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[Lapack] dsyev and dsyevd


I have been using subroutines dsyev and dsyevd to compute the eigenvectors 
and eigenvalues of a symmetric 8x8 matrix. the matrix formed by these 
eigenvectors should form a unitary matrix and therefore be the basis of a 
unitary transformation. When I test the unitariy of the resulting matrix I 
find that it is quite inaccurate. In particular I find that the scalar 
product of two of the eigenvectors is 3.8D-5. this is far less accurate 
than I would like. As I need to perform at least 40,000 unitary 
transformations in my dynamical calculations such a large error allows a 
large build-up of errors and renders my calculations invalid.

All my variables are double precision. Can you advise as to how I can 
improve the accuracy of my eigenvectors.

Gabriel Balint-Kurti
Professor of Theoretical Chemistry

Gabriel Balint-Kurti
Centre for Computational Chemistry
School of Chemistry
University of Bristol
Bristol, BS8 1TS
Tel: +44 117 928 7662
fax: +44 117 925 1295

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