A low-storage filter-diagonalization method to calculate expectation values of operators non-commutative with the Hamiltonian: Vibrational assignmentof HOC1

A method based on the Hellmann-Feynman theorem is proposed for calculating expectation values of operators non-commutative with the system Hamiltonian , (H) over cap(0). It is applicable in both the low-storage filter-diagonal ization (LSFD) and Lanczos methods. Specifically, LSFD is used to determine eigenvalues of H, and a perturbed Hamiltonian consisting of (H) over cap(0 ), and the non-commutative operator (H) over cap': (H) over cap(lambda) = ( H) over cap(0) + lambda (H) over cap'. The difference between the two spect ra yields the expectation values of (H) over cap' without explicit calculat ing the eigenstates. For an appropriate choice of (H) over cap', informatio n about vibrational assignments can be obtained. As an example, the vibrati onal assignment of HOCl and its Fermi resonances are analyzed. (C) 1999 Els evier Science B.V. All rights reserved.