Dear ALPS users,
I've been using the DMRG algorithm provided by ALPS to calculate energies and spin gaps for simple quantum systems. I've noticed that the value calculated for the truncation error comes out to be negative for some of the iterations. These specific instances have very small values, typically 10^-16, 10^-17 and 10^-18. My understanding of the truncation error is that it is the sum of the probabilities of the truncated states, thus always producing a positive value. So I'm not sure how a negative truncation error is occurring. It seems to occur on any DMRG program I run, even the simple ones provided in the DMRG tutorials. Perhaps it is a quirk of the algorithm or how the computer is handling some rounding. I found message on the mailing list archives about this exact question but it was never answered.
I've attached some output data. I used the program 'spin_one_half.py' straight from the first DMRG tutorial herehttp://alps.comp-phys.org/mediawiki/index.php/ALPS_2_Tutorials:DMRG-01_DMRG, unmodified, to produce the provided output. Any insight and advice is much appreciated. Thanks.
Regards, Tyler