Basis vector importance sampling for Hamiltonian lattice spectrum calculations

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Abstract
Different numerical methods for accurate calculation of low-lying eigenvalues of lattice Hamiltonians are proposed and critically compared. A dynamical procedure, called basis vector importance sampling, is shown to select the relevant subspace of the Hilbert space very effectively. This method is used to compute the mass gap of O(2) symmetric quantum chains up to a length of nine sites. Kosterlitz-Thouless type freezing transitions of Z(p) symmetric chains are also studied via the spectrum of quantum kinks.