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Using Group Theory to Obtain Eigenvalues of Nonsymmetric Systems by Symmetry Averaging
Department of Chemistry, The University of Texas at El Paso, El Paso, Texas 79968-0513, USA
Received: 10 July 2009; in revised form: 5 August 2009 / Accepted: 6 August 2009 / Published: 6 August 2009
Abstract: If the Hamiltonian in the time independent Schrödinger equation, HΨ = EΨ, is invariant under a group of symmetry transformations, the theory of group representations can help obtain the eigenvalues and eigenvectors of H. A finite group that is not a symmetry group of H is nevertheless a symmetry group of an operator Hsym projected from H by the process of symmetry averaging. In this case H = Hsym + HR where HR is the nonsymmetric remainder. Depending on the nature of the remainder, the solutions for the full operator may be obtained by perturbation theory. It is shown here that when H is represented as a matrix [H] over a basis symmetry adapted to the group, the reduced matrix elements of [Hsym] are simple averages of certain elements of [H], providing a substantial enhancement in computational efficiency. A series of examples are given for the smallest molecular graphs. The first is a two vertex graph corresponding to a heteronuclear diatomic molecule. The symmetrized component then corresponds to a homonuclear system. A three vertex system is symmetry averaged in the first case to Cs and in the second case to the nonabelian C3v. These examples illustrate key aspects of the symmetry-averaging process.
Keywords: Hamiltonian symmetry; group theory; symmetry-adapted basis; reduced matrix elements; symmetry-averaging
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MDPI and ACS Style
Ellzey, M.L., Jr. Using Group Theory to Obtain Eigenvalues of Nonsymmetric Systems by Symmetry Averaging. Symmetry 2009, 1, 10-20.
Ellzey ML, Jr. Using Group Theory to Obtain Eigenvalues of Nonsymmetric Systems by Symmetry Averaging. Symmetry. 2009; 1(1):10-20.
Ellzey, Marion L., Jr. 2009. "Using Group Theory to Obtain Eigenvalues of Nonsymmetric Systems by Symmetry Averaging." Symmetry 1, no. 1: 10-20.