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Title: A Systematic Multireference Perturbation-Theory Study of the Low-Lying State of SiC3

Abstract

No abstract prepared.

Authors:
; ; ;
Publication Date:
Research Org.:
Ames Laboratory (AMES), Ames, IA
Sponsoring Org.:
USDOE Office of Science and Technology (OST) - (EM-50)
OSTI Identifier:
877133
Report Number(s):
IS-J 7074
TRN: US200606%%708
DOE Contract Number:
W-7405-Eng-82
Resource Type:
Journal Article
Resource Relation:
Journal Name: The Journal of Chemical Physics; Journal Volume: 124; Journal Issue: 3
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; PERTURBATION THEORY; SILICON CARBIDES; ENERGY LEVELS

Citation Formats

Jamie M. Rintelman, Mark S. Gordon, Graham D. Fletcher, and Joseph Ivanic. A Systematic Multireference Perturbation-Theory Study of the Low-Lying State of SiC3. United States: N. p., 2006. Web.
Jamie M. Rintelman, Mark S. Gordon, Graham D. Fletcher, & Joseph Ivanic. A Systematic Multireference Perturbation-Theory Study of the Low-Lying State of SiC3. United States.
Jamie M. Rintelman, Mark S. Gordon, Graham D. Fletcher, and Joseph Ivanic. Wed . "A Systematic Multireference Perturbation-Theory Study of the Low-Lying State of SiC3". United States. doi:.
@article{osti_877133,
title = {A Systematic Multireference Perturbation-Theory Study of the Low-Lying State of SiC3},
author = {Jamie M. Rintelman and Mark S. Gordon and Graham D. Fletcher and Joseph Ivanic},
abstractNote = {No abstract prepared.},
doi = {},
journal = {The Journal of Chemical Physics},
number = 3,
volume = 124,
place = {United States},
year = {Wed Jan 18 00:00:00 EST 2006},
month = {Wed Jan 18 00:00:00 EST 2006}
}
  • Near-equilibrium two-dimensional ([ital C][sub 2][ital v] symmetry) potential energy functions of the first seven electronic states of ClO[sub 2][sup +] and the ground [ital X] [sup 1][ital A][sub 1] electronic state of ClO[sub 2][sup [minus]] have been calculated using internally contracted multireference configuration interaction (CMRCI) wave functions. In the case of ClO[sub 2][sup +], five electronic states ([sup 3][ital B][sub 2], [sup 3][ital A][sub 2], [sup 1][ital A][sub 2], [sup 3][ital B][sub 1], and [sup 1][ital B][sub 1]) are predicted to lie within 3 eV of the [ital X] [sup 1][ital A][sub 1] ground state, while a [sup 1][ital B][submore » 2] state has a calculated [ital T][sub [ital e]] of just over 5 eV. In the [ital X] state, the equilibrium geometry is calculated by CMRCI to be [ital r][sub [ital e]]=1.423 A and [theta][sub [ital e]]=120.8[degree]. Harmonic vibrational frequencies of 1012 ([omega][sub 1]), 511 ([omega][sub 2]), and 1283 cm[sup [minus]1] ([omega][sub 3]) were derived from the computed potential energy function, and [omega][sub 1] and [omega][sub 2] were found to be in good agreement with the values estimated from the photoelectron spectrum of ClO[sub 2]. For the [ital X] [sup 1][ital A][sub 1] state of ClO[sub 2][sup [minus]], calculated spectroscopic constants include [ital r][sub [ital e]]=1.573 A, [theta][sub [ital e]]=113.9[degree], [omega][sub 1]=789 cm[sup [minus]1], [omega][sub 2]=378 cm[sup [minus]1], and [omega][sub 3]=848 cm[sup [minus]1]. Electric dipole moment functions have also been derived for both species in their electronic ground states. The calculated equilibrium dipole moments for ClO[sub 2][sup +] and ClO[sub 2][sup [minus]] 1.46 and 2.55 D, respectively, make them good candidates for observation by pure rotational spectroscopy.« less
  • Multi-reference (MR) electronic structure methods, such as MR configuration interaction or MR perturbation theory, can provide reliable energies and properties for many molecular phenomena like bond breaking, excited states, transition states or magnetic properties of transition metal complexes and clusters. However, owing to their inherent complexity, most MR methods are still too computationally expensive for large systems. Therefore the development of more computationally attractive MR approaches is necessary to enable routine application for large-scale chemical systems. Among the state-of-the-art MR methods, second-order N-electron valence state perturbation theory (NEVPT2) is an efficient, size-consistent, and intruder-state-free method. However, there are still twomore » important bottlenecks in practical applications of NEVPT2 to large systems: (a) the high computational cost of NEVPT2 for large molecules, even with moderate active spaces and (b) the prohibitive cost for treating large active spaces. In this work, we address problem (a) by developing a linear scaling “partially contracted” NEVPT2 method. This development uses the idea of domain-based local pair natural orbitals (DLPNOs) to form a highly efficient algorithm. As shown previously in the framework of single-reference methods, the DLPNO concept leads to an enormous reduction in computational effort while at the same time providing high accuracy (approaching 99.9% of the correlation energy), robustness, and black-box character. In the DLPNO approach, the virtual space is spanned by pair natural orbitals that are expanded in terms of projected atomic orbitals in large orbital domains, while the inactive space is spanned by localized orbitals. The active orbitals are left untouched. Our implementation features a highly efficient “electron pair prescreening” that skips the negligible inactive pairs. The surviving pairs are treated using the partially contracted NEVPT2 formalism. A detailed comparison between the partial and strong contraction schemes is made, with conclusions that discourage the strong contraction scheme as a basis for local correlation methods due to its non-invariance with respect to rotations in the inactive and external subspaces. A minimal set of conservatively chosen truncation thresholds controls the accuracy of the method. With the default thresholds, about 99.9% of the canonical partially contracted NEVPT2 correlation energy is recovered while the crossover of the computational cost with the already very efficient canonical method occurs reasonably early; in linear chain type compounds at a chain length of around 80 atoms. Calculations are reported for systems with more than 300 atoms and 5400 basis functions.« less
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  • The effect of the reference space on the convergence of Rayleigh--Schroedinger perturbation series within the molecular-orbitals framework is studied for the ground X /sup 1/..sigma../sup +//sub g/ and excited EF /sup 1/..sigma../sup +//sub g/ electronic states of H/sub 2/ over a wide range of internuclear separations. Near the ground-state equilibrium distance of 1.4 bohr each state is well described by a single spin-adapted configuration function. This no longer holds for the EF /sup 1/..sigma../sup +//sub g/ state around 3.0 bohr because of forbidden curve crossing. In general, the quasidegeneracy increases with the internuclear distance due to improper dissociation of molecularmore » orbitals. A rigorous approach to define a proper reference space is discussed. It is based on analysis of convergence with emphasis on identifying intruder states. A reference space of nine spin-adapted functions is adequate in the range 1.4--8.0 bohr; giving third-order results within less than 1 x 10/sup -3/ hartree from the basis limit. Other findings are: (i) Epstein--Nesbet breakup of the Hamiltonian usually gives faster convergence as compared with the Moller--Plesset scheme. (ii) Pade approximants improve the results but only when the reference space is capable of describing the state. When this is not the case the Pade sequence is erratic and physically meaningless. (iii) With suitably defined reference space there is only a marginal difference in the results between different breakups of the Hamiltonian, and little improvement is gained by employing Pade approximants.« less
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