Abstract:In this paper we propose a new quadrature scheme for computing vibrational spectra and apply it, using a Lanczos algorithm, to CH(3)CN. All 12 coordinates are treated explicitly. We need only 157'419'523 quadrature points. It would not be possible to use a product Gauss grid because 33 853 318 889 472 product Gauss points would be required. The nonproduct quadrature we use is based on ideas of Smolyak, but they are extended so that they can be applied when one retains basis functions θ(n(1))(r(1))···θ(n(D))(r(… Show more
“…When quadrature is used, all potential terms are treated together, and there is no need to evaluate matrix-vector products term by term. There is also no need to approximate the potential as a sum of terms depending on one, two, three, etc coordinates [52,53,54]. When one uses the full potential, and therefore multidimensional quadrature, the size of the quadrature grid is an important problem.…”
Section: Mathematical Methods For High-dimensional Problemsmentioning
“…When quadrature is used, all potential terms are treated together, and there is no need to evaluate matrix-vector products term by term. There is also no need to approximate the potential as a sum of terms depending on one, two, three, etc coordinates [52,53,54]. When one uses the full potential, and therefore multidimensional quadrature, the size of the quadrature grid is an important problem.…”
Section: Mathematical Methods For High-dimensional Problemsmentioning
“…At each step, the n c and k c indices are constrained among themselves. Everything is clearly explained in [90]. The Smolyak grid is a sum of smaller direct product grids and therefore has structure that makes it possible to evaluate MVPs by doing sums sequentially.…”
Section: Using Pruning To Reduce Both Basis and Grid Sizementioning
I review some computational methods for calculating vibrational spectra. They all use iterative eigensolvers to compute eigenvalues of a Hamiltonian matrix by evaluating matrix-vector products (MVPs). A direct-product basis can be used for molecules with five or fewer atoms. This is done by exploiting the structure of the basis and the structure of a direct product quadrature grid. I outline three methods that can be used for molecules with more than five atoms. The first uses contracted basis functions and an intermediate (F) matrix. The second uses Smolyak quadrature and a pruned basis. The third uses a tensor rank reduction scheme.
We propose an iterative method for computing vibrational spectra that significantly reduces the memory cost of calculations. It uses a direct product primitive basis, but does not require storing vectors with as many components as there are product basis functions. Wavefunctions are represented in a basis each of whose functions is a sum of products (SOP) and the factorizable structure of the Hamiltonian is exploited. If the factors of the SOP basis functions are properly chosen, wavefunctions are linear combinations of a small number of SOP basis functions. The SOP basis functions are generated using a shifted block power method. The factors are refined with a rank reduction algorithm to cap the number of terms in a SOP basis function. The ideas are tested on a 20-D model Hamiltonian and a realistic CH 3 CN (12 dimensional) potential. For the 20-D problem, to use a standard direct product iterative approach one would need to store vectors with about 10 20 components and would hence require about 8 × 10 11 GB. With the approach of this paper only 1 GB of memory is necessary. Results for CH 3 CN agree well with those of a previous calculation on the same potential.
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