying degrees over a wide range of PES, including regions of real or avoided curve-crossing. Motivated by its success, we have suggested and explored in this paper a suite of physically motivated coupled Electron-Pair Approximations (SS-MRCEPA) like methods, which are designed to capture the essential strength of the parent SS-MRCC method without significant sacrificing its accuracy. These SS-MRCEPA theories, like their CC counterparts, are based on complete active space, treat all the reference functions on the same footing and provide a descrip...

  • An orbital-invariant coupled Electron-Pair like approximant to a state-specific multi-reference coupled cluster formalism
    Chemical Physics Letters, 2004
    Co-Authors: Dola Pahari, Sudip Chattopadhyay, Avik Deb, Debashis Mukherjee
    Abstract:

    Almost all the coupled Electron Pair Approximation (CEPA)-like developments based on multi-reference functions do not satisfy invariance with respect to separate unitary transformation among doubly occupied, active and virtual orbitals. Those, which do satisfy this criterion, use empirical CEPA-like corrections involving suitably averaged Pair-correlation terms. We develop in this Letter a CEPA-like approximant to the state-specific multi-reference coupled cluster (SS-MRCC) formalism [J. Chem. Phys. 100, (1999) 6171] which includes all the exclusion principle violating terms rigorously, and display manifest orbital-invariance. The theory, termed as SS-MRCEPA(I), is also rigorously size-extensive, size-consistent and bypasses intruders in a natural manner.

  • state specific multi reference coupled Electron Pair Approximation like methods formulation and molecular applications
    Chemical Physics Letters, 2002
    Co-Authors: Sudip Chattopadhyay, Uttam Sinha Mahapatra, Barnali Datta, Debashis Mukherjee
    Abstract:

    We present two variants of state-specific multi-reference coupled Electron-Pair type approximants (SS-MRCEPA) of our recently formulated state-specific multi-reference coupled-cluster (SS-MRCC) theory. Just like the parent SS-MRCC theory, these are formulated with a complete active space, and are rigorously size-extensive and size-consistent. They also bypass the intruder problem very efficiently. The efficacy of the methods is illustrated with the computation of the ground state potential energy surface of the trapezoidal H4 model, where the ground state requires a two-determinantal model space and the effective hamiltonian methods face intruders.

  • Coupled Electron Pair Approximation Calculation of the Electric Dipole Moment of Atomic Yb
    arXiv: Atomic Physics, 2001
    Co-Authors: Angom Dilip Singh, B. P. Das, Debashis Mukherjee
    Abstract:

    The existence of a finite electric dipole moment (EDM) d_a of the closed-shell atom Yb implies parity and time reversal violations involving the nuclear sector. An important effect which can contribute to the Yb EDM is the tensor-pseudotensor Electron-nucleus interaction characterized by the coupling constant C_T. Within the Standard Model (SM) of particle physics C_T=0, as this form of interaction is not allowed. If a finite d_a of Yb is observed in experiments, then an estimate of C_T can be obtained by combining with the theoretical calculations. A non-zero C_T implies physics beyond the standard model. In this paper we present the result of our ab initio calculation of the EDM Yb using different many-body methods.

Peter Botschwina - One of the best experts on this subject based on the ideXlab platform.

  • Calculated vibrational structure of the photoElectron spectra of free radicals
    Berichte der Bunsengesellschaft für physikalische Chemie, 1995
    Co-Authors: M. Horn, M. Oswald, Rainer Oswald, Peter Botschwina
    Abstract:

    The vibrational structure of the first bands of the photoElectron (PE) spectra of the radicals SiH 3 , CF 3 , CH 2 CN and CH 2 NC has been calculated by means of the Coupled Electron Pair Approximation. Excellent agreement with experiment is obtained for SiH 3 . A long progression in the umbrella bending vibration is calculated for CF 3 . Furthermore, two combination tone series should be observable in the PE spectrum at higher resolution. The PE spectra of CH 2 CN and CH 2 NC are dominated by the adiabatic peaks. Adiabatic ionization potentials of 8.98 ±0.05 eV, 10.20±0.05 eV and 9.36±0.03 eV are predicted for CF 3 , CH 2 CN and CH 2 NC

  • Calculated spectroscopic properties for NH3…HC4H
    Chemical Physics Letters, 1995
    Co-Authors: Bernd Schulz, Peter Botschwina
    Abstract:

    Abstract The hydrogen-bonded hetero-dimer NH 3 …HC 4 H has been investigated by means of the coupled Electron Pair Approximation (CEPA), making use of a basis set of 210 contracted Gaussian-type orbitals. The equilibrium dissociation energy is estimated to be 17.5±1.0 kJ mol −1 and the equilibrium dipole moment is μ e = −2.468 D, with the positive end at the site of the ammonia protons. Compared to the IR-active antisymmetric CH stretching vibration of free diacetylene the wavenumbers of the CH vibrations in the complex are reduced by 126.6 cm −1 (inner CH stretch) and increased by 4.7 cm −1 (outer CH stretch). The corresponding intensity ratio is 5.8. The pseudosymmetric acetylenic CC vibration has a large intensity of 20 km mol −1 . The intensity of the symmetric NH stretching vibration is smaller than in free NH 3 by a factor of 2.

  • An ab initio calculation of the vibrational structure of the first band of the photoElectron spectrum of CCl3
    Chemical Physics Letters, 1994
    Co-Authors: M. Horn, Peter Botschwina
    Abstract:

    Abstract Making use of the coupled Electron Pair Approximation (CEPA-1), two-dimensional potential energy functions have been calculated for the Electronic ground states of CCl 3 and CCl + 3 . Therefrom, vibrational term energies and wavefunctions were obtained variationally and were used to compute the rather complex vibrational structure of the first band of the photoElectron spectrum within the Franck—Condon Approximation. The adiabatic ionization energy is calculated to be 7.990 eV which compares well with the experimental values of 8.109 ± 0.005 eV.

  • Calculated spectroscopic constants and the equilibrium geometry of HCCCl
    J. Chem. Soc. Faraday Trans., 1993
    Co-Authors: M. Horn, Peter Botschwina, J. Flügge
    Abstract:

    Making use of experimental values for the ground-state rotational constants of 12 different isotopomers and vibration–rotation coupling constants calculated by the coupled Electron Pair Approximation (CEPA) an accurate equilibrium geometry has been determined for monochloroacetylene: re(CH)= 1.0605(5)A, R1e(CC)= 1.2030(2)A and R2e(CCl)= 1.6353(1)A. The equilibrium dipole moment is calculated by CCSD(T) using a basis set of 165 contracted Gaussian-type orbitals: µe=–0.433 D.

  • Equilibrium geometry of HC3N
    Molecular Physics, 1993
    Co-Authors: Peter Botschwina, M. Horn, S. Seeger, J. Flügge
    Abstract:

    A near-equilibrium potential energy surface has been calculated for cyanoacetylene, HC3N, by means of the coupled Electron Pair Approximation (CEPA) using a basis set of 118 contracted Gaussian-type orbitals, and from it vibration-rotation coupling constants αv and 1-type doubling constants qt and qJ t have been calculated for various isotopomers by standard perturbation theory. By the combination of experimental B 0 and theoretical αv values for six different isotopomers we were able to determine an accurate equilibrium geometry: r e(HC(1)) = 1·0624 A, R 1e(C(1)C(2)) = 1·2058 A, R2e(C(2)C(3)) = 1·3764 A and R 3e(C(3)N) = 1·1605 A. A conservative estimate of the error in the equilibrium bond lengths is 0·0005 A.

Hongjiang Zhang - One of the best experts on this subject based on the ideXlab platform.

  • Ability of a coupled Electron Pair Approximation to treat single bond breakings
    Chemical Physics Letters, 2010
    Co-Authors: Jean-paul Malrieu, Hongjiang Zhang
    Abstract:

    The self-consistent size-consistent CI ((SC)2CI), which considers all exclusion principle violating (EPV) diagrams, is able to treat single bond breakings, despite its single-reference character. It is shown here that a CEPA-3 (coupled Electron Pair Approximation) dressing, considering only the EPVs through the occupied molecular orbitals, treats satisfactorily the single bond breaking processes, while the uniform dressings, e.g. CEPA-0, the averaged coupled Pair functional (ACPF) or averaged quadratic coupled cluster (AQCC) lead to non-convergence or spurious energy lowering in such processes. A number of test calculations illustrate this rationalizable feature.

  • Ability of a coupled Electron Pair Approximation to treat single bond breakings
    Chemical Physics Letters, 2010
    Co-Authors: Jean-paul Malrieu, Hongjiang Zhang
    Abstract:

    International audienceThe self-consistent size-consistent CI ((SC)2CI), which considers all exclusion principle violating (EPV) diagrams, is able to treat single bond breakings, despite its single-reference character. It is shown here that a CEPA-3 (coupled Electron Pair Approximation) dressing, considering only the EPVs through the occupied molecular orbitals, treats satisfactorily the single bond breaking processes, while the uniform dressings, e.g. CEPA-0, the averaged coupled Pair functional (ACPF) or averaged quadratic coupled cluster (AQCC) lead to non-convergence or spurious energy lowering in such processes. A number of test calculations illustrate this rationalizable feature